MEDICAL 


Florence  J.   Chubb 
Memorial. 


A 


MANUAL  OF  ZOOLOGY 


BY 


RICHARD  HERTWIG 

Professor  of  Zoology  in  the^University  at  Munich 


SECOND  AMERICAN  EDITION  FROM  THE 
FIFTH  GERMAN  EDITION 


TRANSLATED    AND   EDITED    BY 

J.  S.  KINGSLEY 

Professor  of  Zoology  in  Tufts  College 


NEW  YORK 
HENRY  HOLT   AND   COMPANY 


Copyright,  1902, 

BY 
HENRY  HOLT  &  CO. 


ROBERT   DRUMMOND,    PRINTER,  NEW    YORK. 


PEEFACE. 

ON  account  of  its  clearness  and  breadth  of  view,  its  comparatively 
simple  character  and  moderate  size,  Professor  Richard  Hertwig's 
'  Lehrbuch  der  Zoologie '  has  for  ten  years  held  the  foremost  place 
in  German  schools.  The  first  or  general  part  of  the  work  was, 
translated  in  1896  by  Dr.  George  AY.  Field,  and  the  cordial  recep- 
tion which  this  has  had  in  America  has  led  to  the  present  reproduc- 
tion of  the  whole. 

This  American  edition  is  not  an  exact  translation.  "With  the 
consent  of  the  author  the  whole  text  has  been  edited  and  modified 
in  places  to  accord  with  American  usage.  For  these  changes  the 
translator  alone  can  be  held  responsible.  Some  of  the  alterations 
are  slight,  but  others  are  very  considerable.  Thus  the  group  of 
Yermes  of  the  original  has  been  broken  up  and  its  members  dis- 
tributed among  several  phyla;  the  account  of  the  Arthropoda  has 
been  largely  rewritten  and  the  classification  materially  altered ;  while 
the  Tunicata  and  the  Enteropneusti  have  been  removed  from  their 
position  as  appendices  to  the  Vermes  and  united  with  the  Yerte- 
brata  to  form  the  phylum  Chordata.  Other  changes,  like  those  in 
the  classification  of  the  Reptilia  and  the  nephridial  system  of  the 
vertebrates,  are  of  less  importance. 

A  large  number  of  illustrations  have  been  added,  either  to  make 
clearer  points  of  structure  or  to  aid  in  the  identification  of  American 
forms.  Except  in  the  Protozoa,  American  genera  have  in  most 
cases  been  indicated  by  an  asterisk.  Numerous  genera  have  been 
mentioned  so  that  the  student  may  see  the  relationships  of  forms 
described  in  morphological  literature. 

In  the  translation  the  word  Anlage,  meaning  the  embryonic 
material  from  which  an  organ  or  a  part  is  developed,  has  been 
transferred  directly.  As  our  language  is  Germanic  in  its  genius, 
there  can  be  no  valid  objection  to  the  adoption  of  the  word. 

As  this  work  is  intended  for  beginners,  no  bibliography  has  been 
given.  A  list  of  literature  to  be  of  much  value  would  have  been  so 
large  as  to  materially  increase  the  size  of  the  volume.  Experience 

iii 


~  II 


IV  PREFACE. 

has  shown  that  beginners  rarely  go  to  the  original  sources.  This 
omission  is  the  less  important  since  in  all  schools  where  the  book  is 
likely  to  be  used  other  works  containing  good  bibliographies  are 
accessible.  Eeference  might  here  be  made  to  those  in  the  Anat- 
omies of  Lang  and  Wiedersheim,  the  Embryologies  of  Balfour, 
Korschelt  and  Heider,  Minot,  and  Hertwig,  and  Wilson's  work  on 
The  Cell. 

The  editor  must  here  return  his  thanks  to  Dr.  George  W.  Field 
for  his  kindness  in  allowing  the  use  of  his  translation  of  the  first 
part  of  the  book  as  the  basis  of  the  present  edition. 

J.  S.  KINGSLEY. 
TUFTS  COLLEGE,  MASS.,  Sept.  19, 1902. 


TABLE  OF  CONTENTS. 

PAGE 

INTRODUCTION x 

HISTORY  OF  ZOOLOGY 7 

DEVELOPMENT   OF  SYSTEMATIC  ZOOLOGY 8 

DEVELOPMENT  OF   MORPHOLOGY 12 

REFORM  OF  THE  SYSTEM !8 

HISTORY  OF  THE  THEORY  OF  EVOLUTION 19 

DARWIN'S  THEORY  OF  THE  ORIGIN  OF  SPECIES 25 

GENERAL  MORPHOLOGY  AND  PHYSIOLOGY 57 

GENERAL  ANATOMY 58 

The  Morphological  Units  of  the  Animal  Body 58 

The  Tissues  of  the  Animal  Body 71 

Epithelial  Tissues 73 

Connective  Tissues. 83 

Muscular  Tissues 90 

Nervous  Tissues 94 

Summary 97 

The  Combination  of  Tissues  into  Organs 99 

Vegetative  Organs 102 

Organs  of  Assimilation 102 

Digestive  Tract 103 

Respiratory  Organs 107 

Circulatory  Apparatus 109 

Excretory  Organs 115 

Sexual  Organs 117 

Animal  Organs 121 

Organs  of  Locomotion 121 

Nervous  System 122 

Sense  Organs 125 

Summary 131 

Promorphology 133 

GENERAL  EMBRYOLOGY 139 

Spontaneous  Generation 139 

Generation  by  Parents 140 

Asexual  Reproduction 140 

Sexual  Reproduction 142 

Combined  Methods  of  Reproduction 143 

General  Phenomena  of  Sexual  Reproduction 145 

Maturation  of  the  Egg 146 

Fertilization 148 

V 


vi  TABLE  OF  CONTENTS. 

PAGE 

Cleavage  Processes 151 

Formation  of  the  Germ  Layers 156 

Different  Forms  of  Sexual  Development 160 

Summary 162 

RELATION  OF  ANIMALS  TO  ONE  ANOTHER 164 

Relations  between  Individuals  of  the  Same  Species 164 

Relations  between  Individuals  of  Different  Species 167 

ANIMAL  AND  PLANT 171 

GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS 174 

DISTRIBUTION  OF  ANIMALS  IN  TIME 1 80 

SPECIAL  ZOOLOGY 182 

Phylum  I.  PROTOZOA 183 

Class  I.  Rhizopoda 187 

Order  I.  Monera 189 

Order  II.  Lobosa 189 

Order  III.  Heliozoa 190 

Order  IV.  Radiolaria 192 

Order  V.  Foraminifera 196 

Order  VI.  Mycetozoa '. 198 

Class  II.  Flagellata 200 

Order  I.  Autoflagellata 200 

Order  II.  Dinoflagellata 203 

Order  III.  Cystoflagellata 203 

Class  III.  Ciliata 204 

Order  I.  Holotricha 209 

Order  II.  Heterotricha 209 

Order  III.  Peritricha 210 

Order  IV.  Hypotricha 211 

Order  V.  Suctoria 212 

Class  IV.  Sporozoa 213 

Order  I.  Gregarinida 213 

Order  II.  Coccidiae 215 

Order  III.  Haemosporida 216 

Order  IV.  Myxosporida 217 

Order  V.  Sarcosporida 218 

SUMMARY 218 

METAZOA 221 

Phylum  II.  PORIFERA 221 

Order  I.  Calcispongiae 225 

Order  II.  Silicispongiae 226 

SUMMARY 227 

Phylum  III.  CcELENTERATA 228 

Class  I.  Hydrozoa 230 

Order  I.  Hydraria 240 

Order  II.  Hydrocorallinae 241 

Order  III.  Tubulariae  =  Anthomedusae 241 

Order  IV.  Campanulariae  =  Leptomedusae 242 

Order  V.  Trachomedusae 242 

Order  VI.  Narcomedusae 242 

Order  VII.  Siphonophora 243 


TABLE  OF  CONTENTS.  vii 

PAGE 

Class  II.  Scyphozoa 245 

Order  I.  Stauromedusae 250 

Order  II.  Peromedusae  250 

Order  III.  Cubomedusae 250 

Order  IV.  Discomedusae 250 

Class  III.  Anthozoa 251 

Order  I.  Tetracoralla , 258 

Order  II.  Octocoralla   258 

Order  III.  Hexacoralla 259 

Class  IV.  Ctenophora  261 

SUMMARY 265 

Phylum  IV.  PLATHELMINTHES 267 

Class  I.  Turbellaria 268 

Order  I.  Polycladidea 271 

Order  II.  Tricladidea 271 

Order  III.  Rhabdocoelida 271 

Class  II.  Trematoda 271 

Order  I.  Polystomiae 273 

Order  II.   Distomiae 274 

Class  III.  Cestoda 278 

Class  IV.  Nemertini 289 

SUMMARY 292 

Phylum  V.    RoTiFERA 293 

Phylum  VI.  CCELHELMINTHES 295 

Class  I.  Chaetognathi 296 

Class  II.  Nemathelminthes 298 

Order  I.  Nematoda 298 

Order  II.  Gordiacea 304 

Order  III.  Acanthocephala 304 

Class  III.  Annelida 305 

Sub  Class  I.  Chaetopoda 306 

Order  I.  Polychaetae 311 

Order  II.  Oligochaetae .  314 

Sub  Class  II.  Gephyraea 316 

Order  I.  Chsetiferi 317 

Order  II.  Inermes 317 

Order  III.  Priapuloidea 317 

Sub  Class  III.  Hirudinei 318 

Order  I.  Gnathobdellidae 321 

Order  II.  Rhynchobdellidae 321 

Class  IV.  Polyzoa 321 

Sub  Class  I.   Entoprocta 321 

Sub  Class  II.  Ectoprocta 322 

Class  V.  Phoronida 325 

Class  VI.  Brachiopoda  325 

Order  I.  Ecardines 328 

Order  II.  Testicardines 328 

SUMMARY 328 

Phylum  VII.  ECHINODERMA 329 

Class  I.  Asteroidea 333 


viii  TABLE  OF  CONTENTS. 

PAGE 

Class  II.  Ophiuroidea 337 

Class  III.  Crinoidea 338 

Sub  Class  I.  Eucrinoidea 342 

Sub  Class  II.  Edrioasteroidsa 342 

Sub  Class  III.  Cystidea   342 

Sub  Class  IV.  Blastoidea 342 

Class  IV.  Echinoidea 343 

Order  I.  Palechinoidea 345 

Order  II.  Cidaridae 345 

Order  III.  Clypeastroidea 346 

Order  IV.  Spatangoidea 346 

Class  V.  Holothuroidea 346 

Order  I.  Actinopoda 349 

Order  II.  Paractinopoda 349 

SUMMARY 350 

Phylum  VIII.  MOU.USCA 351 

Class  I.  Ampliineura 356 

Sub  Class  I.  Placophora ' .  .    356 

Sub  Class  II.  Solenogastres 358 

Class  II.  Acephala 358 

Order  I.  Protoconchiae  365 

Order  II.  Heteroconchiae  367 

Class  III.  Scaphopoda 369 

Class  IV.  Gasteropoda 369 

Order  I.  Prosobranchiata 378 

Order  II.  Opisthobranchiata 381 

Order  III.  Pulmonata 383 

Class  V.  Cephalopoda 384 

Order  I.  Tetrabranchia 394 

Order  II.  Dibranchia 394 

SUMMARY - 395 

Phylum  IX.  ARTHROPODA 398 

Qlass  I.  Crustacea 408 

Sub  Class  I.  Trilobitae 414 

Sub  Class  II.   Phyllopoda 415 

Order  I.  Branchiopoda 416 

Order  II.  Cladocera 417 

Sub  Class  III.  Copepoda 417 

Order  I.  Eucopepoda 42 1 

Order II.  Siphonostomata 422 

Sub  Class  IV.  Ostracoda 422 

Sub  Class  V.  Cirripedia 423 

Order  I.  Lepadidae 425 

Order  II.  Balanidae 425 

Order  III.  Rhizocephala 426 

Sub  Class  VI.  Malacostraca 426 

Legion  I.  Leptostraca 427 

Legion  II.  Thoracostraca 427 

Order  I.  Schizopoda  428 

Order  II.  Stomatopoda 429 


TABLE  OF  CONTENTS.  ix 

PAGE 

Order  III.  Decapoda 429 

Order  IV.  Cumacia 437 

Legion  III.  Arthrocostraca 438 

Order  I.  Amphipoda 438 

Order  II.  Isopoda 440 

Class  II.  Acerata 442 

Sub  Class  I.  Gigantostraca 443 

Order  I.  Xiphosura 444 

Order  II.  Eurypterida 444 

Sub  Class  II.  Arachnida 444^ 

Legion  I.  Arthrogastrida 447 

Order  I.  Scorpionida 447 

Order  II.  Phrynoidea 448 

Order  III.  Microthelyphorida  448 

Order  IV.  Solpugida 449 

Order  V.  Pseudoscorpii 450 

Order  VI.  Phalangida 450 

Legion  II.  Sphaerogastrida 451 

Order  I.  Araneina 451 

Order  II.  Acarina 453 

Order  III.  Linguatulida 454 

Tardigrada  455 

Pycnogonida 456 

Class  III.  Malacopoda 456 

Class  IV.  Insecta 458 

Sub  Class  I.  Chilopoda 460 

Sub  Class  II.  Hexapoda 461 

Order  I.  Apterygota 477 

Order  II.  Archiptera 477 

Order  III.  Orthoptera 480 

Order  IV.  Neuroptera 481 

Order  V.  Strepsiptera 483 

Order  VI.  Coleoptera  483 

Order  VII.  Hymenoptera 485 

Order  VIII.  Rhynchota 489 

Order  IX.   Diptera 491 

Order  X.  Aphaniptera 493 

Order  XI.  Lepidoptera 494 

Class  V.  Diplopoda 496 

SUMMARY 497 

Phylum  X.  CHORDATA 501 

Sub  Phylum  I.  •  Leptocardii 502 

Sub  Phylum  II.  Tunicata 505 

Order  I.  Copelatae 506 

O'rder  II.  Tethyoidea 508 

Order  III.  Thaliacea 510 

Sub  Phylum  III.  Enteropneusta 512 

Sub  Phylum  IV.  Vertebrata  514 

Series  I.  Ichthyopsida 555 

Class  I.  Cyclostomata 555 


TABLE  OF  CONTENTS. 

PAGE 

Sub  Class  I.   Myzontes 556 

Sub  Class  II.  Petromyzontes 557 

Class  II.  Pisces 557 

Sub  Class  I.  Elasmobranchii 569 

Order  I.  Selachii 570 

Order  II.  Holocephali 572 

Sub  Class  II.  Ganoidei 572 

Order  I.  Crossopterygii 573 

Order  II.  Chondrostei 573 

Order  III.  Holostomi   573 

Sub  Class  III.  Teleostei  574 

Order  I.  Physostomi 575 

Order  II.  Pharyngognathi 576 

Order  III.  Acanthopteri 577 

Order  IV.  Anacanthini 577 

Order  V.  Lophobranchii 578 

Order VI.  Plectognathi 578 

Sub  Class  IV.    Dipnoi 579 

Class  III.  Amphibia 580 

Order  I.  Stegocephali 586 

Order  II.  Gymnophiona 587 

Order  III.  Urodela 587 

Order  IV.  Anura 588 

Series  II.  Amniota 588 

Class  I.  Reptilia 588 

Order  I.  Theromorpha ..  .  . 594 

Order  II.  Plesiosauria 594 

Order  III.  Ichthyosauria 594 

Order  IV.  Chelonia  594 

Order  V.  Rhynchocephalia 595 

Order  VI.  Dinosauria 595 

Order  VII.  Squamata 596 

Order  VIII.  Crocodilia 601 

Order  IX.  Pterodactylia 602 

Class  II.   Aves 603 

Order  I.  Saururae 612 

Order  II.  Odontornithes 612 

Order  III.  Ratitae  612 

Order  IV.  Carinatse 613 

Class  III.  Mammalia 617 

Sub  Class  I.  Monotremata 631 

Sub  Class  II.   Marsupialia 632 

Order  I.  Polyprotodonta 633 

Order  II.   Diprotodonta 633 

Sub  Class  III.  Placentalia 634 

Order  I.  Edentata 635 

Order  II.  Insectivora 636 

Order  III.  Chiroptera 637 

Order  IV.  Rodentia 638 

Order  V    Ungulata 639 


TABLE  OF  CONTENTS.  xi 

PAGE 

Order  VI.  Proboscidia   643 

Order  VII.  Hyracoidea 644 

Order  VIII.  Sirenia 644 

Order  IX.  Cetacea 645 

Order  X.  Carnivora 646 

Order  XI.  Prosimiae 648 

Order  XII.  Primates 649 

SUMMARY 652 


GENERAL    PRINCIPLES    OP    ZOOLOGY. 


INTRODUCTION. 

Man's  Relation  to  Other  Animals. — The  man  who  has  learned 
to  observe  nature  in  a  disinterested  manner  sees  himself  in  the 
midst  of  a  manifold  variety  of  organisms,  which  in  their  structure, 
and  even  more  in  their  vital  phenomena,  disclose  to  him  a  simi- 
larity to  his  own  being.  This  similarity,  with  many  of  the 
mammals,  especially  the  anthropoid  apes,  has  the  sharpness  of  a 
caricature.  In  the  invertebrate  animals  it  is  softened;  yet  even 
in  the  lowest  organisms,  for  our  knowledge  of  which  we  are 
indebted  to  the  microscope,  it  is  still  to  be  found :  although  here 
the  vital  processes  which  have  reached  such  an  astonishing  com- 
plexity and  perfection  in  ourselves  can  only  be  recognized  in  their 
simplest  outlines.  Man  is  part  of  a  great  whole,  the  Animal 
Kingdom,  one  form  among  the  many  thousand  forms  in  which 
animal  organization  has  found  expression. 

Purpose  of  Zoological  Study. — If  we  would,  therefore,  fully 
understand  the  structure  of  man,  we  must,  as  it  were,  look  at  it 
upon  the  background  which  is  formed  by  the  conditions  of 
organization  of  the  other  animals,  and  for  this  purpose  we  must 
investigate  these  conditions.  To  such  endeavors  the  scientific 
knowledge  of  animal  life,  or  Zoology,  owes  its  origin  and  continued 
advancement.  But  meanwhile  the  subject  of  zoology  has  widened; 
for,  apart  from  its  relations  to  man,  zoology  has  to  explain  the 
organization  of  animals  and  their  relations  to  one  another.  This 
is  a  rich  field  for  scientific  activity;  its  enormous  range  is  a  conse- 
quence, on  the  one  hand,  of  the  well-nigh  exhaustless  variety  of 
animal  organization,  and,  on  the  other  hand,  of  the  diiferent 
points  of  view  from  which  the  zoologist  enters  upon  the  solution 
of  his  problem. 


2  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

In  the  first  half  of  the  last  century  the  conception,  which  is 
still  held  by  the  public  at  large,  was  prevalent,  if  not  quite  uni- 
versal, in  scientific  circles,  that  the  aim  of  zoology  is  to  furnish 
every  animal  with  a  name,  to  characterize  it  according  to  some 
easily  recognizable  features,  and  to  classify  it  in  a  way  to  facilitate 
quick  identification.  By  Natural  History  was  understood  the 
classification  of  animals,  that  is  to  say,  only  one  .part  of  zoology, 
indeed  a  part  of  minor  importance,  which  can  pretend  to  scientific 
value  only  when  it  is  brought  into  relation  with  other  problems 
(geographical  distribution,  evolution).  This  conception  has 
during  the  past  five  decades  become  more  and  more  subordinated. 
The  ambition  to  describe  the  largest  possible  number  of  new  forms 
and  to  shine  by  means  of  an  extensive  knowledge  of  species  belongs 
to  the  past.  In  fact  there  is  a  tendency  to  undue  neglect  of 
classification.  Morphology  and  Physiology  to-day  dominate  the 
sphere  of  the  zoologist's  work. 

Morphology,  or  the  study  of  form,  begins  with  the  appearances 
of  animals,  and  has  first  to  describe  all  which  can  be  seen  exter- 
nally, as  size,  color,  proportion  of  parts.  But  since  the  external 
appearance  of  an  animal  cannot  be  understood  without  knowledge 
of  the  internal  organs  which  condition  the  external  form,  the 
morphologist  must  make  these  accessible  by  the  aid  of  dissection, 
of  Anatomy,  and  likewise  describe  their  forms  and  methods  of 
combination.  In  his  investigation  he  only  stops  when  he  has 
arrived  at  the  morphological  elements  of  the  animal  body,  the 
cells.  Everywhere  the  morphologist  has  to  do  with  conditions  of 
form:  the  only  difference  lies  in  the  instruments  by  means  of 
which  he  obtains  his  insight,  according  to  whether  he  gathers  his 
knowledge  through  immediate  observation,  or  after  a  previous 
dissection  with  scalpel  and  scissors,  or  by  use  of  the  micro- 
scope. Therefore  we  cannot  contrast  Morphology  and  Anatomy, 
and  ascribe  to  the  former  the  description  of  only  the  external,  and 
to  the  latter  of  only  the  internal  parts.  The  distinction  is  not 
logically  correct,  since  the  kind  of  knowledge  and  the  mental 
processes  are  the  same  in  both  cases.  The  distinction,  too,  is 
unnatural,  since  in  many  instances  organs  which  in  some  cases  lie 
in  the  interior  of  the  body,  and  must  be  dissected  out,  belong  in 
other  cases  to  the  surface  of  the  body,  and  are  accessible  for  direct 
description.  Further,  on  account  of  their  transparency  the  in- 
ternal parts  of  many  animals  can  be  studied  without  dissection. 

Comparative  Anatomy. — For  morphology,  as  for  every  science, 
the  proposition  is  true  that  the  mere  accumulation  of  facts  is  not 


INTRODUCTION.  3 

sufficient  to  give  the  subject  the  character  of  a  science ;  an  addi- 
tional mental  elaboration  of  this  material  is  necessary.  Such  a 
result  is  reached  by  comparison.  The  morphologist  compares 
animals  with  each  other  according  to  their  structure,  in  order  to 
ascertain  what  parts  of  the  organization  recur  everywhere,  what 
only  within  narrow  limits,  possibly  restricted  to  the  representatives 
of  a  single  species.  He  thus  gains  a  double  advantage:  (1)  an 
insight  into  the  relationships  of  animals,  and  hence  the  foundation 
for  a  Natural  System ;  (2)  the  evidence  of  the  laws  which  govern 
organisms.  Any  organism  is  not  a  structure  which  has  arisen 
independently  and  which  is  hence  intelligible  by  itself:  it  stands 
rather  in  a  regular  dependent  relation  to  the  other  members  of  the 
animal  kingdom.  We  can  only  understand  its  structure  when  we 
compare  it  with  the  closely  and  the  more  distantly  related  animals, 
e.g.,  when  we  compare  man  with  the  other  vertebrates  and  with 
many  lower  invertebrate  forms.  Here  we  have  to  consider  one  of 
the  most  mysterious  phenomena  of  the  organic  world,  the  path  to 
the  full  explanation  of  which  was  first  broken  by  the  Theory  of 
Evolution,  as  will  be  shown  in  another  chapter. 

Ontogeny. — To  morphology  belongs,  as  an  important  integral 
part,  Ontogeny  or  Embryology.  Only  a  few  animals  are  com- 
pletely formed  in  all  their  parts  at  the  beginning  of  their  individual 
existence;  most  of  them  arise  from  the  egg,  a  relatively  simple 
body,  and  then  step  by  step  attain  their  permanent  form  by  com- 
plicated changes.  The  morphologist  must,  with  the  completest 
possible  series,  determine  by  observation  the  different  stages,  com- 
pare them  with  the  mature  animals,  and  with  the  structure  and 
developmental  stages  of  other  animals.  Here  is  revealed  to  him 
the  same  conformity  to  law  which  dominates  the  mature  animals, 
and  a  knowledge  of  this  conformity  is  of  fundamental  importance 
as  well  for  classification  as  for  the  causal  explanation  of  the  animal 
form.  The  df3velopmental  stages  of  man  show  definite  regular 
agreements,  not  only  with  the  structure  of  the  adult  human  being, 
which  in  and  of  itself  would  be  intelligible,  but  also  with  the 
structure  of  lower  vertebrates,  like  the  fishes,  and  even  with  many 
of  the  still  lower  animals  of  the  invertebrate  groups. 

Physiology. — In  the  same  way  as  the  morphologist  studies  the 
structure,  the  physiologist  studies  the  vital  phenomena  of  animals 
and  the  functions  of  their  organs.  Formerly  life  was  regarded  as 
the  expression  of  a  special  vital  force  peculiar  to  organisms,  and 
any  attempt  at  a  logical  explanation  of  the  vital  processes  was 
thereby  renounced.  Modern  physiology  has  abandoned  this  theory 


4  GENERAL  PRINCIPLES   OF  ZOOLOGY. 

of  vital  force;  it  has  begun  the  attempt  to  explain  life  as  the 
summation  of  extremely  complicated  chemico-physical  processes, 
and  thus  to  apply  to  the  organic  world  those  explanatory  princi- 
ples which  prevail  in  the  inorganic  realm.  The  results  obtained 
show  that  it  is  the  correct  method. 

Since  each  organic  form  is  the  product  of  its  development, 
since,  further,  the  development  represents  to  us  the  summation  of 
most  complicated  vital  processes,  the  explanation  of  the  organic 
bodily  form  is,  therefore,  in  ultimate  analysis  a  physiological 
problem;  though  of  course  a  problem  whose  solution  lies  still  in 
the  indefinitely  distant  future.  What  has  been  actually  accom- 
plished in  this  direction  is  only  the  smallest  beginning,  even  in 
comparison  with  that  which  many  falsely  regard  as  already  attained. 

Biology. — According  as  the  relations  of  each  organism  to  the 
external  world  are  brought  about  through  its  vital  phenomena, 
there  belongs  to  physiology,  or  at  least  is  connected  with  it,  the 
study  of  the  conditions  of  animal  existence,  (Ecology  or  Biology. 
This  branch  of  the  science  has  of  late  attained  a  very  considerable 
importance.  How  animals  are  distributed  over  the  globe,  how 
climate  and  conditions  influence  their  distribution,  how  by  known 
factors  the  structure  and.  the  mode  of  life  become  changed,  are 
questions  which  are  to-day  discussed  more  than  ever  before. 

Paleontology. — Finally  in  the  realm  of  zoology  belongs  also 
Paleozoology  or  Paleontology,  the  study  of  the  extinct  animals. 
For  between  the  extinct  and  the  living  animals  there  exists  a 
genetic  relationship :  the  former  are  the  precursors  of  the  latter, 
and  their  fossil  remains  are  the  most  trustworthy  records  of  the 
history  of  the  race,  or  Phylogeny.  As  in  human  affairs  the 
present  conditions  can  only  be  completely  understood  by  the  aid 
of  history,  so  in  many  cases  the  zoologist  must  draw  upon  the 
results  of  paleontology  for  an  explanation  of  the  living  animal 
world. 

The  science  of  zoology  would  be  subdivided  in  the  above-men- 
tioned manner  if  we  wished  to  proceed  entirely  on  a  scientific  basis. 
Yet  practical  considerations  have  made  many  modifications  neces- 
sary. On  account  of  their  paramount  importance  to  the  medical 
profession  human  anatomy  and  embryology  have  been  raised  to 
independent  branches  of  science.  In  comparative  physiology  only 
the  most  general  foundations  have  been  laid;  a  more  special* 
physiology  exists  only  for  man  and  the  higher  vertebrates;  this, 
too,  for  the  above-named  reasons  has  been  made  a  special  branch 
of  science.  Paleontology  also  has,  in  addition  to  its  specific 


INTRODUCTION.  5 

zoological  tasks,  attained  importance  as  a  scientific  aid  to  geology, 
since  it  furnishes  the  materials  for  characterizing  and  fixing  the 
various  geological  ages  and  the  earth's  history  during  those  ages. 
When,  therefore,  at  the  present  day  we  speak  of  zoology,  we 
usually  refer  to  morphology  and  classification  of  living  animals 
with  consideration  of  their  general  vital  phenomena. 

The  views  here  given  of  the  character  of  zoology  have  not  been 
the  same  in  all  time.  Like  every  science  zoology  has  developed 
gradually;  it  has  varied  with  each  epoch  and  tendency,  according 
as  the  systematic  or  the  morphological  or  the  physiological  point 
of  view  was  the  prevailing  one.  It  will  now  be  interesting  to  take 
a  hasty  glance  at  the  most  important  phases  in  the  development 
of  zoology.  The  reader  will  better  understand  the  questions  which 
now  dominate  zoological  inquiry,  if  he  know  how  these  have  arisen 
historically. 


HISTORY  OF  ZOOLOGY. 

Methods  of  Zoological  Study. — In  the  history  of  zoology  we 
can  distinguish  two  great  currents,  which  have  been  united  in  a 
few  men,  but  which  on  the  whole  have  developed  independently, 
nay,  more  often  in  pronounced  opposition  to  each  other;  these  are 
on  the  one  side  the  systematic,  on  the  other  the  morphologico- 
physiological  mode  of  studying  animals.  In  this  brief  historical 
summary  they  will  be  kept  distinct  from  one  another,  although  in 
the  commencement  of  zoological  investigation  there  was  no  oppo- 
sition between  the  two  points  of  view,  and  even  later  this  has  in 
many  instances  disappeared. 

Aristotle,  the  great  Greek  philosopher,  has  been  distinguished 
as  the  Father  of  Natural  History,  which  means  that  his  predeces- 
sors' fragmentary  knowledge  of  zoology  could  not  be  compared 
with  the  well-arranged  order  in  which  Aristotle  had  brought 
together  his  own  and  the  previously  existing  knowledge  of  the 
nature  of  animals.  In  Aristotle  favorable  external  conditions  were 
united  with  more  favorable  mental  ability.  Equipped  with  the 
literary  aid  of  an  extensive  library  and  the  pecuniary  means  then 
more  indispensable  than  now  for  natural-history  investigation,  he 
pursued  the  inductive  method,  the  only  one  which  is  capable  of 
furnishing  secure  foundations  in  the  realm  of  natural  science.  It 
is  a  matter  for  great  regret  that  there  have  been  preserved  only 
parts  of  his  three  most  important  zoological  works,  "  Kistoria 
animalium,"  "  De  partibus,"  and  "De  generatione,"  works  in 
which  zoology  is  founded  as  a  universal  science,  since  anatomy  and 
embryology,  physiology  and  classification  find  equal  consideration. 
How  far  Aristotle,  notwithstanding  many  errors,  attained  to  a 
correct  knowledge  of  the  structure  and  embryology  of  animals,  is 
shown  by  the  fact  that  many  of  his  discoveries  have  been  confirmed 
only  within  a  century.  Thus  it  was  known  to  Aristotle,  though 
only  lately  rediscovered  by  Johannes  Miiller,  that  many  sharks  are 
not  only  viviparous,  but  that  also  in  their  case  the  embryo  becomes 
fixed  to  the  maternal  uterus  and  there  is  formed  a  contrivance  for 

7 


8  GENERAL  PRINCIPLES  OF  ZOO  LOOT. 

nutrition  resembling  the  mammalian  and  even  the  human  pla- 
centa; he  knew  the  difference  between  male  and  female  cephalo- 
pods,  and  that  the  young  cuttlefish  has  a  preoral  yolk-sac. 

The  position  which  Aristotle  took  in  reference  to  the  classifica- 
tion of  animals  is  of  great  interest;  he  mentions  in  his  writings  the 
very  considerable  number  of  about  five  hundred  species.  Since  he 
does  not  mention  very  well-known  forms,  like  the  badger,  dragon- 
fly, etc.,  we  can  assume  that  he  knew  many  more,  but  did  not 
regard  it  necessary  to  give  a  catalogue  of  all  the  forms  known  to 
him,  and  that  he  mentioned  them  only  if  it  was  necessary  to  refer 
to  certain  physiological  or  morphological  conditions  found  in  them. 

This  neglect  of  the  systematic  side  is  further  shown  in  the  fact 
that  the  great  philosopher  is  satisfied  with  two  systematic  cate- 
gories, with  eidos,  species  or  kind,  and  yeros  or  group.  His 
eight  yevrj  juey  terra  would  about  correspond  with  the  Classes  of 
modern  zoology;  they  have  been  the  starting-point  for  all  later 
attempts  at  classification,  and  may  therefore  be  enumerated  here  : 

1.  Mammals  (CcporoKOvvra  ev  avrois). 

,       2.  Birds  (opnOes). 

3.  Oviparous  quadrupeds  (rerpanodoi  cooroKovvra). 

4.  Fishes 

5.  Molluscs 

6.  Crustaceans 

7.  Insects  (errata). 

8.  Animals  with  shells 


Aristotle  also  noticed  the  close  connexion  of  the  first  four 
groups,  since  he,  without  indeed  actually  carrying  out  the  divi- 
sion, has  contrasted  the  animals  with  blood,  evai^a  (better, 
animals  with  red  blood),  with  the  bloodless,  avai}jia  (better, 
animals  with  colorless  blood  or  with  no  blood  at  all). 


DEVELOPMENT   OF   SYSTEMATIC  ZOOLOGY. 

Pliny.  —  It  is  a  remarkable  fact  that  after  the  writings  of 
Aristotle,  in  which  classification  is  much  subordinated  and  only 
serves  to  express  the  anatomical  relationships  in  animals,  an 
exclusively  systematic  direction  should  have  been  taken.  This  is 
explicable  only  when  we  consider  that  the  mental  continuity  of 
investigation  was  completely  broken  on  the  one  hand  by  the 
decline  and  ultimate  complete  collapse  of  ancient  classic  civiliza- 
tion, and  on  the  other  by  the  triumphant  advance  of  Christianity. 


Ill S TORT   OF  ZOOLOGY.  9 

The  decay  of  zoological  investigation,  that  had  only  just  begun  to 
bloom,  begins  in  the  writings  of  Pliny.  Although  this  Roman 
general  and  scholar  was  long  lauded  as  the  foremost  zoologist  of 
antiquity,  he  is  now  given  the  place  of  a  not  even  fortunate  com- 
piler, who  collected  from  the  writings  of  others  the  accurate  and 
the  fabulous  indiscriminately,  and  replaced  the  natural  classifica- 
tion of  animals  according  to  structure  by  the  unnatural,  purely 
arbitrary  division  according  to  their  place  of  abode  (flying  animals, 
land  animals,  water  animals). 

Zoology  of  the  Middle  Ages. — The  rise  of  Christianity  resulted 
in  the  complete  annihilation  of  natural  science  and  investigation. 
The  world-renouncing  character,  which  originally  was  peculiar  to 
the  Christian  conception,  led  naturally  to  a  disposition  hostile  to 
any  mental  occupation  with  natural  things.  Then  came  a  time 
when  answers  to  questions  capable  of  solution  by  the  simplest 
observation  were  sought  by  painstaking  learned  rummaging  of  the 
works  of  standard  authors.  How  many  teeth  the  horse  has,  was 
debated  in  many  polemics,  which  would  have  led  to  bloodshed  if 
one  of  the  authors  had  not  taken  occasion  to  look  into  a  horse's 
mouth.  Significant  of  this  mental  bias  which  prevailed  through- 
out the  entire  Middle  Ages  is  the  i  Physiologus'  or  '  Bestiarius/  a 
book  from  which  the  zoological  authors  of  the  Middle  Ages  drew 
much  material.  The  book  in  its  various  editions  names  about 
seventy  animals,  among  them  many  creatures  of  fable :  the  dragon, 
the  unicorn,  the  phoenix,  etc.  Most  of  the  accounts  given  of 
various  animals  are  fables,  intended  to  illustrate  religious  or 
ethical  teachings.  In  a  similar  way  the  religious  element  played 
an  important  role  in  the  many-volumed  Natural  History  of  the 
Dominican  Albertus  Magnus,  and  Viiicentius  Bellovacensis,  and 
of  the  Augustine  Thomas  Cantimpratensis,  although  these  used 
as  a  foundation  for  their  expositions  the  Latin  translation  of 
Aristotle,  the  works  of  Pliny  and  other  authors  of  antiquity. 

Wotton. — Under  such  conditions  we  must  regard  it  as  an  im- 
portant advance  that  at  the  close  of  the  Middle  Ages,  when  the 
interest  in  scientific  investigation  awoke  anew,  Aristotle's  concep- 
tions were  taken  up  and  elaborated  from  a  scientific  standpoint. 
In  this  sense  we  can  call  the  Englishman  Wotton  the  successor  of 
Aristotle.  In  1552  he  published  his  work  "De  differentiis 
animalium,"  in  which  he  essentially  copied  the  system  of  Aristotle, 
except  that  he  admitted  the  new  group  of  plant-animals  or 
zoophytes.  However,  the  title,  '  On  the  Distinguishing  Characters 
of  Animals/  shows  that  of  the  rich  treasury  of  Aristotelian  knowl- 


10  GENERAL  PRINCIPLES   OF  ZOOLOGY. 

edge  the  systematic  results  obtained  the  chief  recognition,  and 
thus  Wotton's  work  inaugurated  the  period  of  systematic  zoology, 
which  in  the  Englishman  Ray.  but  even  more  in  Linnaeus,  has 
found  its  most  brilliant  exponents. 

Linnaeus,  the  descendant  of  a  Swedish  clergyman,  whose 
family  name  Ingemarsson  had  been  changed  after  a  linden-tree 
near  the  parsonage,  to  Lindelius,  was  born  in  Eashult  in  1707. 
Pronounced  by  his  teachers  to  be  good  for  nothing  at  study,  he 
was  saved  from  the  fate  of  learning  the  cobbler's  trade  through 
the  influence  of  a  physician,  who  recognized  the  fine  abilities  of 
the  boy,  and  won  him  for  medical  studies.  He  studied  at  Lund 
and  Upsala;  at  the  age  of  twenty-eight  he  made  extended  tours  on 
the  Continent,  and  at  that  time  gained  recognition  from  the  fore- 
most men  in  his  profession.  In  1741  he  became  professor  of  medi- 
cine in  Upsala,  some  years  later  professor  of  natural  history.  He 
died  in  1778. 

Improvement  of  Zoological  Nomenclature  by  Linnaeus.— 
Linnaeus's  most  important  work  is  his  "  Systema  Naturae,"  which, 
first  appearing  in  1735,  up  to  1766-68  passed  through  twelve 
editions;  after  his  death  there  came  out  a  thirteenth,  edited  by 
Gmelin.  This  has  become  the  foundation  for  systematic  zoology, 
since  it  introduces  for  the  first  time  (1)  a  sharper  division  into  the 
system,  (2)  a  definite  scientific  terminology,  the  binomial  nomen- 
clature, and  (3)  brief,  comprehensive,  clear  diagnoses.  In  classi- 
fication Linnaeus  employed  four  categories;  he  divided  the  entire 
Animal  Kingdom  into  Classes,  the  Classes  into  Orders,  these  into 
Genera,  the  Genera  finally  into  Species.  The  term  Family  was 
not  employed  in  the  " ;  Sy sterna  Naturae."  Still  more  important 
was  the  binomial  nomenclature.  Hitherto  the  common  names  were 
in  use  in  the  scientific  world,  and  led  to  much  confusion;  the  same 
animals  had  different  names,  and  different  animals  had  the  same 
names;  in  the  naming  of  newly  discovered  animals  there  prevailed 
no  generally  accepted  principle.  This  inconvenience  was  entirely 
obviated  by  Linnaeus  in  the  tenth  edition  of  his  Systema  by  the 
introduction  of  a  scientific  nomenclature.  The  first  word,  a  noun, 
designates  the  genus  to  which  the  animal  belongs,  the  following 
word,  usually  an  adjective,  the  species  within  the  genus.  The 
names  Canis  familiaris,  Canis  lupus,  Canis  vitlpes,  indicate  that 
the  dog,  wolf,  and  fox  are  related  to  one  another,  since  they  belong 
to  the  same  genus,  the  genus  of  doglike  animals,  of  which  they  are 
different  species.  Linnaeus's  method  of  naming  was  particularly 
valuable  in  the  description  of  new  species,  inasmuch  as  it  at  the 


HISTORY  OF  ZOOLOGY.  11 

outset  informed  the  reader  to  what  position  of  relationship  the  new 
species  was  to  be  assigned. 

In  his  characterization  of  the  various  systematic  groups  Linnaeus 
broke  completely  with  the  hitherto-prevailing  custom.  His 
predecessors  (as  Gessner,  Aldrovandus)  in  their  Natural  Histories 
had  given  a  verbose  and  detailed  description  of  each  animal,  from 
which  the  beginner  was  scarcely  able  to  see  what  was  specially 
characteristic  for  that  animal,  a  matter  which  should  have  been 
emphasized  in  the  definition.  Linnaeus,  011  the  other  hand,  intro- 
duced brief  diagnoses,  which  in  a  few  words,  never  in  sentence 
form,  gave  only  what  was  necessary  for  recognition.  Thus  a  way 
was  found  which  insured  comprehensibility  in  the  enormously 
increasing  number  of  known  animals. 

Influence  of  the  Linnean  System. — But  in  the  great  superiority 
of  the  Linnean  System  lay  at  the  same  time  the  germ  of  the  one- 
sided development  which  zoology  came  to  take  under  his  influence. 
The  logical  perfecting  of  the  system,  which  undoubtedly  had 
become  necessary,  gave  that  a  brilliant  aspect,  and  hid  the  fact 
that  classification  is  not  the  ultimate  purpose  of  investigation,  but 
only  an  important  and  indispensable  aid  to  it.  In  the  zeal  for 
naming  and  classifying  animals,  the  higher  goal  of  investigation, 
knowledge  of  the  nature  of  animals,  was  lost  sight  of,  and  the 
interest  in  anatomy,  physiology,  and  embryology  flagged. 

From  these  reproaches  we  can  scarcely  spare  Linnaeus  himself, 
the  father  of  this  tendency.  For  while  in  his  "  Systema  Naturae  " 
he  treated  of  a  much  larger  number  of  animals  than  any  earlier 
zoologist,  he  brought  about  no  deepening  of  our  knowledge.  The 
manner  in  which  he  divided  the  animal  kingdom,  in  comparison 
with  the  Aristotelian  system,  is  rather  a  retrogression  than  an 
advance.  Linnaeus  divided  the  animal  kingdom  into  six  classes: 
Mammalia,  Aves,  Amphibia,  Pisces,  Insecta,  Yermes.  The  first 
four  classes  correspond  to  Aristotle's  four  groups  of  animals  with 
blood.  In  the  division  of  the  invertebrated  animals  into  Insecta 
and  Vermes  Linnaeus  stands  undoubtedly  behind  Aristotle,  who 
attempted,  and  in  part  successfully,  to  set  up  a  larger  number  of 
groups. 

But  in  his  successors,  even  more  than  in  Linnaeus  himself,  we 
see  the  damage  wrought  by  the  systematic  method.  The  diagnoses 
of  Linnaeus  were  for  the  most  part  models,  which,  mutatis 
mutandis,  could  be  employed  for  new  species  with  little  trouble. 
There  was  needed  only  some  exchanging  of  adjectives  to  express 
the  differences.  With  the  hundreds  of  thousands  of  different 


12  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

species  of  animals  there  was  no  lack  of  material,  and  so  the  arena 
was  opened  for  that  spiritless  zoology  of  species-making  which  in 
the  first  half  of  the  last  century  brought  zoology  into  such  discredit. 
Zoology  would  have  been  in  danger  of  growing  into  a  Tower  of 
Babel  of  species-describing  had  not  a  counterpoise  been  created  in 
the  strengthening  of  the  physiologi  co-anatomical  side. 

DEVELOPMENT  OF  MOKPHOLOGY. 

Anatomists  of  Classic  Antiquity. — Comparative  anatomy — for 
this  chiefly  concerns  us  here — for  a  long  time  owed  its  development 
to  the  students  of  human  anatomy;  this  is  due  to  the  fact  that  even 
up  to  a  recent  date  comparative  anatomy  was  assigned  to  the 
medical  faculty,  while  zoology  belonged  to  the  philosophical 
faculty,  as  if  it  were  an  entirely  separate  study.  The  disciples  of 
Hippocrates  had  previously  studied  animal  anatomy  for  the  pur- 
pose of  obtaining  an  idea  of  human  organization,  from  the  struc- 
ture of  other  mammals,  and  thus  to  gain  a  secure  foundation  for 
the  diagnosis  of  human  diseases.  The  work  of  classical  antiquity 
most  prominent  in  this  respect,  the  celebrated  Human  Anatomy 
by  Claudius  Galenus  (131-201  A.D.),  is  based  chiefly  upon  obser- 
vations upon  dogs,  monkeys,  etc. ;  for  in  ancient  times,  and  even 
in  the  Middle  Ages,  men  showed  considerable  repugnance  to 
making  the  human  cadaver  a  subject  of  scientific  investigation. 

Middle  Ages. — The  first  thousand  years  in  which  Christianity 
formed  the  ruling  power  in  the  mental  life  of  the  people  was  quite 
fruitless  for  anatomy;  in  the  main  men  held  to  the  writings  of 
Galen  and  the  works  of  his  commentators,  and  seldom  took  occa- 
sion to  prove  their  correctness  by  their  own  observations.  With 
the  ending  of  the  Middle  Ages  the  interest  in  independent  scien- 
tific research  first  broke  its  bounds. 

Vesal  (1514-1564),  the  creator  of  modern  anatomy,  had  the 
courage  carefully  to  investigate  the  human  cadaver  and  to  point 
out  numerous  errors  in  Galen's  writings  which  had  arisen  through 
the  unwarranted  application  to  human  anatomy  of  the  discoveries 
made  upon  other  animals.  By  his  corrections  of  Galen,  Vesal  was 
drawn  into  a  violent  controversy  with  his  teacher,  Sylvius,  an 
energetic  defender  of  Galen's  authority,  and  with  his  renowned 
contemporary  Eustachius,  which  did  much  for  the  development  of 
comparative  anatomy.  At  first  animals  were  dissected  only  for 
the  purpose  of  disclosing  the  cause  of  Galen's  mistakes,  but  later 
through  a  zeal  and  love  for  facts.  It  was  natural  that  first  of  all 


HISTORY  OF  ZOOLOGY.  13 

vertebrates  found  consideration,  since  they  stand  next  to  man  in 

structure.     Thus  there  appeared  in  the  same  century  with  VesaFs 

> 
Human   Anatomy   drawings  of   skeletons  of  vertebrates  by   the 

Nuremberg  physician  Goiter;  the  anatomical  writings  of  Fabricius 
ab  Aquapendente,  etc. 

Beginning  of  Zootomy. — But  later  attention  was  turned  also  to 
insects  and  molluscs,  indeed  even  to  the  marine  echinoderms, 
ccelenterates,  and  Protozoa.  Here,  above  all,  three  men  who  lived 
at  the  end  of  the  seventeenth  century  deserve  mention,  the  Italian 
Malpighi  and  the  Dutchmen  Swammerdam  and  Leeuwenhoek. 
The  former's  "  Dissertatio  de  bombyce"  was  the  pioneer  for  insect 
anatomy,  since  by  the  discovery  of  the  vasa  Malpighii,  the  heart, 
the  nervous  system,  the  tracheae,  etc.,  an  extraordinary  extension 
of  our  knowledge  was  brought  about.  Of  Swammerdam/s  writings 
attention  should  be  called  particularly  to  "  The  Bible  of  Nature," 
a  work  to  which  no  other  of  that  time  is  comparable,  since  it  con- 
tains discoveries  of  great  accuracy  on  the  structure  of  bees,  May- 
. flies,  snails,  etc.  Leeuwenhoek,  finally,  was  a  most  fortunate 
discoverer  in  the  field  of  microscopic  research,  by  him  introduced 
into  science.  Besides  other  things  he  studied  especially  the 
minute  inhabitants  of  the  fresh  waters,  the  '  infusion-animalcules/ 
a  more  careful  investigation  of  which  has  led  to  a  complete  reversal 
of  our  conception  of  the  essentials  of  animal  organization. 

The  Dawn  of  Independent  Observation. — The  great  service  of 
the  men  named  above  consists  chiefly  in  that  they  broke  away  from 
the  thraldom  of  book-learning  and,  relying  alone  upon  their  own 
eyes  and  their  own  judgment,  regained  what  had  been  lost,  the 
blessing  of  independent  and  unbiassed  observation.  They  spread 
the  interest  in  observation  of  nature  over  a  wide  circle  so  that  in 
the  eighteenth  century  the  number  of  independent  natural-history 
writings  had  increased  enormously.  There  were  busy  with  the 
study  of  insect  structure  and  development,  de  Geer  in  Sweden, 
Reaumur  in  France,  Lyonet  in  Belgium,  Rosel  von  Rosenhof  in 
Germany;  the  latter  besides  wrote  a  monograph  on  the  indigenous 
batrachia,  which  is  still  worth  reading.  The  investigation  of  the 
infusoria  formed  a  favorite  occupation  for  the  learned  and  the  laity, 
as  Wrisberg,  von  Gleichen-Russwurm,  Schaffer,  Eichhorn,  and 
0.  F.  Miiller.  In  most  of  the  writings  the  religious  character  of 
the  contemplations  of  nature  are  extraordinarily  emphasized,  and 
since  we  find  that  among  these  writers  numerous  clergymen 
(Eichhorn  in  Danzig,  Goeze  in  Quedlinburg,  Schaffer  in  Regens- 
burg)  attained  distinction,  we  have  a  sign  that  a  reconciliation 


14  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

had  taken  place  between  Christianity  and  natural  science.  As  a 
criterion  of  the  progress  made  in  comparison  with  the  earlier 
centuries,  a  mere  glance  at  the  illustrations  is  sufficient.  Any  one 
will  at  the  first  glance  recognize  the  difference  between  the  shabby 
drawings  of  an  Aldrovandus  and  the  masterly  figures  of  a  Lyonet 
or  a  Rosel  von  Rosenhof. 

Period  of  Comparative  Anatomy. — Thus  through  the  zeal  of 
numerous  men  filled  with  a  love  of  nature  a  store  of  anatomical 
facts  was  collected,  which  needed  only  a  mental  reworking ;  and 
this  mental  reworking  was  brought  about,  or  at  least  entered  upon, 
by  the  great  comparative  anatomists  who  lived  at  the  end  of  the 
eighteenth  and  the  beginning  of  the  nineteenth  century.  Among 
these  the  French  zoologists  Lamarck,  Savigny,  Geoffroy  St.  Hilaire, 
Cuvier,  and  the  Germans  Meckel  and  Goethe  are  especially  to  be 
named. 

Correlation  of  Parts. — When  the  various  animals  were  com- 
pared with  one  another  with  reference  to  their  structure  there  was 
obtained  a  series  of  important  fundamental  laws,  particularly  the 
law  of  the  Correlation  of  Parts  and  the  law  of  the  Homology  of 
Organs.  The  former  established  the  fact  that  there  exists  a 
dependent  relation  between  the  organs  of  the  same  animal,  that 
local  changes  in  one  single  organ  also  lead  to  corresponding 
changes  at  some  distant  part  of  the  body,  and  that  therefore  from 
the  constitution  of  certain  parts  an  inference  can  be  drawn  as  to 
the  constitution  of  another  part  of  the  body.  Cuvier  particularly 
made  use  of  this  principle  in  reconstructing  the  form  of  extinct 
animals. 

Homology  and  Analogy. — Still  more  important  was  the  theory 
of  the  Homology  of  Organs.  In  the  organs  of  animals  a  distinction 
was  drawn  between  an  anatomical  and  a  physiological  character; 
the  anatomical  character  is  the  sum  of  all  the  anatomical  features, 
as  found  in  form,  structure,  position,  and  mode  of  connection  of 
organs;  the  physiological  character  is  their  function.  Anatomically 
similar  organs  in  closely  related  animals  will  usually  have  the  same 
functions,  as,  for  example,  the  liver  of  all  vertebrates  has  the 
function  of  producing  gall;  here  anatomical  and  physiological 
characteristics  go  hand  in  hand.  But  this  need  not  necessarily  be 
the  case;  very  often  it  may  happen  that  one  and  the  same  function 
is  possessed  by  organs  anatomically  different;  as,  for  example,  the 
respiration  of  vertebrates  is  carried  on  in  fishes  by  gills,  in  mammals 
by  lungs.  Conversely,  anatomically  similar  organs  may  have 
different  functions,  as  the  lungs  of  mammals  and  the  swim-bladder 


HISTORY  OF  ZOOLOGY.  15 

of  fishes;  similar  organs  may  also  undergo  a  change  of  function 
from  one  group  to  another;  the  hydrostatic  apparatus  of  fishes  has 
come  to  be  the  seat  of  respiration  in  the  mammals.  Organs  with 
like  functions  —  physiologically  equivalent  organs  —  are  called 
'  analogous ' ;  organs  of  like  anatomical  constitution — anatomically 
equivalent  organs — are  called  'homologous/  It  is  the  task  of 
comparative  anatomy  to  discover  in  the  various  parts  of  animals 
those  which  are  homologous,  i.e.  those  anatomically  equivalent, 
and  to  follow  the  changes  in  them  conditioned  by  a  change  of 
function. 

Cuvier. — The  foremost  representative  of  comparative  anatomy 
was  Georges  Dagobert  Cuvier.  He  was  born  in  1769  in  the  town 
of  Mompelgardt  (Montbeillard),  then  belonging  to  Wiirtemberg, 
and  obtained  his  early  training  in  the  Karlschule  at  Stuttgart. 
where,  through  the  influence  of  his  teacher  Kielmeyer,  he  was  led 
to  the  study  of  comparative  anatomy.  The  opportunity  of  going 
to  the  seashore  which  was  offered  to  him  as  private  instructor  to 
Count  d'Hericy  he  employed  for  his  epoch-making  investigations 
upon  the  structure  of  molluscs.  In  1794,  upon  the  persuasion 
largely  of  the  man  who  afterwards  became  his  great  opponent, 
Geoffroy  St.  Hilaire,  he  moved  to  Paris,  where  he  was  made  at 
first  Professor  of  Natural  History  in  the  central  school  and  in  the 
College  of  France,  later  Professor  of  Comparative  Anatomy  in  the 
Jardin  des  Plantes.  As  a  sign  of  the  great  regard  in  which  Cuvier 
was  held,  it  should  be  noticed  that  he  was  repeatedly  intrusted  with 
high  educational  positions  and  was  made  a  French  peer.  As  such 
he  died  in  1832, 

Type  Theory. — Cuvier's  investigations,  apart  from  the  mol- 
luscs, extended  to  the  coelenterates,  arthropods,  and  vertebrates, 
living  and  fossil.  He  collected  his  extensive  observations  into  his 
two  chief  works  "  Le  regne  animal  distribue  d'*apres  son  organiza- 
tion "  and  "  Lecons  d'anatomie  comparee."  Of  quite  epoch-making 
importance  was  his  little  pamphlet  "  Sur  un  rapprochement  a  etablir 
entre  les  differentes  classes  des  animaux,"  in  which  he  founded  his 
celebrated  type  theory,  and  which  in  1812  introduced  a  complete 
reform  of  classification.  The  Cuvierian  division,  which  has  become 
the  starting-point  for  all  later  classifications,  differed,  broadly 
speaking,  from  all  the  earlier  systems  in  this,  that  the  classes  of 
mammals,  birds,  reptiles,  and  fishes  were  brought  together  into  a 
higher  grade  under  the  name,  introduced  by  Lamarck,  of  '  verte- 
brate animals';  that  further  the  so-called  ' invertebrate  animals ' 
were  divided  into  three  similar  grades,  each  equal  to  that  of  the 


16  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

vertebrate  animals,  viz.,  Mollusca,  Articulata,  and  Radiata.  Cuvier 
called  these  grades  standing  above  the  classes,  provinces  or  chief 
branches  (embranchements),  for  which  later  the  name  Types  was 
introduced  by  Blainville.  But  still  more  important  are  the  differ- 
ences which  appear  in  the  structural  basis  of  the  system.  Instead 
of,  like  the  earlier  systematists,  using  a  few  external  character- 
istics for  the  division,  Cuvier  built  upon  the  totality  of  internal 
organization,  as  expressed  in  the  relative  positions  of  the  most 
important  organs,  especially  the  position  of  the  nervous  system, 
as  determining  the  arrangement  of  the  other  organs.  "  The 
type  is  the  relative  position  of  parts"  (von  Baer).  Thus  for  the 
first  time  comparative  anatomy  was  employed  in  the  formation  of 
a  natural  system  of  animals. 

Lastly  the  type  theory  established  an  entirely  new  conception 
of  the  arrangement  of  animals.  Cuvier  found  prevalent  the  theory 
that  all  animals  formed  a  single  connected  series  ascending  from 
the  lowest  infusorian  to  man;  within  this  series  the  position  of 
each  animal  was  definitely  determined  by  the  degree  of  its  organi- 
zation. On  the  other  hand  Cuvier  taught  that  the  animal  kingdom 
consisted  of  several  co-ordinated  unities,  the  types,  which  exist 
quite  independently  side  by  side,  within  which  again  there  are 
higher  and  lower  forms.  The  position  of  an  animal  is  determined 
by  two  factors :  first,  by  its  conformity  to  a  type,  by  the  structural 
plan  which  it  represents;  second,  by  its  degree  of  organization,  by 
the  stage  to  which  it  attains  within  its  type. 

Comparative  Embryology. — Evolution  vs.  Epigenesis. — The 
same  results  which  Cuvier  reached  by  the  way  of  comparative 
anatomy  were  attained  two  decades  later  by  C.  E.  von  Baer  by  the 
aid  of  embryology.  Embryology  is  the  youngest  branch  of 
zoology.  What  Aristotle  really  knew,  what  was  written  by 
Fabricius  ab  Aquapendente  and  Malpighi  upon  the  embryology  of 
the  chick,  did  not  rise  above  the  range  of  aphorisms,  and  were  not 
of  sufficient  value  to  make  a  science.  The  difficulties  of  observa- 
tion, due  to  the  delicacy  and  the  minuteness  of  the  developmental 
stages,  were  lessened  by  the  invention  of  the  microscope  and 
microscopical  technique.  Further,  the  prevailing  philosophical 
conceptions  placed  hindrances  in  the  way;  there  was  no  belief  in 
Embryology  in  the  present  sense  of  the  word;  each  organism  was 
thought  to  be  laid  down  from  the  first  complete  in  all  its  parts, 
and  only  needed  growth  to  unfold  its  organs  (evolutio  *) ;  eithei  the 

*  This  original  meaning  of  '  evolution '  is  different  from  that  prevailing 
at  present. 


BISTORT  OF  ZOOLOGY.  17 

spermatozoon  must  be  the  young  creature  which  found  favorable 
conditions  for  growth  in  the  store  of  food  in  the  egg,  or  the  egg 
represents  the  individual  and  was  stimulated  to  the  '  evolutio '  by 
the  spermatozoon.  This  theory  led  to  the  doctrine  of  inclusion, 
which  taught  that  in  the  ovary  of  Eve  were  included  the  germs  of 
all  human  beings  who  have  lived  or  ever  will  live. 

Caspar  Friedrich  Wolff  combated  this  idea  with  his  "  Theoria 
geuerationis"  (1759);  he  sought  to  prove  by  observation  that  the 
hen's  egg  at  the  beginning  is  without  any  organization,  and  that 
gradually  the  various  organs  appear  in  it.  In  the  embryo  there  is 
a  new  formation  of  all  parts,  an  Epigenesis.  This  first  assault 
upon  the  evolutionist  school  was  entirely  without  result,  chiefly 
because  Albrecht  von  Haller,  the  most  celebrated  physiologist  of 
the  eighteenth  century,  by  his  influence  suppressed  the  idea  of 
epigeuesis.  Wolff  was  not  able  to  establish  himself  in  scientific 
circles  in  Germany,  and  was  obliged  to  emigrate  to  Russia.  Only 
after  his  death  did  his  writings  find,  through  Oken  and  Meckel, 
proper  recognition. 

Von  Baer. — Thus  it  remained  for  Carl  Ernst  von  Baer  in  his 
classic  work,  "Die  Entwicklung  des  Huhnchens,  Beobachtung 
und  Reflexion"  (1832),  to  establish  embryology  as  an  independent 
study.  Baer  confirmed  Wolff's  doctrine  of  the  appearance  of 
layerlike  Anlagen,  from  which  the  organs  arose;  and  on  account 
of  the  accuracy  with  which  he  proved  this  he  is  considered  the 
founder  of  the  germ-layer  theory.  Further,  he  came  to  the  con- 
clusion that  each  type  had  not  only  its  peculiar  structural  plan, 
but  also  its  peculiar  course  of  development;  that  for  vertebrates 
an  evolutio  bigemina  was  characteristic,  for  the  articulates  the 
evolutio  gemina,  for  the  molluscs  the  evolutio  contorta,  and  for 
the  radiates  the  evolutio  radiata.  Here  we  meet  for  the  first  time 
the  idea  that  for  the  correct  solution  of  the  questions  of  relation- 
ship of  animals,  and  therefore  a  basis  for  a  natural  classification, 
comparative  embryology  is  indispensable;  an  idea  which  in  recent 
years  has  proved  exceedingly  fruitful. 

Cell  Theory. — Of  fundamental  importance  for  the  further 
growth  of  comparative  anatomy  and  embryology  was  the  proof 
that  all  organisms,  as  well  as  their  embryonic  forms,  were  com- 
posed of  the  same  elements,  the  cells.  This  knowledge  is  the 
quintessence  of  the  cell  theory,  which  during  the  third  decade  of 
the  last  century  was  advanced  by  Schleiden  and  Schwann,  and 
which  two  decades  later  was  completely  remodelled  by  the  proto- 
plasm theory  of  Max  Schultze.  In  the  cell  theory  a  simple  prin- 


18  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

ciple  of  organization  was  found  for  all  living  creatures,  for  highly 
and  for  lowly  organized  plants  and  animals,  and  the  wide  realm  of 
histology  was  laid  open  for  scientific  treatment. 

KEFORM   OF   THE   SYSTEM. 

Foundation  of  Modern  Zoology. — With  the  establishment  of 
comparative  anatomy  and  embryology  and  the  application  of  these 
to  classification,  and  with  the  development  of  the  cell  theory  and 
of  histology,  which  is  connected  with  it,  we  may  say  that  the 
foundation  of  zoology  was  laid.  Wonderful  advances  were  made 
in  vertebrate  anatomy  by  the  classic  researches  of  Owen,  Johannes 
Miiller,  Rathke,  Gegenbaur,  and  others;  our  conceptions  of  organ- 
ization have  been  completely  altered  by  the  work  of  Dujardin, 
Max  Schultze,  Haeckel,  and  others,  who  have  proved  the  unicellu- 
larity  of  the  lowest  animals.  The  germ -layer  theory  was  further 
elaborated  by  Remak  and  Kolliker;  and  applied  to  the  invertebrate 
animals  by  Kowalewsky,  Haeckel,  and  Huxley.  It  is  beyond  the 
limits  of  this  brief  historical  summary  to  go  into  what  has  been 
accomplished  in  regard  to  the  other  branches  of  the  animal  king- 
dom; it  must  here  be  sufficient  to  mention  the  most  important 
changes  which  the  Cuvierian  system  has  undergone  under  the 
influence  of  increasing  knowledge. 

The  Division  of  the  Radiata. — Of  the  four  types  of  Cuvier  the 
branch  Radiata  was  undoubtedly  the  one  of  whose  representatives 
he  had  the  least  knowledge;  it  was  therefore  the  least  natural, 
since  it  comprised,  besides  the  radially  symmetrical  coelenterates 
and  echinoderms,  other  forms,  which,  like  the  worms,  were 
bilaterally  symmetrical,  or,  like  many  infusorians,  were  asym- 
metrical. Thus  it  came  about  that  most  reforms  have  here  found 
their  point  of  attack. 

C.  Th.  von  Siebold  was  the  originator  of  the  first  important 
reform.  He  limited  the  type  Radiata,  or,  as  ho  termed  them,  the 
Zoophytes,  to  those  animals  with  radially  symmetrical  structure 
(Echinoderms  and  the  Plant-animals) ;  separating  all  the  others, 
he  formed  of  the  unicellular  organisms  the  branch  of  '  primitive 
animals'  or  Protozoa;  the  higher  organized  animals  he  grouped 
together  as  worms  or  Vermes;  at  the  same  time  he  transferred  a 
part  of  the  Articulata,  the  annelids,  to  the  worm  group,  and  pro- 
posed for  the  other  articulates,  crabs,  millipedes,  spiders,  and 
insects,  the  term  Arthropoda. 

Leuckart,  about   the  same  time   (1848),   divided  the  branch 


HISTORY  OF  ZOOLOGY.  19 

Eadiata  into  two  branches  differing  greatly  in  structure.  The 
lower  forms,  in  which  no  special  body-cavity  is  present,  the 
interior  of  the  body  consisting  only  of  a  system  of  cavities  serving 
for  digestion,  the  alimentary  canal,  he  called  the  Ccelentera 
(essentially  the  Zoophyta  of  the  older  zoologists) ;  to  the  rest,  in 
which  the  alimentary  canal  and  the  body-cavity  occur  as  two 
separate  cavities,  he  gave  the  name  Echinoderma. 

The  Present  System. — Thus  there  resulted  seven  classes: 
i  Protozoa,  Coelentera,  Echinoderma,  Vermes,  Arthropoda,  Mol- 
lusca,  and  Vertebrata.  Still  this  arrangement  does  not  meet  the 
requirements  of  a  natural  system  and  hence  is  more  or  less  unsat- 
isfactory. Some  zoologists  are  returning  to  the  Cuvierian  classifi- 
cation to  the  extent  of  uniting  the  segmented  worms  with  the 
arthropods  in  a  group  Articulate.  Upon  the  ground  of  important 
anatomical  and  embryological  characters  the  Brachiopoda,  the 
Bryozoa,  and  the  Tunic^ita  have  been  separated  from  the  Mollusca; 
they  form  the  subject  of  diverse  opinions.  The  relationships  of 
the  first  two  groups  have  not  yet  been  settled :  of  the  Tunicata  we 
know  indeed  that  they  are  related  to  the  Vertebrata,  but  the 
diiferences  are  such  that  they  cannot  be  included  in  that  group. 
The  only  way  out  of  the  difficulty  is  to  unite  vertebrates,  tunicates, 
and  some  other  forms  in  a  larger  division,  Chordata.  The  Vermes, 
too,  must  be  divided,  as  will  appear  in  the  second  part  of  this 
volume. 


HISTORY    OF   THE   THEORY   OF   EVOLUTION. 

Importance  of  the  Subject. — Before  closing  the  historical  intro- 
duction we  must  consider  the  historical  development  of  a  question 
whose  importance  might,  on  a  superficial  examination,  be  under- 
rated, but  which  from  a  small  beginning  has  grown  into  a  problem 
completely  dominating  zoological  research,  and  has  occupied  not 
only  zoologists,  but  all  interested  in  science  generally.  This  is  the 
question  of  the  logical  value  of  the  systematic  conceptions  species, 
genus,  family,  etc. 

The  Nature  of  Species. — In  nature  we  find  only  separate 
animals :  how  comes  it  that  we  classify  them  into  larger  and  smaller 
groups  ?  Are  the  single  species,  genera,  and  the  other  divisions 
which  the  systematist  distinguishes,  fixed  quantities,  as  it  were 
fundamental  conceptions  of  nature,  or  a  Creator's  thoughts,  which 
find  expression  in  the  single  forms  ?  Or  are  they  abstractions 
which  man  has  brought  into  nature  for  the  purpose  of  making  it 


20  GENERAL  PRINCIPLES  OF  ZOOLOO  Y. 

comprehensible  to  his  mental  capabilities  ?  Are  the  specific  and 
generic  names  only  expressions  which  have  become  necessary,  from 
the  nature  of  our  mental  capacity,  for  the  gradation  of  relation- 
ship in  nature,  which  in  and  for  themselves  are  not  immutable, 
and  hence  can  undergo  a  gradual  change  ?  Practically  speaking, 
the  problem  reads:  are  species  constant  or  changeable?  What  is 
true  for  species  must  necessarily  be  true  for  all  other  categories  of 
the  system,  all  of  which  in  the  ultimate  analysis  rest  upon  the 
conception  of  species. 

Ray's  Conception  of  Species. — One  of  the  first  to  consider  the 
conception  of  species  was  Linnaeus's  predecessor,  the  Englishman 
John  Ray.  In  the  attempt  to  define  what  should  be  understood 
as  a  species  he  encountered  difficulties.  In  practice,  animals  which 
differ  little  in  structure  and  appearance  from  one  another  are 
ascribed  to  the  same  species;  this  practical  procedure  cannot  be 
carried  out  theoretically;  for  there  are  males  and  females  within 
the  same  species  which  differ  more  from  one  another  than  do  the 
representatives  of  different  species.  Thus  John  Ray  reached  the 
genetic  definition  when  he  said:  for  plants  there  is  no  more 
certain  criterion  of  specific  unity  than  their  origin  from  the  seeds 
of  specifically  or  individually  like  plants;  that  is  to  say,  generalized 
for  all  organisms :  to  one  and  the  same  species  belong  individuals 
which  spring  from  similar  ancestors. 

The  « Cataclysm  Theory.1— With  Ray's  definition  an  entirely 
uncontrollable  element  was  brought  into  the  conception  of  species, 
since  no  systematist  usually  knew  anything,  nor  indeed  could  he 
know  anything,  as  to  whether  the  representatives  of  the  species 
placed  before  him  sprang  from  similar  parents.  It  was  therefore 
only  natural  that  the  conception  of  species  put  on  a  religious  garb, 
since  by  resting  upon  theological  ideas  it  found  a  firmer  support. 
Linnaeus  said:  "Tot  sunt  species  quot  ab  initio  creavit  infinitum 
Ens";  with  this  he  built  up  a  conception  of  species  upon  the 
tradition  cf  the  Mosaic  history  of  creation,  a  procedure  quite 
unjustified  upon  grounds  of  natural  science,  since  it  drew  one  of 
its  iundamental  ideas  from  transcendental  conceptions,  not  from 
the  experience  of  natural  science.  Linnaeus's  definition  showed 
itself  untenable,  as  soon  as  paleontology  began  to  make  accessible 
a  vast  quantity  of  extinct  animals  deposited  as  fossils.  With  an 
odd  fancy,  the  fossils,  being  inconvenient  for  study,  were  for  a 
long  time  regarded  as  outside  the  pale  of  scientific  research.  They 
might  be  sports  of  nature,  it  was  said,  or  remains  of  the  Flood,  or 
of  the  influence  of  the  stars  upon  the  earth,  or  products  of  an  aura 


HISTORY  OF  ZOOLOGY.  21 

seiniiialis,  a  fertilizing  breath,  which,  if  it  fell  upon  organic  bodies, 
led  to  the  formation  of  animals  and  plants,  but  if  it  strayed  upon 
inorganic  materials  gave  rise  to  fossils.  The  foundation  of 
scientific  paleontology  by  Cuvier  put  an  end  to  such  empty  specu- 
lations. Cuvier  proved  beyond  a  doubt  that  these  fossils  were  the 
remains  of  animals  of  a  previous  time.  Just  as  the  formation  of 
the  earth's  crust  by  successive  overlying  layers  made  possible  the 
recognition  of  different  periods  in  the  earth's  history,  so  paleon- 
tology taught  how  to  recognize  also  the  different  periods  in  the 
vegetable  and  animal  world  of  life  on  our  globe.  Each  geological 
age  was  characterized  by  a  special  world  of  animals  quite  peculiar 
to  it;  and  these  animal  worlds  differed  the  more  from  the  present, 
the  older  the  period  of  the  earth  to  which  they  belonged.  All 
these  generalizations  led  Cuvier  to  his  cataclysm  theory,  that  a 
great  revolution  brought  each  period  of  the  earth's  history  to  an 
end,  destroying  all  life,  and  that  upon  the  newly  formed  virgin 
earth  a  new  organic  world  of  immutable  species  sprang  up. 

Objections  to  the  Cataclysm  Theory. — By  the  supposition  of 
numerous  acts  of  creation  the  Linnean  conception  of  species 
seemed  to  be  rescued,  though,  to  be  sure,  by  summoning  to  its  aid 
hypotheses  which  had  neither  foundation  in  science  nor  justifica- 
tion in  theology.  The  logical  results  of  Cuvier's  cataclysm  theory 
were  conceptions  of  a  Creator  who  built  up  an  animal  world  only 
for  the  purpose  of  destroying  it  after  a  time  as  a  troublesome  toy; 
it  has  therefore  at  no  time  found  warm  supporters,  at  least  among 
geologists,  for  whom  it  was  intended.  Of  the  prominent  zoologists 
there  is  only  to  be  mentioned  Louis  Agassiz,  who  till  the  end  of 
his  life  remained  faithful  to  this  theory. 

Under  these  conditions  it  is  readily  understood  how  thinking 
naturalists,  who  felt  the  necessity  of  explaining  the  character  of 
organic  nature  simply  and  by  a  natural  law  capable  of  general 
application,  began  to  doubt  the  fixity  of  species,  and  were  led  to 
the  theory  of  change  of  form,  the  Theory  of  Descent,  or  Evolution. 

Darwin's  Predecessors. — Even  in  Cuvier's  time  there  prevailed 
a  strong  current  in  favor  of  this  theory.  It  found  expression  in 
England  in  the  writings  of  Erasmus  Darwin  (grandfather  of  the 
renowned  Charles  Darwin);  in  Germany  in  the  works  of  Goethe, 
Oken,  and  the  disciples  of  the  ' natural  philosophical'  school;  in 
France  the  genealogical  theory  was  developed  particularly  by 
Buffon,  Geoffroy  St.  Hilaire,  and  Lamarck.  Its  completest  ex- 
pression was  found  in  Lamarck's  "  Philosophic  zoologique  "  (1809) ; 
its  arguments  will  be  considered  in  the  following  paragraphs. 


22  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

Lamarck  (Jean  Baptiste  de  Monet,  Chevalier  de  Lamarck, 
born  in  Picardy,  1744,  died,  Professor  at  the  Jardin  des  Plantes, 
1829)  taught  that  on  the  earth  at  first  organisms  of  the  simplest 
structure  arose  in  the  natural  way  through  spontaneous  generation 
from  non-living  matter.  From  these  simplest  living  creatures  have 
developed,  by  gradual  changes  in  the  course  of  an  immeasurably 
vast  space  of  time,  the  present  species  of  plants  and  animals, 
without  any  break  in  the  continuity  of  life  upon  oitr  globe;  the 
terminal  point  of  this  series  is  man;  the  other  animals  are  the 
descendants  of  those  forms  from  which  man  has  developed. 
Lamarck,  in  accordance  with  the  then  prevailing  conceptions, 
regarded  the  animal  kingdom  as  a  single  series  grading  from  the. 
lowest  primitive  animal  up  to  man.  Among  the  causes  which  may 
influence  th6  change  and  perfecting  of  organisms,  Lamarck 
emphasized  particularly  use  and  disuse;  the  giraffe  has  obtained  a 
long  neck  because  by  a  special  condition  of  life  he  was  compelled 
to  stretch,  in  order  to  browse  the  leaves  on  high  trees;  conversely, 
the  eyes  of  animals  which  live  in  the  dark  have  degenerated  from 
lack  of  use  into  functionless  structures.  The  direct  influence  of 
the  external  world  must  be  unimportant;  the  changes  in  the  sur- 
roundings (Geoffrey  St.  Hilaire's  le  monde  ambient)  must  for  the 
most  part  act  indirectly  upon  animals  by  altering  the  conditions 
for  the  use  of  organs. 

Evolution  vs.  Creation. — Lamarck's  ingenious  work  remained 
almost  unnoticed  by  his  contemporaries.  On  the  other  hand  there 
arose  a  violent  controversy  between  the  defenders  and  the 
opponents  of  the  evolution  theory  when  [1830]  Geoffroy  St.  Hilaire 
in  a  debate  in  the  Academy  at  Paris  defended  against  Cuvier  the 
thesis  of  a  near  relationship  of  the  vertebrates  and  the  insects,  and 
set  up  the  proposition  that  the  latter  were  "  vertebrates  running 
on  their  backs."  The  conflict  ended  in  the  complete  overthrow  of 
the  theory  of  evolution;  the  defeat  was  so  complete  that  the 
problem  vanished  for  a  long  time  from  scientific  discussion,  and 
the  theory  of  the  fixity  of  species  again  became  dominant.  This 
error  was  occasioned  by  many  causes.  Above  all,  the  theory  of 
Geoffroy  St.  Hilaire  and  Lamarck  was  rather  a  clever  conception 
than  founded  011  abundant  facts;  besides,  it  had  in  it  as  a  funda- 
mental error  the  doctrine  of  the  serial  arrangement  of  the  animal 
world.  *  Opposed  to  this  stood  Cuvier's  great  authority  and  his 
extensive  knowledge,  the  latter  making  it  easy  for  him  to  show 
that  the  animal  kingdom  was  made  up  of  separate  co-ordinated 
groups,  the  types. 


HISTORY  OF  ZOOLOGY.  23 

Lyell. — In  the  same  year  in  which  Cuvier  obtained  his  victory 
over  Geoffroy  St.  Hilaire,  his  theory  of  the  succession  of  numerous 
animal  worlds  upon  the  globe  received  its  first  destructive  blow. 
Cuvier's  cataclysm  theory  had  two  sides,  a  geological  and  a 
biological.  Cuvier  denied  the  continuity  of  the  various  terrestrial 
periods,  as  well  as  the  continuity  of  the  fauna  and  flora  belonging 
to  them.  In  1830-32  appeared  the  "Principles  of  Geology"  by 
Lyell,  an  epoch-making  work,  which,  in  the  realm  of  geology, 
completely  set  aside  the  cataclysm  theory.  Lyell  proved  that  the 
supposition  of  violent  revolutions  on  the  earth  was  not  necessary 
in  order  to  explain  the  changes  of  the  earth's  surface  and  the 
superposition  of  its  strata;  that  rather  the  constantly  acting 
forces,  elevations  and  depressions,  the  erosive  action  of  water,  be 
it  as  ebb  and  flow  of  the  tide,  as  rain,  snow,  or  ice,  or  as  the  flow 
of  rivers  and  brooks  rushing  as  torrents  towards  the  sea,  are  suffi- 
cient to  furnish  a  complete  explanation.  Very  gradually  in  the 
course  of  a  vast  space  of  time  the  earth's  surface  has  changed,  and 
passed  from  one  period  into  the  next,  and  still  at  the  present  day 
the  constant  process  of  change  is  going  on.  The  continuity  in  the 
geological  history  of  the  earth,  here  postulated  for  the  first  time, 
has  since  then  become  one  of  the  fundamental  axioms  of  Geology; 
on  the  other  hand  the  discontinuity  of  living  creatures,  although 
the  geological  support  of  this  was  frail,  was  for  a  long  time 
regarded  as  correct. 

Darwin. — It  is  the  great  merit  of  Charles  Darwin  that  he  took 
up  the  theory  of  descent  anew  after  it  had  rested  a  decade,  and 
later  brought  it  into  general  recognition.  With  this  began  the 
most  important  period  in  the  history  of  zoology,  a  period  in  which 
the  science  not  only  made  such  an  advance  as  never  before,  but 
also  began  to  obtain  a  permanent  influence  upon  the  general  views 
of  men. 

Charles  Darwin  was  born  at  Shrewsbury,  Eng.,  in  1809.  After 
studying  at  the  universities  of  Edinburgh  and  Cambridge,  he  joined 
as  naturalist  the  English  war-ship  "  Beagle."  In  its  voyage  from 
1831  to  36  around  the  globe,  Darwin  recognized  the  peculiar 
character  of  island  faunas,  particularly  of  the  Galapagos  Islands, 
and  the  remarkable  geological  succession  of  edentates  in  South 
America;  these  facts  formed  for  him  the  germ  of  his  epoch-making 
theory.  Further  results  of  this  journey  were  his  beautiful  mono- 
graph on  the  Cirripedia,  and  the  classic  investigation  of  coral-reefs. 
After  his  return  to  England  Darwin  lived,  entirely  devoted  to 
scientific  work,  chiefly  in  the  hamlet  of  Down,  county  Kent,  up 


24  GENERAL  PRINCIPLES   OF  ZOO  LOOT. 

to  the  time  of  his  death  in  1882.  He  was  incessantly  busy  in 
developing  his  conception  of  the  origin  of  species,  and  in  collect- 
ing for  this  a  constantly  increasing  array  of  facts.  The  first 
written  notes,  the  fundamental  ideas  of  which  he  communicated 
to  friends,  particularly  the  geologist  Lyell  and  the  botanist 
Hooker,  were  made  in  1844,  but  the  author  was  not  persuaded  to 
give  them  publicity.  Not  until  1858  did  Darwin  decide  to  make 
his  first  contribution  to  science.  In  this  year  he  received  an  essay 
sent  by  the  traveller  Wallace,  which  in  its  most  important  points 
coincided  with  his  own  views.  At  the  same  time  with  Wallace's 
manuscript  an  abstract  of  Darwin's  theory  was  published.  In  the 
next  year  (1859)  appeared  the  most  important  of  his  writings, 
"  On  the  Origin  of  Species  by  means  of  Natural  Selection/'  and 
in  rapid  succession  a  splendid  series  of  works,  the  fruit  of  many 
years,  of  preparatory  labors.  For  the  history  of  the  theory  the 
most  important  of  these  are:  (1)  "Upon  the  Variation  of  Plants 
and  Animals  under  Domestication/'  two  volumes,  which  chiefly 
contain  a  collection  of  material  for  proofs;  (2)  on  "The  Descent 
of  Man,"  a  work  which  gives  the  application  of  the  theory  to  man. 

No  scientific  work  of  this  century  has  attracted  so  much  atten- 
tion in  the  zoological,  we  may  even  say  in  the  whole  educated 
world,  as  "The  Origin  of  Species."  It  was  generally  received  as 
something  entirely  new,  so  completely  had  the  scientific  tradition 
been  lost.  In  professional  circles  it  was  stoutly  combated  by  one 
faction,  with  another  it  found  well-wishing  but  hesitating  accept- 
ance. Only  a  few  men  placed  themselves  from  the  beginning  in 
a  decided  manner  on  the  side  of  the  great  British  investigator. 
There  was  a  lively  scientific  battle,  which  ended  in  a  brilliant  vic- 
tory for  the  theory  of  evolution.  At  the  present  time  all  our 
scientific  thoughts  are  so  permeated  with  the  idea  of  evolution  that 
we  can  scarcely  speak  of  any  considerable  opposition  to  it. 

Post-Darwinian  Writers. — Among  the  men  who  have  most 
influenced  this  rapid  advance  is  to  be  mentioned,  besides  A.  K. 
Wallace,  the  co-founder  of  Darwinism,  above  all  Ernst  Haeckel, 
who  in  his  "General  Morphology"  and  his  "Natural  History  of 
Creation"  has  done  much  towards  the  extension  of  the  theory. 
Among  other  energetic  defenders  of  the  theory  in  Germany  should 
be  mentioned  Fritz  Miiller,  Carl  Vogt,  Weismann,  Moritz  Wagner, 
and  Nageli,  even  if  they  have  taken  special  standpoints  in  refer- 
ence to  the  causes  conditioning  the  changes  of  form.  Among  the 
English  naturalists  are  to  be  named  particularly  Huxley,  Hooker, 
and  Lyell.  In  America  Gray,  Cope,  "and  Hyatt  were  early  sup- 


HISTORY  OF  ZOOLOGY.  25 

porters.      Darwinism   was   long   in   obtaining   an   entrance   into 
France. 

DARWIN'S   THEORY   OF   THE   ORIGIN   OF   SPECIES. 

Before  Darwin  wrote  the  idea  of  fixity  of  species  prevailed 
among  systematists.  It  was  recognized  that  all  the  individuals  of 
a  species  are  not  alike,  and  that  more  or  less  pronounced  variability 
occurs,  so  that  it  was  possible  to  distinguish  races  and  varieties 
within  the  species,  but  it  was  believed  that  the  variations  never 
transcended  specific  bounds. 

The  Problem  Stated. — Darwin  begins  with  a  criticism  of  the 
term  species.  Is  the  conception  of  species  on  the  one  side  and 
that  of  race  and  variety  on  the  other  something  entirely  different  ? 
Are  there  special  criteria  for  determining  beyond  the  possibility 
of  a  doubt  whether  in  a  definite  case  we  have  to  do  with  a  variety 
of  a  species  or  with  a  different  species  ?  or  do  the  conceptions  in 
nature  pass  into  one  another  ?  Are  species  varieties  which  have 
become  constant,  and  precisely  in  the  same  manner  are  varieties 
species  in  the  process  of  formation  ? 

Morphological  Characters. — A.  Distinction  between  Species  and 
Variety. — For  the  settlement  of  this  fundamental  question  morpho- 
logical and  physiological  characters  can  be  considered.  In  the 
practice  of  the  systematists  usually  the  morphological  characters 
prevail  exclusively;  for  that  reason  they  will  be  here  considered 
first.  If,  among  a  great  number  of  forms  similar  to  one  another, 
two  groups  can  be  adduced  which  differ  considerably  from  one 
another,  if  the  difference  between  them  be  obliterated  by  no  inter- 
mediate forms,  and  if  in  several  successive  generations  they 
remain  constant,  then  the  systematist  speaks  of  a  '  good  species '  ; 
on  the  other  hand  he  speaks  of  varieties  of  the  same  species  when 
the  differences  are  slight  and  inconstant,  and  when  they  lose  their 
importance  through  the  existence  of  intermediate  forms.  A 
definite  application  of  this  rule  discloses  great  incongruities,  many 
animal  and  vegetable  groups  being  regarded  by  one  set  of  systema- 
tists as  good  species,  by  another  only  as  'sports/  i.e.,  as  varieties 
of  the  same  species.  The  differences  between  the  '  sports '  of  our 
domestic  animals  are  in  many  instances  so  considerable  that 
formerly  they  were  regarded  not  only  as  sufficient  for  the  founda- 
tion of  good  species,  but  even  of  genera  and  families.  In  the 
fantail  pigeon  the  number  of  tail-feathers,  formerly  only  12-14, 
has  increased  to  30-42  (fig.  Ic) ;  among  the  other  races  of  pigeons 


26  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

enormous  variations  are  found  in  the  size  of  the  beak  and  feet  in 
comparison  with  the  rest  of  the  body  (figs.  IA,  IB);  even  the 
skeleton  itself  participates  in  this  variation,  as  is  shown  by  the  fact 


FIG.  IA.— English,  carrier-pigeon.    (After  Darwin.) 


FIG.  IB.— English  tumbler-pigeon.    (After  Darwin.) 

that  the  total  number  of  vertebrae  varies  from  38  (in  the  carrier- 
pigeon)  to  43  (in  the  pouter),  the  number  of  sacral  vertebra  from 
14  to  11. 

B.  Variation  within  the  Species. — Now  in  respect  to  the  occur- 
rence of  transitional  forms  and  the  constancy  of  differences,  there 
is  within  one  and  the  same  <  good  species '  the  greatest  conceivable 
difference.  In  many  very  variable  species  the  extremes  are  united 


HISTORY  OF  ZOOLOGY. 


27 


"by  many  transitions;  in  other  cases  sharply  circumscribed  groups 
of  forms,  or  races,  can  be  distinguished  within  the  same  species. 
In  the  race,  the  peculiar  characteristics  are  inherited  from  genera- 


Fio.  lc.— English  fantail  pigeon.    (After  Darwin.) 

tion  to  generation  with  the  same  constancy  as  in  good  species. 
This  is  shown  in  the  human  races,  and  in  many  pure,  cultivated 
races  of  domesticated  animals. 

Physiological  Characters. — A.  Crossing  of  Species  and  Varie- 
ties.— A  critical  examination  leads  to  the  conclusion  that  Mor- 
phology is  indeed  useful  for  grouping  animals  into  species  and 
varieties,  but  that  it  leaves  us  completely  in  the  lurch  when  it  is 
Called  upon  to  show  the  distinctions  between  what  should  be  called 
a  species  and  what  a  variety.  Therefore  there  remains  open  to 
the  systematist  only  one  resource,  i.e.,  to  summon  Physiology 
to  his  aid.  This  has  been  done,  and  it  has  disclosed  considerable 
distinctions  in  reproduction.  We  should  expect  a  priori  that  the 
individuals  of  different  species  would  not  reproduce  with  each 
other;  on  the  other  hand  under  normal  conditions  the  individuals 
of  one  and  the  same  species,  even  though  they  are  of  different 
varieties  or  races,  should  be  entirely  fertile.  One  must  beware  of 
arguing  in  a  circle  in  proof  of  these  two  propositions;  it  would  be 
an  argument  in  a  circle  if  an  experimenter  should  regard  two 
animals  as  representatives  of  one  species  only  because  they  proved 
to  be  fertile  together,  while  under  their  former  relations  he 
assigned  them  to  different  species.  Bather  the  question  for  him 
must  read:  does  physiological  experiment  lead  to  the  same 


28  GENERAL  PRINCIPLES   OF  ZOOLOGY. 

systematic  distinctions  as  does  the  common  systematic  experience, 
viz.,  the  depreciation  of  constancy  and  the  divergence  of  distin- 
guishing characters  ? 

B.  The  Intercrossing  of  Species. — This  field  is  as  yet  far  from 
being  sufficiently  investigated  experimentally;  yet  some  general 
propositions  can  be  set  up:  (1)  that  not  a  few  so-called  'good 
species'  can  be  crossed  with  one  another;  (2)  that  in  general  the 
difficulty  of  crossing  increases,  the  more  distant  the  systematic 
relationship  of  the  species  used;  (3)  that  these  difficulties  are  by 
no  means  directly  proportional  to  the  systematic  divergence  of  the 
species.  The  most  favorable  material  for  research  is  furnished  by 
those  animals  in  which  artificial  fertilization  can  be  carried  out, 
i.e.,  of  which  one  can  take  the  eggs  and  spermatozoa  and  mix 
them  independently  of  the  will  of  the  animals.  Thus  hybrids 
have  been  obtained  from  species  which  belong  to  quite  different 
genera,  while  very  often  nearly-related  species  will  not  cross. 
Among  fishes  we  know  hybrids  of  Abramis  brama  and  Blicca 
bjorkna,  of  Trutta  solar  (salmon)  and  Trutta  far io  (trout);  among 
sea-urchins  the  spermatozoa  of  Strongylocentrotus  lividus  fertilize 
with  great  readiness  the  eggs  of  Echinus  microtuberculatus,  but 
only  rarely  the  eggs  of  Sphcerechinus  granularis,  which  is  nearer 
in  the  system.  It  also  happens  that  crossing  in  one  direction 
(male  of  A  and  female  of  B)  is  easily  accomplished,  but  in  the 
other  direction  (male  of  B  and  female  of  A)  it  completely  fails;  as, 
for  example,  the  sperm  of  Strongylocentrotus  lividus  fertilizes  well 
the  eggs  of  Echinus  microtuberculatus,  but,  conversely,  the  sperm 
of  E.  microtuberculatus  does  not  fertilize  the  eggs  of  S.  lividus. 
Even  better  known  is  the  fact  that  salmon  eggs  are  fertilized  by 
trout  sperm  but  not  trout  eggs  by  salmon  sperm.  Eggs  have  been 
fertilized  by  sperm  belonging,  to  different  families,  orders,  and 
possibly  classes.  Eggs  of  Pleuronectes  platessa  and  Labrus  rnpestris 
by  sperm  of  the  cod  (Gadus  morrhua),  frogs'  eggs  (Rana  arvalis) 
by  sperm  of  two  species  of  Triton,  eggs  of  a  starfish  (Asterias 
forbesi)  by  milt  from  a  sea-urchin,  Arbacia  pustulosa  (??).  In 
these  extreme  cases,  it  is  true,  the  hybrids  die  during  or  at  the 
close  of  segmentation,  before  the  embryo  is  outlined. 

In  the  case  of  animals  where  copulation  is  necessary  the  diffi- 
culties of  experimentation  increase,  since  here  often  between  males 
and  females  of  different  species  there  exists  an  aversion  which 
prevents  any  union  of  the  sexes.  Yet  in  this  case  we  know  crosses 
of  different  species;  among  the  vertebrates  crossing  takes  place, 
e.g.,  between  the  horse  and  the  ass;  our  domestic  cattle  and  the 


HISTORY  OF  ZOOLOGY.  29 

zebu;  ibex  (or  wild  buck)  and  she-goat;  sheep  and  goats;  dog  and 
jackal;  dog  and  wolf :  hare  and  rabbit  (Lepus  darwini);  American 
bison  and  domestic  cattle;  etc.;  among  birds,  between  different 
species  of  finches  and  of  grouse;  mallard  (Anas  boschas)  and  the 
pintail  duck  (Dafila  acutd)'3  the  European  goose  and  the  Chinese 
goose  (Anser  ferus  and  A.  cygnoides}.  Among  the  insects, 
especially  the  Lepidoptera,  the  cases  are  many,  but  the  resulting 
eggs  produce  larvae  of  slight  vital  force  only  in  the  case  of  Actias 
luna  and  A.  isabellce. 

C.  Fertility  of  Hybrids  and  Mongrels. — Since  many  hybrids, 
as  the  mule,  have  been  known  for  thousands  of  years,  the  criterion 
is,  as  it  were,  pushed  back  one  stage ;  if  the  infertility  in  cases  of 
crosses  in  many  species  is  not  immediately  noticeable,  yet  it  may 
be  apparent  in  the  products  of  the  cross.     While  the  products  of 
the  crossing  of  varieties,  the  '  mongrels/  always  have  a  normal, 
often   an   increased,   fertility,    the   products   of   the   crossing   of 
species,  the  hybrids,  should  always  be  sterile.     But  even  this  is 
a  rule,  not  a  law.     The  mule  (which  only  very  rarely  reproduces) 
and  many  other  hybrids  are  indeed  sterile,  but  there  are  not  a  few 
exceptions,  although  the  number  of  experiments  in  reference  to 
this  point  is  very  small.     Hybrids  of  hares  and  rabbits  have  con- 
tinued  fruitful    J:or  generations;    the   same   is    true    of   hybrids 
obtained  from  the  wild  buck  and  the  domesticated  she-goat;  from 
Anser  cygnoides  and  A.  domesticus;   from  Salmo  salvelinus   and 
S.  fontinalis;   Cyprinus  carpio    and   Carassius  vulgar  is;  Bombyx 
cynthia  and  B.  arrindia. 

D.  Inbreeding. — Even  the  second  of  the  above  statements,  that 
individuals  of  a  species,  provided  they  are  sound,  always  reproduce 
with  one  another,  needs  limitation.     Breeders  of  animals  have  long 
known  the  disastrous  consequences  of  inbreeding — that  the  repro- 
ductive  power    is    reduced    even   to   sterility   if,    for   breeding, 
descendants  of  a  single  pair  be  continually  chosen.     Darwin  has 
collected  not  a  few  cases  where  undoubted  members  of  the  same 
species  have  been  completely  sterile  with  one  another;  as  certain 
forms  of  primrose  and  other  di-  and  tri-morphic  species.     Exam- 
ples of  the  sterility  of  mongrels  are  known  only  in  botany  (certain 
varieties  of  maize  and  mullein). 

Conditions  Governing  Fertility  in  Sexual  Reproduction.— 
When  we  look  over  these  facts  it  would  seem  as  if  continued 
fertility  in  sexual  reproduction  were  guaranteed  by  a  not  too  con- 
siderable diiference  in  the  sexual  products.  Too  great  similarities, 
as  these  exist  in  inbreeding,  and  too  great  differences,  as  in  the 


30  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

hybridization  of  different  species,  are  injurious  and  are  abhorred 
by  Nature.  Sexual  reproduction  possesses  an  optimum;  if  this  be 
departed  from  in  either  direction,  diminution  gradually  follows. 
But  for  that  reason  it  has  already  been  said  that  here  gradual  and 
not  primary  differences  exist,  and  therefore  this  character  cannot 
be  employed  as  a  primary  distinction*  bet  ween  species  and  varieties. 

Difficulties  in  Classification. — The  final  result  of  all  this  dis- 
cussion may  be  summed  up  as  follows:  up  to  the  present  time, 
neither  by  physiological  nor  by  morphological  evidence  has  there 
been  successfully  fixed  in  a  clear  and  generally  applicable  way  a 
criterion  which  can  guide  the  systematist  in  deciding  whether 
certain  series  of  forms  are  to  be  regarded  as  good  species  or  as 
varieties  of  a  species.  Zoologists  are  guided  rather  in  practice  by 
a  certain  tact  for  classification,  which,  however,  in  difficult  cases 
leaves  them  in  the  lurch,  and  thus  the  opinions  of  various  investi- 
gators vary. 

Change  of  Varieties  into  Species. — The  conditions  above  dis- 
cussed find  their  natural  explanation  in  the  assumption  that  sharp 
distinctions  between  species  and  variety  do  not  exist;  that  species 
are  varieties  which  have  become  constant,  and  varieties  are  incipient 
species.  The  meaning  of  the  above  can  be  made  clear  by  explana- 
tion of  a  concrete  case.  Individuals  of  a  species  begin  to  vary, 
i.e.,  compared  with  one  another  they  attain  a  greater  or  less- 
difference  in  character.  So  long  as  the  extreme  differences  are 
bridged  by  transitional  forms  we  speak  of  varieties  of  a  species ; 
if,  on  the  other  hand,  the  intermediate  transitions  have  died  out, 
and  the  differences  have  in  the  course  of  a  long  space  of  time 
become  fixed,  and  so  very  much  intensified  that  a  sexual  union  of 
the  extreme  forms  results  either  in  complete  sterility  or  at  least  in 
a  marked  tendency  towards  sterility,  then  we  speak  of  different 
species. 

Species  may  be  Related  to  each  other  in  Unequal  Degrees.— 
In  favor  of  this  view,  that  varieties  will  in  longer  time  become 
species,  is  the  great  agreement  which  in  the  large  majority  of 
cases  exists  between  the  two.  In  genera  which  comprise  a  remark- 
able number  of  species,  the  species  usually  show  also  many 
varieties;  the  species  are  then  usually  grouped  in  sub-genera,  i.e., 
they  are  related  to  each  other  in  unequal  degrees,  since  they  form 
small  groups  arranged  around  certain  species.  In  regard  to  the 
varieties  also  the  case  is  similar.  In  such  genera  the  formation  of 
species  is  in  active  progress;  but  each  species  formation  presup- 
poses a  high  degree  of  variability. 


HISTORY  OF  ZOOLOGY.  31 

Phylogeny. — It  is  now  clear  that  what  has  here  been  worked 
out  in  the  case  of  the  species  must  also  apply  to  the  other  cate- 
gories of  the  system.  Just  as  by  divergent  development  varieties 
become  species,  so  must  species  by  continued  divergence  become 
so  far  removed  from  one  another  that  we  distinguish  them  as 
genera.  It  will  be  only  a  question  of  time  when  these  differences 
will  become  still  greater,  and  cause  the  establishment  of  orders, 
classes,  and  branches,  just  as  the  tender  shoots  of  the  young 
plantlet  become  in  the  strong  tree  the  chief  branches  from  which 
spring  lateral  branches  and  twigs.  If  we  pursue  this  train  of 
thought  to  its  ultimate  consequences,  we  reach  the  conception  that 
all  the  animals  and  plants  living  at  present  have  arisen  by  means 
of  variation  from  a  few  primitive  organisms.  Inasmuch  as  at  least 
many  thousands  of  years  are  required  for  the  formation  of  several 
new  species  through  the  variability  of  one,  there  must  then  have 
been  necessary  for  this  historical  development  of  the  animal  and 
vegetable  kingdoms  a  space  of  time  greater  than  our  mental 
capacity  can  grasp.  Since  for  the  idea  of  the  individual  develop- 
ment (embryology)  of  an  animal  the  term  Ontogeny  has  been 
chosen,  it  has  also  proved  convenient  to  apply  to  the  historical 
development  of  animals — though  this  has  not  been  observed,  but 
only  inferred — the  term  History  of  the  Race  or  Phylogeny. 

Spontaneous  Generation.— If  we  attempt  to  derive  all  living 
animals  from  a  common  primitive  form,  we  are  compelled  to 
assume  that  this  was  of  extremely  simple  organization,  that  it  was 
unicellular;  for  the  simpler,  the  less  specialized,  the  organization, 
so  -  much  the  greater  is  its  capacity  for  variation.  Only  from 
simple  organisms  can  the  lower  unicellular  organisms,  the 
Protozoa,  be  derived.  Finally,  for  the  simple  organisms  alone 
can  we  conceive  a  natural  origin.  Since  there  was  undoubtedly  a 
time  upon  our  earth  when  temperatures  prevailed  which  made  life 
impossible,  life  must  at  some  time  have  arisen,  either  through  an 
act  of  creation  or  in  a  natural  way  through  spontaneous  generation. 
If,  in  agreement  with  the  spirit  of  natural  science,  we  invoke  for 
the  explanation  of  natural  facts  only  the  forces  of  nature,  we  are 
driven  to  the  hypothesis  of  spontaneous  generation,  namely,  that 
by  a  peculiar  combination  of  materials  without  life,  the  compli- 
cated mechanism  which  we  call  '  life '  has  arisen.  This  hypothesis 
also  supposes  that  the  first  organisms  possessed  the  simplest  con- 
ceivable structure. 

Variability  not  proven  to  be  a  Universal  Principle. — Starting 
from  a  basis  of  facts,  by  generalization  we  reach  a  simple  concep- 


32  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

tion  of  the  origin  of  the  animal  kingdom,  but  we  have  in  equal 
measure  departed  from  the  results  of  direct  observation.  Observa- 
tions only  show  us  that  species  are  capable  of  changes  and  can 
from  themselves  produce  new  species.  That  this  capacity  for 
variation  is  a  universal  principle,  a  principle  which  explains  to  us 
the  origin  of  the  animal  world,  needs  further  demonstration. 

Proofs  of  Phylogeny. — The  rise  of  the  existing  animal  world 
is  a  process  which  has  taken  place  in  the  thousands  of  years  long 
past,  but  is  no  longer  accessible  for  direct  observation,  and  there- 
fore it  can  never  be  proved  in  the  sense  that  we  explain  the  indi- 
vidual development  of  an  organism.  In  regard  to  the  conception 
of  the  simple  evolution  of  animals  we  can  merely  prove  the 
probability;  yet  it  is  shown  that  all  our  observations  of  accessible 
facts  not  only  agree  with  this  conception,  but  find  in  it  their  only 
simple  explanation.  Such  facts  are  furnished  to  us  by  the  classi- 
fication of  animals,  paleontology,  geographical  distribution,  com- 
parative anatomy,  and  comparative  embryology. 

(1)  Proofs  from  Classification. — For  a  long  time  it  has  been 
recognized,  and  in  recent  times  finds  ever-increasing  confirmation, 
that  if  we  wish  to  express  graphically  the  relationships  of  animals, 
their  classes,  orders,  genera,  and  species,  simple  co-ordination  and 
subordination  are  not  sufficient,  but  one  must  select  a  treelike 
arrangement,  in  which  the  principal  divisions,  more  closely  or  dis- 
tantly related  to  one  another, — the  branches,  phyla,  or  types,— 
represent  the  main  limbs,  while  the  smaller  branches  and  twigs 
correspond  to  the  several  classes,  orders,  etc.     This  is,  in  fact, 
the  arrangement  to  which  the  theory  of  evolution,  as  seen  above, 
necessarily  leads. 

(2)  Paleontological  Demonstration  approaches  nearest  to  what 
one  might  call  direct  proof;  for  paleontology  gives  us  the  only 
traces  of  existence  which  the  predecessors  of  the  present  animal 
world  have  left.     Even  here  a  hypothetical  element  has  crept  into 
the  demonstration.     We  can  only  observe  that  various  grades  of 
forms  of  an  animal  group  are  found  in  successive  strata;  if  we 
.unite  these  into  a  developmental  series,  and  regard  the  younger 
as  derived  from  the  older  by  variation,  we  depart,  strictly  speak- 
ing, from  the  basis  of  fact.      But  the  value   of   paleontological 
evidence  is  weakened  much  more  by  its  extreme  incompleteness. 
In  fossils  only  the  hard  parts  are  generally  preserved;   the  soft 
parts,  on  the  other  hand,  which  alone  are  present,  or  at  least  make 
up  the  most  important  part  of  many  animals,  are  almost  always 
lost.     Only  rarely  are  the  soft  parts  (muscle  of  fishes,  ink-bag  of 


HISTORY   OF  ZOOLOGY. 


33 


cephalopods,  outlines  of  medusae)  preserved  in  the  rocks.  Even 
the  hard  parts  remain  connected  only  under  exceptionally  favor- 
able conditions.  If  further  we  take  into  consideration  the  fact 
that  these  treasures  are  buried  in  the  bosom  of  the  earth,  and  are 
usually  obtained  only  by  accident,  in  quarrying  and  road-build- 
ing, and  besides  only  extremely  seldom  excavated  with  scientific 
care,  it  becomes  sufficiently  clear  how  little  is  to  be  expected  from 
the  past  and  indeed  future  material  of  paleontology. 

Examples    of  Paleontological    Proof. — Yet    paleontology   has 
already  furnished  many  important  proofs  of  the  theory  of  descent. 


Fio.  2.—  Archceopteryx  lithnijraphica.     (After  Zittel.)    cl,  clavicle;  co,  coracoid;  7i, 
humerus ;  r,  radius;  w,  ulna ;  c,  carpus ;  I-IV,  digits  ;  sc,  scapula. 

It  has  shown  that  the  lower  forms  appeared  first,  and  the  more 
highly  organized  later.  Among  animals  in  general  the  latest  to 
appear  were  the  vertebrates,  and  of  these  the  mammals;  among 
the  mammals  man.  For  smaller  groups  genealogical  material  has 


34:  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

fortunately  been  found.  Transitional  forms  connect  the  single- 
toed  horse  of  the  present  with  the  four-toed  Eohippos  of  the 
eocene;  for  all  the  hoofed  animals  a  common  starting-point  or 
ancestral  form  has  been  found  in  the  Condylarthra.  Transitional 
forms  have  also  been  found  between  the  greater  divisions,  as,  e.g., 
between  reptiles  and  birds,  the  remarkable  toothed  birds,  and  the 
Archceopteryx  (fig.  2),  a  bird  with  a  long,  feathered,  lizard-like 
tail. 

(8)  Morphological  Proofs.  —  When  we  employ  comparative 
anatomy  and  embryology  in  support  of  evolution,  we  find  that  the 
two  studies  have  so  many  points  in  common  that  they  can  best  be 
treated  together. 

Cuvier  and  von  Baer  taught  that  the  separate  types  of  the 
animal  kingdom  are  units,  each  with  a  special  structure  and  plan 
of  development  peculiar  to  it;  farther,  that  there  are  no  similari- 
ties in  structure  and  in  the  development  forming  a  bridge  from 
type  to  type.  The  first  of  these  two  propositions  is  still  regarded 
as  correct,  but  the  second,  which  alone  is  important  for  the  theory 
of  evolution,  has  become  quite  untenable.  All  animals  have  a 
common  organic  basis  in  the  cell  and  are  thereby  brought  close 
to  one  another;  all  multicellular  animals  agree  in  the  principal 
points  during  the  first  stages  of  their  development,  during  the 
fertilization,  cleavage  of  the  egg,  and  the  formation  of  the  first 
two  germ-layers,  and  vary  from  one  another  only  in  such  differ- 
ences as  may  occur  within  one  and  the  same  type.  Also  the 
peculiarities  which  distinguish  each  type  in  structure  and  in  the 
mode  of  development  are  not  without  intermediate  phases. 
Especially  from  the  branch  of  the  worms  there  lead  off  transitional 
forms  to  the  other  branches :  Balanoglossus  to  both  echinoderms 
and  chordates,  the  annelids  and  Peripatus  to  the  arthropods,  the 
tunicates  and  Amphioxus  to  the  vertebrates.  In  some  representa- 
tives of  each  type  the  structure  and  the  mode  of  development  are 
simpler,  thereby  approaching  to  the  conditions  which  obtain  in 
the  other  types.  The  existence  of  such  transitional  forms  is  one 
of  the  most  important  proofs  in  favor  of  the  theory  of  evolution, 
and  speaks  against  the  assumption  of  a  rigid  unvarying  type  in 
Cuvier's  sense. 

Fundamental  Law  of  Biogenesis. — A  fact  that  weighs  heavily 
in  the  balance  in  favor  of  the  theory  of  evolution  is  the  fact  that 
the  structure  and  mode  of  development  of  animals  is  ruled  by  a 
law  which  at  present  can  only  be  explained  by  the  assumption  of 
a  common  ancestry.  Each  animal  during  its  development  passes 


HISTORY  OF  ZOOLOGY. 


35 


through  essentially  the  stages  which  remain  permanent  in  the  case 
of  lower  or  at  least  more  primitive  animals  of  the  same  branch,  as 

432        1 


ell 


FIG.  3.— Human  Embryo,  about  third  or  fourth  week.  1-4,  visceral  arches  with  gill- 
slits  between  them  :  1,  mandibular  arch  ;  2,  hyoid  arch  ;  3  and  4,  first  and  second 
gill-arches,  a,  eye ;  n,  nasal  pit :  ft,  cardiac  region  ;  e  land  e  II,  fore  and  hind 
extremities;  rn,  mesodermal  somites. 


FIG.  4.— Tadpoles  of  Rana  tempnraria.  TM,  mouth  ;  g,  upper  jaw;  2,  lower  ja^;  «, 
sucking-disc  ;  hb,  external  gills ;  ifc,  region  of  the  internal  gills ;  w,  nose  ;  a,  eye; 
o,  auditory  vesicle  ;  /i,  cardiac  region  ;  c/,  operculum. 

the  three  following  examples  will  show:  (1)  In  the  early  stages  of 
development   the   human   embryo   (fig.   3)    possesses   remarkable 


36 


GENERAL  PRINCIPLES   OF  ZOOLOGY. 


resemblances  to  the  lowest  vertebrates,  the  fishes.  Like  these  it 
has  gill-slits,  the  same  arrangement  of  the  heart  and  of  the  arterial 
vessels,  certain  fundamental  features  in  the  development  of  the 
skeleton,  etc.  (2)  Frogs  in  their  tadpole  stage  have  an  organiza- 
tion similar  to  that  which  remains  permanent  in  the  case  of 
certain  Amphibia,  the  Perennibranchiata  (fig.  5),  which  stand 


FIG.  5.— Siredon  pisciformis  (larva  of  Amblystoma  tigrinum).    (After  Dum6ril  and 

Bibron.) 

lower  in  the  system;  they  have  a  swimming  tail  and  tuft-like  gills, 
which  are  lacking  in  the  adult  frog.  (3)  There  are  certain  para- 
sitic Crustacea,  which  live  upon  the  gills  of  fishes,  and  seem  not 


FIG.  6. — Achtheres  percarum.    a,  nauplius-,  h,  cyclops-stage  ;  c,  adult  female. 
(After  Claus.) 

at  all  like  their  relatives.  They  are  shapeless  masses  which  were 
formerly  regarded  as  parasitic  worms.  Their  systematic  position 
was  only  determined  by  their  embryology  (fig.  6).  Here  it  is 


HISTORY  OF  ZOO  LOOT. 


37 


shown  that  they  pass  through  a  nauplius-stage  (fig.  6«),  charac- 
teristic of  most  Crustacea,  and  that  they  then  assume  the  shape  of 
small  Crustacea  (fig.  6,  b),  like  Cyclops  (fig.  7,  A),  so  widely  dis- 


ctu 


FIG.  7.— Cyclops  coronatus  (A)  and  also  the  nauplius  in  lateral  (B)  and  in  ventral  view 
(G).  I,  head;  II- F,  the  five  thoracic,  and  behind  these  the  five  abdominal  seg- 
ments ;  F,  furca  ;  1,  the  first,  2,  the  second,  antennae  ;  3,  mandibles  ;  4,  maxillae: 
5,  maxillipeds  ;  6-9,  the  first  four  pairs  of  biramous  feet,  while  the  rudimentary 
fifth  pair  are  hidden ;  cm,  eye ;  o,  upper  lip ;  e,  egg-sacs  ;  (7,  gut ;  m,  muscle. 

tributed  in  fresh  waters.  Very  often  the  males  make  a  halt  in  the 
cy clops-stage  while  the  female  develops  farther  and  assumes  a 
shapeless  form,  so  that  there  arises  a  very  remarkable  sexual 
dimorphism  (fig.  8).  All  these  examples,  which  can  be  multiplied 
by  hundreds,  can  be  explained  in  the  same  way.  The  higher  forms 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


Flmai8eTaPf?ef  ffius  x 
(after  Bergsoe),  x  13. 


pass    through   the   stage   of   organization    of   the  lower,    because 

they  spring  from  ancestors  which  were 
more  or  less  similar  to  the  latter.  Man 
in  his  embryological  development  passes 
through  the  fish  stage,  the  frog  the  per- 
ennibranchiate  stage,  the  parasitic  crus- 
tacean first  the  nauplius-  and  then  the 
cyclops-stage,  because  their  ancestors 
were  once  fish-like,  perennibranchiate- 
like,  nauplius-  and  cyclops-like.  Here 
is  expressed  a  general  phenomenon 
which  Haeckel  has  stated  in  a  general 
proposition  under  the  name  of  'the 
Fundamental  Law  of  Biogenesis.  '  '  '  The 
development  history  (ontogeny)  of  an 
individual  animal  briefly  recapitulates 
the  history  of  the  race  (phylogeny); 
i.  e.  ,  the  most  important  stages  of  organi- 
zation  which  its  ancestors  have  passed 
through  appear  again,  even  if  somewhat  modified,  in  the  develop- 
ment of  individual  animals." 

Examples  of  the  Application  of  this  Law.  —  Hie  Nervous 
System.  —  This  law  applies  as  well  to  single  organs  as  to  entire 
animals.  The  central  nervous  system  of  the  lower  animals 
(echinoderms,  coelenterates,  many  worms)  forms  part  of  the  skin; 
in  its  first  appearance  it  belongs  to  the  surface  of  the  body,  because 
it  has  to  mediate  the  relations  with  the  external  world.  In  the 
case  of  higher  animals,  e.  g.  ,  the  vertebrates,  the  brain  and  spinal 
cord  lie  deeply  embedded  in  the  interior  of  the  body;  but  in  the 
embryo  it  is  laid  down  likewise  as  a  part  of  the  skin  (medullary 
plate)  and  which  gradually  through  infolding  and  cutting  off  from 
this  comes  to  lie  internally.  One  can  demonstrate  this  change  oi 
position  by  cross-sections  through  the  dorsal  region  of  embryos  of 
different  ages  of  any  vertebrate  (fig.  9). 

The  Skeletal  System.  —  The  skeleton  of  vertebrates  is  a  further 
example.  In  the  lowest  chordates,  amphioxus  and  the  cyclostomes, 
the  vertebrae  are  lacking,  and  in  their  place  we  find  a  cylindrical 
cord  of  tissue,  the  chorda  dorsalis  (notochord).  In  the  fishes  and 
Amphibia  the  notochord  usually  persists;  but  it  is  partially 
reduced  and  constricted  by  the  vertebrae,  which  in  the  lower  forms 
consist  of  cartilage,  and  in  the  higher  of  bone  or  a  combination  of 
bone  and  cartilage.  Mature  birds  and  mammals  finally  have  a 


HISTORY  OF  ZOOLOGY. 


39 


FIG.  ^.-Cross-sections  through  the  dorsal  region  of  Triton  embryos  at  .different  ages 
(from  O.  Hertwig)    In  T  the  medullary  plate  fanlage  of  spinal  cord)  rn»  is 
off  from  the  skin  (epidermis,  ep}  bv  the  medullary  folds  (wf).    In  II  the 
lary  plate  by  inrolling  of  the  medullary  folds  is  converted  into  a  groove 


In  11 


the  groove  has  closed  into  a  tube  <n\  the  spinal  cord,  which  has  separated  trom 
the  rest  of  the  ectoderm  (epidermis).  C,  body  cavity  (coelom)  ;  cfc,  notochord  ; 
cp,  cavity  of  primitive  somite  (myotome);  rfz,  yolk-cells;  i/c,  entoderm;  Ig,  lumen 
of  gut  ;  wfc1,  rnfe*,  somatic  and  splanchnic  layers  of  mesoderm. 


40  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

completely  ossified  vertebral  column;  their  embryos,  on  the  other 
hand,  have  in  the  early  stages  only  the  notochord  (amphioxus 
stage) ;  later  this  notochord  becomes  constricted  by  the  vertebrae 
(fish-amphibian  stage)  and  finally  entirely  replaced;  the  vertebral 
column  is  in  the  beginning  cartilaginous,  only  later  becoming  ossi- 
fied. Comparative  anatomy  and  embryology  thus  give  the  same 
developmental  stage  of  the  axial  skeleton:  (1)  notochord,  (2) 
notochord  and  vertebral  column,  the  latter  at  first  formed  of 
cartilage,  then  of  bone. 

We  have  here  spoken  of  a  parallelism  between  the  facts  of 
comparative  anatomy  and  those  of  embryology.  But  in  reality  we 
should  expect  a  threefold  parallelism:  for  according  to  the  theory 
of  evolution  the  systematic  arrangement  of  animals  is  conditioned 
by  a  third  factor — the  historical  development  of  the  animal  world, 
or  phylogeny.  The  mile-stones  of  phylogenesis,  the  fossils,  should 
give  the  same  progressive  series  in  the  successive  geological  strata 
as  the  stages  of  forms  found  by  comparative  anatomy  and  embry- 
ology. We  actually  know  instances  of  such  threefold  parallelisms. 
Comparative  anatomy  teaches  that  the  lowest  developed  form  of  a 
fish's  tail  is  the  diphycercal  (fig.  10,  ^4);  that  from  this  the 
heterocercal  (B),  and  from  the  heterocercal  the  homocercal  form 
of  tail-fin  ( C,  D)  can  be  derived.  Embryologically  the  most  highly 
developed  fishes  are  first  diphycercal,  later  heterocercal,  and 
finally  become  homocercal.  Last  of  all,  paleontologically  the 
oldest  fishes  are  diphycercal  or  heterocercal,  and  only  later  do 
homocercal  forms  appear. 

What  has  here  been  referred  to  is  only  a  small  fraction  of  the 
weighty  proofs  which  morphology  offers  in  favor  of  evolution;  it 
can  only  serve  to  show  how  morphological  observations  can  be 
employed.  For  the  reflecting  naturalist  the  facts  of  morphology 
are  a  single  great  inductive  proof  in  favor  of  the  theory  of  evolu- 
tion. 

Distribution  of  Animals. — From  Animal  Geography  we  learn 
that  the  present  distribution  of  animals  is  the  product  of  past 
hundreds  and  thousands  of  years.  It  will  therefore  be  possible 
from  this  to  figure  out  many  of  the  earlier  conditions  of  things, 
by  proceeding  with  the  utmost  caution  and  overcoming  extreme 
difficulties. 

If  we  assume  that  from  the  beginning  all  animal  species  were 
constituted  as  they  now  are,  they  would  then  have  been  placed  by 
the  purposeful  Creator  in  the  regions  best  suited  to  their  organiza- 
tion; their  distribution  would  therefore  have  been  determined  by 


HISTORY  OF  ZOOLOGY. 


41* 


favorable  or  unfavorable  conditions  of  life  prevailing  in  the  various 
regions,  as  the  climate,  food-supply,  etc.  If,  on  the  other  hand, 
we  assume  that  the  animal  species  have  arisen  from  one  another 
through  variation,  then  there  must  have  been,  as  an  influence 
determining  the  manner  of  distribution,  besides  the  conditions 
of  existence,  still  a  second  factor,  which  we  will  call  the  geo- 


Fio.  10.— Tail-fins  of  various  fishes.  (From  Zittel.)  A,  Diphycercal  fin  of  Polypterus 
bichir.  (Vertebral  column  and  notochord  divide  the  tail  into  symmetrical  dorsal 
and  ventral  portions.)  Zf,  Heterocercal  tail  of  the  sturgeon.  (As  a  result  of  an 
upward  bending  of  the  notochord  and  vertebral  column  the  fin  has  become 
asymmetrical,  the  ventral  portion  much  larger  than  the  dorsal.)  C,  -D,  Homo- 
cereal  fins,  C,  of  Amia  calva ;  D,  of  Trutta  salar.  (By  a  still  greater  upward  bend- 
ing of  the  notochord  and  vertebral  column  the  dorsal  portion  has  almost  en- 
tirely disappeared  and  the  ventral  portion  almost  alone  forms  the  fin,  externally 
apparently  symmetrical,  but  in  its  internal  structure  very  asymmetrical.)  c/i, 
chorda ;  a,  b,  c,  cover-plates. 

logical.  We  know  that  the  configuration  of  the  earth's  surface 
has  changed  in  many  respects  in  the  course  of  the  enormous  space 
of  time  of  the  geological  periods;  that  land  areas,  which  earlier 
were  united,  have  become  separated  by  the  encroachments  of  the 
sea;  that  by  the  upheaval  of  mountains  important  barriers  to  the 
distribution  of  animals  were  also  formed.  From  the  fact  that 


42  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

these  two  changes — the  changes  in  the  earth's  surface  and  in  the 
animal  world  established  upon  it — have  gone  on  hand  in  hand 
-there  follows  necessarily  the  consequence  that  greater  differences 
in  the  faunal  character  of  two  lands  must  result  the  longer  they 
have  developed  independently  of  one  another,  without  interchange 
of  their  animal  populations,  and  the  longer  the  inhabitants  have 
been  separated  by  impassable  barriers.  For  the  various  groups 
the  character  of  the  barriers  is  different ;  terrestrial  animals,  which 
•cannot  fly,  are  hindered  in  their  distribution  by  arms  of  the  sea; 
marine  forms  by  land  barriers;  for  terrestrial  molluscs  high  moun- 
tain ranges,  which  are  dry  and  barren,  or  constantly  snow-capped, 
are  effectual. 

Instances  of  Proofs. — Since  attention  has  been  called  to  these 
conditions,  many  geographical  facts  favorable  to  the  theory  of 
evolution  have  been  ascertained:  (1)  Of  the  various  continents 
Australia  has  faunally  an  independent  character;  when  discovered 
it  contained  almost  none  of  the  higher  (placental)  mammals., 
except  such  as  can  fly  (Chiroptera),  or  marine  forms  (Cetacea),  or 
.such  as  are  easily  transported  by  floating  wood  (small  rodents),  or 
.such  as  could  be  introduced  by  man  (dingo,  the  Australian  dog); 
instead,  it  had  remarkable  birdlike  animals  (with  beak  and  cloaca), 
.and  the  marsupials,  which  have  become  extinct  in  the  Old  World 
and  the  opossums  excepted,  in  America  as  well.  The  phenomenon 
is  explained  by  the  geological  fact  that  in  the  earth's  history 
Australia,  with  its  surrounding  islands,  was  certainly  the  earliest 
to  lose  its  connexion  with  the  other  continents.  While  in  the 
-other  four  parts  of  the  earth  the  higher  vertebrates,  which  were 
developed  from  the  marsupials  and  their  lower  contemporaries, 
came,  by  way  of  the  lands  connecting  the  various  continents,  to 
have  a  wide  or  even  a  cosmopolitan  distribution,  in  isolated 
Australia  this  process  of  evolution  did  not  go  on,  and  its  ancient 
i'aunal  character  was  preserved.  (2)  As  Wallace  has  shown,  the 
Malay  Archipelago  is  divided  faunally  into  an  eastern  and  a  western 
half;  within  each  group  there  are  islands  which,  in  spite  of  a 
•different  climate,  have  a  very  similar  fauna.  On  the  other  hand, 
the  faunal  boundary  ('  Wallace's  line')  passes  between  the  two 
islands  Bali  and  Lombok,  which  have  the  same  climate  and 
geographically  are  very  close  together.  But  the  depth  of  the  strait 
in  this  region  shows  that  here  runs  a  boundary  of  extraordinarily 
long  geological  duration,  and  that  in  the  earth's  history  Bali  has 
developed  in  connexion  with  the  western,  Lombok  with  the  eastern 
chain  of  islands.  More  recent  studies  make  it  probable  that  there 


HISTORY  OF  ZOOLOGY.  43 

is  an  island  zone  between  the  two  in  which  a  mixture  of  faunas 
occurs.  Celebes  especially  belongs  here.  (3)  A  long  time  before 
Darwin,  the  renowned  geologist  Leopold  von  Buch,  from  the  dis- 
tribution of  plants  on  the  Canary  Islands,  had  come  to  the  conclu- 
sion of  a  change  of  species  into  new  species;  viz.,  on  islands 
peculiar  species  develop  in  secluded  valleys,  because  high  mountain- 
chains  isolate  plants  more  effectually  than  do  wide  areas  of  water. 
M.  Wagner  has  collected  many  instances  which  prove  that  locali- 
ties inhabited  by  certain  species  of  beetles  and  snails  have  been 
sharply  divided  by  wide  rivers  or  by  mountain-chains,  while  in 
neighboring  regions  related  so-called  '  vicarious  species '  are  found. 

Causal  Foundation  of  the  Theory  of  Evolution. — The  Dar- 
winian theory,  so  far  as  the  above  exposition  shows,  is  fundamen- 
tally like  the  theories  of  descent  advocated  at  the  beginning  of  this 
century  by  Lamarck  and  other  zoologists;  it  is  distinguished  from 
these  only  by  its  much  more  extensive  foundation  of  facts,  and 
further  in  that  it  abandoned  the  successional  arrangement  over- 
thrown by  the  type  theory,  and  replaced  it  by  the  branched,  tree- 
like mode  of  arrangement, — the  genealogical  tree.  But  still  more 
important  are  those  advances  of  Darwinism  which  relate  to  the 
causal  foundation  of  the  descent  theory.  The  doctrine  of  causes 
which  has  brought  about  the  change  of  species  forms  the  nucleus 
of  the  Darwinian  theory,  by  which  it  is  especially  distinguished  from 
Lamarckism.  In  order  to  substantiate  causally  the  change  of 
species,  Darwin  proposed  his  highly  important  principle  of 
*  Natural  Selection  by  means  of  the  Struggle  for  Existence. ' 

Artificial  Selection. — In  the  development  of  this  principle 
Darwin  started  from  the  limited  and  hence  easily  comprehended 
subject  of  Domestication,  the  artificial  breeding  of  our  races  of 
domesticated  animals.  Many  of  these  undoubtedly  sprang  from  a 
.single  wild  living  species;  others  arose  from  several  species,  but 
now  have  the  appearance  of  a  single  species.  How  have  arisen 
.such  extraordinarily  different  rapes  of  pigeons — the  fantail,  the 
pouter,  long-  and  short-billed  pigeons,  etc.,  the  long-  and  short- 
horned  cattle,  the  heavy,  slow  Percherons  and  the  slenderly-built, 
fleet-footed  Arabian  horses  ?  Undoubtedly  through  that  same 
more  or  less  conscious  influence  of  man,  which  is  still  employed 
"by  the  skilful  animal-breeder.  If  he  wish  to  obtain  a  particular 
form,  he  chooses  from  his  stock  suitable  animals,  which  he  pairs 
together  if  they  in  ever  so  slight  a  manner  approach  nearer  than 
the  others  to  the  desired  ideal.  By  repetition  of  this  selection 
according  to  plan,  the  breeder  attains  a  slow  but  sure  approx- 


44  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

imation  to  the  goal,  since  he  uses  for  breeding  only  the  suitable 
individuals  from  each  new  generation.  If  he  wish,  for  example, 
to  breed  fantail  pigeons,  he  selects  from  his  stock  animals  with 
the  most  numerous  and  strongest  tail-feathers.  In  the  course  of 
generations,  then,  characteristics  cumulate;  the  number  of  pigeons 
having  an  increased  number  of  tail-feathers  becomes  greater,  and 
thus  material  is  obtained  which  is  adapted  to  a  further  increase  in 
the  number  of  feathers. 

Factors  of  Evolution  in  Breeding. — The  remarkable  results  of 
breeding  which  are  well  known  to  every  observer  of  our  domesti- 
cated animals  depend  mainly  upon  three  factors:  (1)  Variability; 
the  descendants  of  one  pair  of  parents  have  the  capability  of 
developing  new  characteristics,  thereby  differing  in  appearance 
from  their  parents.  (2)  Hereditalility  of  newly-acquired  charac- 
ters. This  consists  in  the  tendency  of  the  daughter-generation  to 
transmit  the  newly-developed  characteristic  to  the  succeeding 
generation.  (3)  Artificial  selection;  man  selects  for  breeding  pur- 
poses suitable  individuals,  and  prevents  a  new  character  which  has 
arisen  through  variation  from  disappearing  through  crossing  again 
with  animals  of  the  opposite  variational  tendencies. 

Factors  of  Evolution  in  Nature. — If  we  compare  with  the  facts 
of  domestication  the  conditions  of  animals  living  in  the  state  of 
nature,  we  find  again  variability  and  heredity,  as  efficient  forces, 
inherent  in  all  organisms,  though  the  former  is  not  everywhere  of 
the  same  intensity.  There  are  many  species  which  vary  only 
slightly  or  not  at  all,  and  therefore  have  remained  unchanged  for 
thousands  of  years.  But  contrasted  with  these  conservative  species 
are  in  every  group  progressive  species,  active  species,  which  are 
in  the  process  of  rapid  change,  and  these  alone  are  of  importance 
in  causing  the  appearance  of  new  species.  Since  heredity  is 
present  in  all  organisms,  there  is  only  lacking  a  factor  correspond- 
ing to  artificial  selection,  and  this  Darwin  discovered  in  the 
so-called  '  natural  selection. ' 

Natural  Selection :  Struggle  for  Existence. — Natural  selection 
finds  its  basis  in  the  enormous  number  of  descendants  which  every 
animal  produces.  There  are  animals  (e.g.,  most  fishes)  which 
produce  many  thousands  of  young  in  the  course  of  their  lives;  not 
to  mention  parasites,  whose  eggs  are  numbered  by  millions.  For 
the  development  of  this  animal  throng  there  is  no  room  on  the 
earth;  for  even  if  we  compute  upon  the  basis  of  a  slowly-multiply- 
ing animal,  like  the  elephant,  and  assume  that  all  the  progeny  live 
and  reproduce  normally,  it  would  only  be  a  few  centuries  before 


BISTORT  OF  ZOOLOGY.  45 

the  entire  earth  would  be  occupied  by  herds  of  elephants.  In 
order  to  preserve  the  equilibrium  in  nature  great  numbers  of 
unfertilized  and  fertilized  eggs,  as  well  as  young  animals  and  many 
thafc  are  mature  but  have  not  yet  attained  their  physiological 
destiny,  must  perish.  Many  individuals  will  undoubtedly  be 
blotted  out  by  purely  accidental  causes;  yet  on  the  whole  those 
individuals  which  are  best  protected  will  best  withstand  adverse 
conditions.  Slight  superiority  in  structure  will  be  of  importance 
in  this  struggle  for  existence,  and  the  possessors  of  this  will  gain 
an  advantage  over  their  companions  of  the  same  species,  just  as  in 
domestication  each  character  which  is  or  is  fancied  to  be  useful  to 
man  insures  advantage  to  the  possessor.  Among  the  numerous 
varieties  that  appear  the  fittest  will  survive,  and  in  the  course  of 
many  generations  the  fortunate  variations  will  increase  by  sum- 
mation, while  destruction  overtakes  the  unsuitable  varieties.  Thus 
will  arise  new  forms,  which  owe  their  existence  to  '  natural  selec- 
tion in  the  struggle  for  existence/ 

The  <  Struggle  for  Existence.' — The  expression  i  struggle  for 
existence '  is  figurative,  for  only  in  rare  cases  does  an  active  con- 
scious struggle  decide  the  question  of  an  animal's  existence;  for 
example,  in  the  case  of  the  beasts  of  prey,  that  one  which  by 
means  of  his  bodily  strength  is  best  able  to  struggle  with  his  com- 
petitors for  his  prey  is  best  provided  in  times  of  limited  food- 
supply.  Much  more  common  is  the  unconscious  struggle:  each 
man  who  attains  a  more  favorable  position  by  special  intelligence 
and  energy,  limits  to  an  equal  degree  the  conditions  of  life  for 
many  of  his  fellow  men,  however  much  he  may  interest  himself  in 
humanity.  The  prey  which  by  special  craft  or  swiftness  escapes 
the  pursuer  turns  the  enemy  upon  the  less  favored  of  its  com- 
panions. It  is  noticeable  that  in  severe  epidemics  certain  men  do 
not  fall  victims  to  the  disease,  because  their  organization  better 
withstands  infection.  Here  the  term  '  survival  of  the  fittest/ 
which  Spencer  has  adopted  in  preference  to  '  struggle  for  exist- 
ence/ is  better. 

Instances  of  the  Struggle  for  Existence. — Although  the  fore- 
going general  considerations  suffice  to  show  that  the  struggle  for 
existence  plays  a  very  prominent  role  in  the  organic  world,  yet  on 
account  of  the  importance  of  this  feature  it  will  be  illustrated  by 
a  few  concrete  examples.  The  migratory  rat  (Mus  decumanus), 
which  swarmed  out  from  Asia  at  the  beginning  of  the  eighteenth 
century,  has  since  then  almost  completely  exterminated  the  house- 
rat  ( Mus  rattus)  in  Europe,  and  has  made  existence  impossible  for 


46  GENERAL  PRINCIPLES  OF  ZOO  LOOT. 

it  in  other  parts  of  the  world.  Several  European  species  of  thistle 
have  increased  so  enormously  in  the  La  Plata  states  that  they  have 
in  places  completely  crowded  out  the  native  plants.  Another 
European  plant  (Hypochceris  radicata)  has  become  a  weed,  over- 
running everything  in  New  Zealand.  Certain  races  of  men,  like 
the  Dravidian  and  Indian,  die  off  to  the  same  degree  that  other 
races  of  men,  like  the  Caucasian,  Mongolian,  and  Negro,  spread. 
The  more  one  attempts  to  explain  that  endlessly  complicated  web 
of  the  relations  of  animals  to  one  another,  the  relations  of  animals 
to  plants  and  to  climatic  conditions,  as  Darwin  has  done,  so  much 
the  more  does  he  learn  to  appreciate  the  methods  and  results  of 
the  struggle  for  existence.  He  will  become  conversant  with  many 
interesting  phenomena,  formerly  unintelligible,  which  immediately 
find  an  explanation  through  this  doctrine.  Islands  lying  in  the 
midst  of  the  ocean  have  a  disproportionately  large  number  of 
species  of  wingless  insects,  because  the  flying  forms  are  easily 
carried  out  to  sea.  For  example,  on  the  Kerguelen  Islands, 
remarkably  exposed  to  storms,  the  insects  are  wingless;  among 
them  one  species  of  butterfly,  several  flies,  and  numerous  beetles. 

Sympathetic  Coloration. — Very  often,  in  regions  which  have  a 
permanent  or  prevailing  uniform  color,  the  coat  of  the  animals  is 
distinguished  by  the  same  or  at  least  by  a  similar  hue;  this 
phenomenon  is  called  sympathetic  coloration.  Inhabitants  of 
regions  of  snow  are  white,  desert  animals  have  the  pale  yellow 
color  of  the  desert,  animals  which  live  at  the  surface  of  the  sea  are 
transparent;  representatives  of  the  most  diverse  animal  branches 
show  the  same  phenomenon.  The  advantages  connected  therewith 
scarcely  need  an  explanation.  Every  animal  may  have  occasion 
to  conceal  himself  from  his  pursuers;  or  it  may  be  his  lot  to 
approach  his  prey  by  stealth:  he  is  much  better  adapted  for  this 
the  closer  he  resembles  his  surroundings.  Natural  selection  fixes 
every  advantage  in  either  of  these  directions,  and  in  the  course  of 
many  generations  these  advantages  increase. 

Mimicry  is  referable  to  the  same  principle,  except  that  the 
imitation  is  not  here  limited  to  the  color,  but  also  influences  form 
and  marking.  Frequently  parts  of  plants  are  imitated,  sometimes 
leaves,  sometimes  stems.  Certain  butterflies  with  the  upper  sur- 
faces of  the  wings  beautifully  colored  escape  their  pursuers  by  the 
rapidity  of  their  flight;  if  they  alight  to  rest,  they  are  protected  by 
their  great  similarity  to  the  leaves  of  the  plants  around  which 
they  chiefly  fly.  When  the  wings  are  folded  over  the  back,  the 
dark  coloring  of  the  under  sides  comes  into  sight  and  the  color  on 


HISTORY  OF  ZOO  LOOT. 


4T 


the  upper  side  is  concealed.  The  parts  are  so  arranged  that  the' 
whole  takes  on  a  leaf -like  form,  and  certain  markings  heighten 
the  imitation  of  the  neuration  of  the  leaf  (fig.  11).  Among  the 
numerous  species  of  leaf-butterflies  there  are  different  grades  of 
completeness  of  mimicry;  in  many  even  the  depredations  of  insects 


B, 


FIG.  11.— Leaf-butterflies.    A,  Kallima  paralecta.  flying;  a,  at  rest.     (After  Wallace.^ 
J3,  Siderone  strigoms,  flying ;  b,  at  rest.    (After  C.  Sterne.) 

are  imitated;  in  others  the  form  and  marking  are  still  incompletely 
leaf-like,  the  marking  being  the  first  to  come  into  existence. 
Among  the  grasshoppers  also  there  are  imitations  of  leaves,  like 
the  '  walking-leaf /  Phyllium  siccifolium,  P.  scythe,  while  other 
nearly  related  forms  more  or  less  completely  approach  the  appear- 
ance of  dried,  sometimes  of  thorny  twigs  (fig.  12,  a  and  I). 

Examples  of  Mimicry. — Very  often  insects  are  copied  by  other 
animals.       Certain   butterflies,    Heliconia,    fly  in  large   swarms,, 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


FIG.  12.— Grasshopper  mimicry,    a,  Acanthoderus  wallacei  $  .    b,  Phyllium  scythe  $ 


FIG.  13  —  Methona  psidii,  a  bad-tasting  Heliconiid,  copied  by  the  Pierid,  Leptalls  arise. 

(After  Wallace.) 


HISTORY  OF  ZOOLOGY. 


4:9 


clumsy  and  yet  unmolested  by  birds,  because  they  contain  bad- 
tasting  fat  bodies.  Another  species  of  butterfly  accompanies  them 
(Pieridae),  which  does  not  taste  bad.  and  yet  are  not  eaten,  because 
in  flight,  in  cut,  and  marking  of  the  wings  they  imitate  the' 
HeliconidB  so  closely  that  even  a  systematist  might  easily  be 
confused  (fig.  13).  In  a  similar  way  bees  and  wasps,  feared  on 
account  of  their  sting,  are  imitated  by  other  insects.  In  Borneo 
there  is  a  large  black  wasp,  whose  wings  have  a  broad  white  spot 


FIG.  14.— a,  Mygnimia  aviculus,  a  wasp  imitated  by  a  beetle:  b,  Coloborhombus  fascia- 
tipennis.    (After  Wallace.)    |  nat.  size. 

near  the  tip  (Mygnimia  aviculus).  Its  imitator  is  a  heterornerous 
beetle  (Coloborhombus  fasciatipennis),  which,  contrary  to  the  habit 
of  beetles,  keeps  its  hinder  wings  extended,  showing  the  white  spot 
at  their  tips,  while  the  wing-covers  have  become  small  oval  scales 
(fig.  14). 

Sexual  Selection  is  a  special  phase  of  natural  selection,  chiefly 
observed  in  birds  and  hoofed  animals.  For  the  fulfilment  of  his 
sexual  instincts  the  male  seeks  to  drive  his  competitors  from  the 
field,  either  in  battle  or  by  impressing  the  female  by  his  special 


50  GENERAL  PRINCIPLES  OF  ZOOLOGY, 

excellences.  With  strong  wings  and  with  spurs  the  cock  main- 
tains possession  of  his  flock,  the  stag  by  means  of  his  antlers,  the 
bull  with  his  horns.  The  birds  of  paradise  by  means  of  beautiful 
coloring  win  the  favor  of  the  females,  most  singing-birds  by  means 
of  song;  many  species  of  the  fowl  by  peculiar  love-dances.  Since 


FIG.  ISA.— Paradisea  apoda,  male.    (After  Levaillant.) 

all  these  characters  belong  chiefly  to  the  male,  and  since  it  is 
only  exceptionally  that  they  are  inherited  by  the  female  (and  even 
then  are  less  pronounced),  it  is  almost  certain  that  in  a  great 
measure  they  have  been  acquired  by  the  males  through  the  struggle 
for  the  female.  In  the  case  of  birds  a  second  factor  has  un- 
doubtedly co-operated  to  impress  distinctly  the  often  enormous 
difference  between  the  feathers  of  the  male  and  of  the  female — as 


HISTORY  OF  ZOOLOGY.  51 

is  shown,  for  example,  in  the  case  of  the  birds  of  paradise 
(fig.  15) ;  for  the  nesting  female  inconspicuous  colors  and  a  close- 
lying  coat  of  feathers  are  necessary  in  order  that,  undisturbed  by 
enemies,  she  may  devote  herself  to  incubation. 

On  the  Efficiency  of  Natural  Selection. — In  the  course  of  the 
last  decade  there  has  been  much  controversy  as  to  how  far  natural 
selection  alone  is  a  species-forming  factor.  A  number  of  objectors 
dispute  the  possibility  of  fortuitous  variations  being  utilized  in  the 
struggle  for  existence.  It  is  not  easy  to  see  how  many  characters, 


FIG.  15s.— Paradisea  apoaa,  female.    (After  Levaillant.) 

especially  such  as  are  used  in  classification,  can  be  of  use  to  their 
owners.  It  can  only  be  said  that  they  have  developed  in  correla- 
tion, that  is  in  necessary  organic  connexion,  with  other  important 
characters.  But  useful  characters  must  be  considerable  in  order 
to  be  seized  upon  by  natural  selection.  Fortuitous  variations  with 
which  Darwinism  deals  are  too  inconsiderable  to  be  utilized  by  the 
organism  and  so  to  be  of  value  in  the  struggle  for  existence.  In. 
most  cases,  too,  alteration  in  one  organ  alone  is  not  enough  to  be 
of  value;  usually  a  whole  series  of  accessory  structures  must  be 
modified.  In  short,  there  must  exist  a  harmonious  co-operation 
of  parts,  which  presupposes  a  progressive  and  well-regulated 
development  extending  through  a  long  space  of  time  during  which 
the  struggle  for  existence  could  have  exerted  no  directing  influ- 
ence. Thus,  for  example,  the  wing  of  a  bird  in  order  to  be  used 
for  flight  must  have  already  reached  a  considerable  size;  the 
muscles  for  moving  it,  the  supporting  skeletal  parts,  the  nerves 
running  to  it  must  have  a  definite  formation  and  arrangement. 
Then  there  are  difficulties  in  that  most  animals  are  bilaterally  or 


52  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

radially  symmetrical,  many  in  addition  segmented.  In  all  these 
cases  the  same  organ  is  repeated  two  or  more  times.  Organs  which 
are  repeated  symmetrically  and  usually  those  which  are  segmental 
agree  in  general  in  structure.  One  must  therefore  admit  that  the 
alterations  of  chance  must  have  occurred  at  at  least  two  points 
simultaneously  and  in  exactly  the  same  way. 

A  further  objection  is  that  the  action  of  natural  selection  would 
under  ordinary  conditions  be  negatived  by  unhindered  crossing  of 
the  varying  forms.  If,  for  example,  we  do  not  isolate  fantails  from 
other  pigeons,  they  will  cross  with  these,  and  their  descendants 
will  soon  resume  the  character  of  common  pigeons.  Finally,  it 
has  been  claimed  that  for  the  formation  of  new  species  a  simple 
variation  of  forms  is  not  sufficient;  it  must  reach  still  farther:  (1) 
a  variation  in  different  directions',  a  divergent  development  of  the 
individual  members  of  a  species;  (2)  the  disappearance  of  the 
transitional  forms  which  unite  the  divergent  forms. 

The  objection  that  the  struggle  for  existence  cannot  bring 
about  the  divergent  development  of  individuals  necessary  for 
improvement  is  of  least  importance.  It  need  only  be  added  that 
of  the  many  variations  appearing  at  the  same  time  in  a  species  two 
or  more  may  be  equally  useful;  that  then  one  set  of  individuals 
will  seize  upon  one,  another  set  upon  the  other  advantage,  and  that 
in  consequence  of  this  both  sets  will  develop  in  different  directions. 
Consequently  the  intermediate  forms  which  are  not  pronounced  in 
the  one  or  the  other  direction  will  be  in  an  unfavorable  position, 
and  must  carry  on  the  struggle  for  existence  with  both  groups  of 
partially  differentiated  companions  of  their  species,  and,  being  less 
completely  adapted,  must  fall. 

More  important  are  the  first  two  objections;  they  have  led  to 
theories  which  originally  seemed  destined  to  complete  the  Dar- 
winian theory,  but  in  the  course  of  discussion  they  have  more  and 
more  raised  the  claim  of  entirely  supplanting  it.  In  the  following 
paragraphs  will  be  found  an  outline  of  these  theories,  but  it  is  to 
be  taken  into  consideration  that,  at  the  present  time,  we  are  still 
in  the  midst  of  the  reform  movement,  and  it  cannot  yet  be  said 
whether  they  will  be  able  to  stand  beside  the  theory  of  the  struggle 
for  existence  or  will  supplant  it, 

Migration  Theory. — To  explain  how  characters  newly  formed 
by  variation  become  fixed,  and  do  not  disappear  again  through 
crossing  with  differently  modified  individuals,  M.  Wagner  has  pro- 
posed the  Theory  of  Geographical  Isolation,  or  the  Migration 
Theory.  New  species  may  arise  if  a  part  of  the  individuals  of  one 


HISTORY   OF  ZOOLOGY.  53 

species  should  take  to  wandering,  or  should  be  transplanted,  and 
thus  come  to  a  new  place,  in  which  crossing  with  the  companions 
of  their  species  who  were  left  behind  is  not  possible.  The  same 
might  occur,  if  the  region  inhabited  by  a  species  should  by 
geological  changes  be  divided  into  two  parts,  between  which  inter- 
change of  forms  would  be  no  longer  possible.  The  animals 
remaining  under  the  old  conditions  would  retain  the  original 
characteristics;  the  wanderers,  on  the  other  hand,  would  change 
into  a  new  species.  Direct  observations  support  this  theory.  A 
litter  of  rabbits  placed  at  the  beginning  of  the  fifteenth  century 
on  the  island  of  Porto  Santo  has  in  the  mean  time  increased 
enormously  and  the  descendants  have  taken  on  the  characteristics 
of  a  new  species.  The  animals  have  become  smaller  and  fiercer, 
have  acquired  a  uniformly  reddish  color,  and  no  longer  pair  with 
the  European  rabbit.  A  further  proof  in  favor  of  the  theory  of 
geographical  isolation  is  the  peculiar  faunal  character  of  regions 
separated  from  adjacent  lands  by  impassable  barriers,  broad  rivers 
or  straits,  or  high  mountains  (comp.  p.  42) ;  especially  instructive 
in  this  regard  is  the  peculiar  faunal  character  of  almost  every 
island.  The  fauna  of  an  island  resembles  in  general  the  fauna  of 
the  mainland  from  which  che  island  has  become  separated  by 
geological  changes;  it  usually  has  not  only  these  but  also  so-called 
'  vicarious  species/  i.e.,  species  which  in  certain  characteristics 
closely  resemble  the  species  of  the  mainland.  Such  vicarious 
species  have  plainly  arisen  from  the  fact  that  isolated  groups  of 
individuals,  scattered  over  the  islands,  have  taken  on  a  develop- 
ment divergent  from  the  form  from  which  they  started.  With  all 
due  recognition  of  the  migration  theory,  it  will  never  be  possible 
by  it  alone  to  explain  the  multiformity  of  the  organic  world.  In 
addition,  it  must  be  assumed  that  formerly  the  earth's  surface 
possessed  an  enormous  capacity  for  change;  but  the  more  recent 
investigations  make  it  probable  that  the  distribution  of  land  and 
water  has  not  varied  to  the  degree  that  was  formerly  believed. 
The  experience  of  botanists,  too,  teaches  that  several  varieties  can 
arise  in  the  same  locality  and  become  constant. 

Lamarckism. — While  the  migration  theory  agrees  with  Dar- 
winism in  this,  that  the  new  characters  appearing  through  varia- 
tion are  to  be  regarded  as  the  products  of  chance,  yet  it  is  just 
this  part  of  the  theory  which  has  been  subjected  to  searching 
criticism.  Many  zoologists  have  again  adopted  the  causal  founda- 
tion of  the  descent  theory  proposed  by  Lamarck  and  believe  that 
the  cause  of  species  formation  is  to  be  found  in  part  in  the 


54:  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

immediate  influence  of  changing  environment,  in  part  in  the 
varying  use  and  disuse  of  organs,  brought  about  by  alterations  in 
the  conditions  of  life.  Both  principles,  they  say,  are  sufficient, 
even  without  the  help  of  the  struggle  for  existence,  to  explain  the 
phylogenesis  of  organisms. 

Influence  of  Environment. — To  what  extent  can  the  environ- 
ment bring  about  a  permanent  change  in  the  structure  of  plants 
and  animals  ?  To  decide  this  is  no  simple  problem,  on  account  of 
the  complexity  of  the  factors  entering  into  the  question. 

In  cases  where  the  food-supply  is  altered,  organisms  change  in 
a  very  remarkable  manner  and  within  a  short  time;  but  these 
changes  (Niigeli's  '  Modifications  through  Nutrition ')  seem  to  have 
no  permanence.  Plants  which,  found  in  nature  in  poor  soil,  are 
transplanted  into  rich  soil,  or  vice  versa,  soon  acquire  quite  a 
different  appearance,  and  preserve  this  through  the  following 
generations,  so  long  as  they  remain  in  the  rich  soil;  but  the  plant 
quickly  returns  to  its  former  appearance  when  replaced  in  its 
previous  surroundings. 

In  general,  a  change  seems  to  be  the  more  permanent  the  more 
slowly  it  has  developed.  In  researches  upon  the  influence  of 
environment,  we  can,  therefore,  rely  soonest  upon  results  if  we 
experiment  with  slowly-working  factors,  such  as  light  and  heat, 
dry  or  moist  air,  different  intensities  of  gravitation,  of  stimuli, 
etc.,  which  can  be  excluded  from  the  environment  of  the  organism. 

Use  and  Disuse. — Regarding  the  efficiency  of  use  and  disuse, 
there  is  no  doubt  that  the  shape  of  an  animal  is  influenced  to  a 
great  extent  by  the  manner  in  which  the  organs  are  used.  The 
•organs  which  are  much  used  will  become  especially  strong  and  vice 
versa  those  which  are  not  used  will  become  weak.  The  only  ques- 
tion is  whether  these,  in  the  strict  sense  of  the  word,  newly-acquired 
•characteristics  are  transmitted  to  the  offspring,  or  whether  the 
descendants,  in  order  to  attain  to  the  same  stage,  must  not  repeat 
in  the  same  way  use  and  disuse.  In  the  latter  case  the  cumulation 
•of  characteristics,  and  with  it  the  possibility  that  these  may 
become  permanent,  is  excluded.  It  is  to  be  regretted  that  accurate 
results  are  still  lacking  on  a  point  so  well  adapted  for  experimental 
treatment.  At  this  time  rudimentary  organs  strongly  favor  the 
Lamarckian  principle;  for  we  see  that  cave  animals,  which  for 
many  generations  have  lived  in  darkness,  are  blind,  either  having 
no  eyes,  or  only  vestiges  of  them,  incapable  of  function.  This 
seems  to  justify  the  view  that  this  condition  is  attributable  to  lack 
of  use,  since  it  has  brought  about  a  functional  and  anatomical 


HISTORY  OF  ZOOLOGY.  55 

incapacity,  which  has  increased  from  generation  to  generation. 
Now  we  must  believe  that  what  is  true  for  disuse  must  express 
itself  in  the  reverse  sense  in  the  case  of  use. 

Nageli's  Principle  of  Progression. — In  conclusion,  there  is  still 
to  be  considered  the  change  of  species  from  internal  .causes,  to 
which  von  Baer  gave  the  poorly  adapted  because  easily  misleading 
term  "  Zielstrebigkeit "  (the  striving  toward  an  ideal),  and  which 
Nageli  has  termed  the  '  perfecting  principle/  or  the  '  principle  of 
progression/  It  cannot,  indeed,  be  denied  that  each  species  is 
compelled,  by  some  peculiar  internal  cause,  to  develop  into  new 
forms,  independently  of  the  environment,  and  up  to  a  certain 
degree,  independently  of  the  struggle  for  existence.  In  all  animal 
branches  we  see  the  progress  from  lower  to  higher  going  on,  very 
often  in  a  quite  similar  way,  in  spite  of  the  fact  that  the  animals 
live  under  very  different  conditions  of  development.  We  see  how 
the  nervous  system  lying  near  the  surface  in  the  lower  animals 
becomes  in  the  higher  animals  concealed  in  the  depths  of  the  body; 
how  the  eye,  at  first  a  simple  pigment-spot,  becomes  in  worms, 
arthropods,  molluscs,  and  vertebrates,  provided  with  accessory 
apparatus,  as  lens,  vitreous  body,  iris,  choroid,  etc.  Here  we  see 
an  energy  for  perfection  which,  since  it  occurs  everywhere,  must 
be  independent  of  the  individual  conditions  of  life,  and  must  have 
its  special  explanation  in  the  character  of  the  living  substance. 

It  is  by  no  means  justifiable  to  call  an  assumption,  as  here 
expressed,  teleological,  and  to  reject  it  as  unscientific;  rather  the 
organism  seems  to  be  just  as  mechanically  conditioned  as  a  billiard- 
ball,  whose  course  is  determined  not  only  by  contact  with  the 
cushions  of  the  billiard- table,  but  also  in  a  large  measure  by  its 
indwelling  force,  imparted  to  it  by  the  stroke  of  the  cue.  An 
organism,  too,  is  a  store  of  energy  which  must  necessarily  from 
itself  develop  more,  but  it  is  of  more  extraordinary  complexity, 
and  to  an  equal  degree  also  is  independent  of  the  external  world. 
A  complete  independence  is  naturally  never  present,  and  Nageli 
has  not  so  maintained.  Along  with  it  rather  goes  always  an 
'action'  of  the  external  world,  a  modifying  influence  which  is 
carried  on  by  the  external  conditions  of  existence,  either  directly 
or  by  the  mediation  of  use  and  disuse. 

This  exposition  of  evolution  has  been  given  in  a  rather  detailed 
way,  because  in  the  history  of  zoology  it  is  undoubtedly  the  most 
important  feature.  No  other  theory  in  the  course  of  the  develop- 
ment of  zoological  investigation  has  gained  such  a  hold,  none  has 
propounded  so  many  new  problems  and  opened  so  many  new  fields 


56  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

for  research.  There  is  no  other  zoological  theory  which  compares 
with  it  in  value  as  a  working  hypothesis.  To  the  many  objections 
which  have  been  made  that  the  theory  is  insufficiently  grounded, 
it  can  only  be  replied  that  in  the  present  state  of  our  knowledge  it 
is  the  only  theory  which  agrees  with  our  experiences  and  explains 
these  in  a  simple  way  and  on  a  scientific  basis.  In  this  sentence 
is  given  the  merit  of  the  theory,  but  at  the  same  time  also  a  limita- 
tion of  its  applicability.  For  on  the  one  side  the  statement  attrib- 
utes the  merit  in  the  applicability  of  the  system  to  the  necessity 
of  the  human  mind  for  simple  explanations  of  the  facts  of  natural 
science,  and  on  the  other  hand  it  makes  the  degree  of  correctness 
dependent  upon  the  state,  whatever  it  may  be,  of  our  knowledge. 
On  both  sides  no  constant  quantities  are  involved.  Many  investi- 
gators see  no  necessity  of  reconciling  paleontology  and  our  knowl- 
edge of  plants  and  animals.  To  such,  therefore,  the  Darwinian 
theory  proves  just  as  little  as  any  opposing  theory.  Meanwhile 
thoughtful  naturalists  will  keep  in  mind  that  our  knowledge  of 
nature  is  making  considerable  advances,  and  is  visibly  becoming 
wider  and  deeper.  It  is  possible,  even  probable,  that  these 
advances  will  lead  to  many  modifications  of  the  theory.  For 
instance,  the  theory  of  the  causes  which  condition  the  formation 
of  new  species  will  undergo  numerous  changes.  On  the  other 
hand,  we  can  affirm  with  great  certainty  that  the  principle  of 
descent,  which  first  obtained  credence  through  Darwinism,  will  be 
a  permanent  landmark  of  zoological  investigation. 


GENERAL  MOEPHOLOGY  AND  PHYSIOLOGY 

General  Zoology :  Animal  Morphology, — In  the  vital  phenom- 
ena of  animals  a  certain  degree  of  similarity  can  be  followed 
through  the  entire  animal  kingdom;  the  way  in  which  animals  are 
nourished  and  reproduce  their  kind,  how  they  move,  and  how  they 
gain  experience,  is  essentially  the  same  in  great  groups,  and  even 
widely  separated  forms  show  many  agreements.  Corresponding 
to  this,  the  apparatus  which  is  concerned  with  the  above-men- 
tioned functions,  the  organs  of  nutrition  and  reproduction,  of 
motion  and  sensation  in  their  grosser  and  finer  structure,  and  in 
their  ontogeny,  must  be  similar  to  one  another  and  show  evidence 
of  some  fundamental  characters  which  always  or  frequently  recur. 
All  this  needs  a  general  explanation  before  we'  can  go  into  a 
description  of  the  separate  branches  of  animals.  This  explanation 
is  the  subject  of  general  zoology,  specially  of  general  anatomy  and 
embryology,  or  animal  morphology. 

(Ecology  or  Biology. — If  by  means  of  anatomy  and  embryology 
we  have  learned  the  general  character  of  the  animal  organism,  we 
must  yet  farther  study  its  relations  to  the  environment.  For  this- 
study  of  the  conditions  of  animal  life,  cacology  or  biology,  we  have 
to  consider  the  geographical  range  of  animals,  their  distribution 
over  the  surface  of  the  earth  and  in  the  different  depths  of  the 
sea;  further,  the  reciprocal  relations  of  animals  and  plants,  and 
of  beast  to  beast,  as  these  find  special  expression  in  colony-build- 
ing, symbiosis,  parasitism,  etc. 

General  Anatomy. — In  the  case  of  General  Anatomy,  with 
which  we  shall  begin,  the  fundamental  proposition  will  be,  How  is 
an  organism  formed  from  its  constituent  parts?  We  shall  thus  in 
spirit  follow  the  opposite  course  from  that  which  anatomy  actually 
takes,  for  this  resolves  the  animal  body  into  its  elementary  parts, 
its  organs,  tissues,  and  cells.  Instead  of  analytical  we  will  pursue 
synthetic  anatomy. 

The  synthesis  of  an  organism,  of  which  by  general  anatomy  we 
can  only  gain  an  idea,  actually  takes  place  in  nature  during  the 

57 


58  GENERAL  PRINCIPLES  OF  ZOO  LOO  Y. 

development  of  every  animal.  Embryologically  every  organism 
is  at  some  time  a  simple  element,  a  cell ;  this  divides  and  gives  rise 
to  tissues;  from  the  tissues  are  formed  organs,  and  from  the 
organs  the  regularly  membered  whole  of  the  animal  body  is  com- 
bined. If  the  general  ontogeny  proceeds  synthetically,  it  then 
agrees  in  its  manifestations  with  the  processes  which  go  on  in 
nature  and  which  are  accessible  to  direct  observation. 

GENERAL  ANATOMY. 

The  Morphological  Units. — The  expression  <  constituent  parts 
of  the  animal  body '  can  be  used  in  a  double  sense.  We  can  speak 
of  the  chemical  units,  the  chemical  combinations,  which  form  the 
tissues;  these  are  the  subject  of  animal  chemistry,  and  may  there- 
fore be  passed  over  here.  But  we  may  also  speak  of  the  constituent 
units  (morphological  units)  of  the  animal  body ;  these  are  the  cells. 
These  and  their  transformation  into  tissues,  organs,  and  entire 
animals  are  for  us  of  vastly  greater  importance. 

I.  THE  MORPHOLOGICAL  UNITS  OF  THE  ANIMAL  BODY. 

The  Cell. —  The  study  of  the  morphological  units  of  the  organic 
body  first  found  a  firm  foundation  in  the  cell  theory.  Every 
scientific  study  of  the  anatomy  of  plants  and  animals  must  there- 
fore take  the  cell  as  its  starting-point. 

History  of  the  Cell  Theory. — The  conception  of  the  cell  of  animals  and 
plants  has  in  the  course  of  time  undergone  many  changes,  which  must 
be  known  to  some  extent  in  order  to  understand  completely  the  name  and 
the  conception.  When,  in  the  seventeenth  century,  Hooker,  Marcello 
Malpighi,  and  Nehemia  Grew  introduced  the  term  into  vegetable  anatomy, 
they  meant  small  chambers  surrounded  by  firm  walls  and  filled  with  air 
or  fluid  contents.  When,  also,  early  in  the  nineteenth  century,  it  was  cor- 
rectly recognized  that  the  cell  is  the  anatomical  and  physiological  vegetable 
unit  from  which  all  the  other  parts  of  the  plant  are  formed,  and  when  the 
English  botanist  Brown  discovered  in  the  interior  of  the  cell  that  small 
body  previously  overlooked,  the  kernel  or  nucleus,  the  old  conception 
remained,  and  as  such  was  accepted  by  Schleiden  in  his  cell  theory. 
Schleiden  added  as  new  a  completely  erroneous  view  of  the  origin  of  cells: 
that  in  a  sort  of  matrix  (the  *  cyroblast ')  first  a  granule,  the  nuclear 
body,  was  formed,  then  around  this  granule  a  membrane,  the  nuclear  mem- 
brane, arose  by  precipitation,  and  around  the  thus  completed  nucleus  a 
larger  membrane  (the  cell  membrane)  was  deposited.  Hence  for  the 
formation  of  the  cell  the  nucleus  would  be  of  most  importance. 

The  Schleiden-Schwann  Cell  Theory. — Since  it  is  the  nuclei  which  are 
most  easily  seen  in  the  animal  body,  and  even  now  are  particularly  useful 


GENERAL  ANA10M7.  59 

in  deciding  questions  concerning  the  presence  of  cells,  it  is  readily  under- 
stood how  Schleiden's  theory,  which  placed  the  nucleus  so  much  in  the 
foreground,  should  have  led  Schwann  to  apply  the  cell  theory  to  the 
animal  kingdom,  and  thus  raise  it  to  a  principle  of  general  application. 
We  usually,  therefore,  speak  of  the  Schleiden-Schwann  cell  theory. 

As  a  result  of  this  theory  the  walls,  the  cell  membrane,  were  regarded 
as  most  important  for  the  function  of  the  cell ;  through  the  cell  mem- 
brane diffusion-currents  must  pass  between  the  surrounding  medium 
and  the  contents  of  the  cell ;  the  character  of  the  membrane  and  of  the 
cell-sap  must  determine  the  condition  of  the  diffusion-currents,  and 
hence  the  functional  character  of  the  cell ;  the  different  appearance  of 
tissues  depends  chiefly  upon  the  fact  that  the  cells,  spherical  in  the 
beginning,  change  their  form  ;  in  the  case  of  fibrillar  connective  tis- 
sue, for  example,  they  increase  enormously  in  length  and  become  fine 
fibrillaB.  Since  the  life  of  an  organism  is  nothing  else  than  the  co-operative 
work  of  all  its  cells,  they  flattered  themselves  that  through  the  cell  theory 
.and  the  discovery,  brought  about  by  it,  of  the  physical  unity  of  the  animal 
-and  vegetable  body  they  had  made  an  important  advance  in  the  great 
problem  of  the  physical  explanation  of  the  phenomena  of  life.  Cell  gene- 
sis also  seemed,  according  to  the  theory,  to  be  just  as  satisfactorily 
explained  on  a  mechanical  basis  as  the  formation  of  a  crystal.  In  the 
'  cytoblast '  the  nuclear  bodies,  nuclear  membrane,  and  cell  membrane 
must  be  formed  by  deposition  just  as  in  the  process  of  crystallization. 

Reform  Movements. — Since  that  time  our  conception  of  the  nature  of 
•cells  has  completely  changed.  The  cell  does  not,  after  the  manner  of  a 
crystal,  arise  as  a  new  formation  in  a  matrix,  but  it  presupposes  the 
existence  of  a  living  mother-cell,  from  which  it  arises  by  division  or  bud- 
ding. Just  so  also  the  cell  is  not  a  physical  unit,  but  is  itself  an  organism 
which  shows  to  us  all  the  enigmas  of  life,  the  physical  basis  of  which  our 
investigations  must  ever  keep  in  view  as  a  goal,  though  it  be  still  indiscern- 
ibly  distant.  The  membrane  and  cell-sap  are  of  quite  subordinate  impor- 
tance for  the  existence  of  the  cell;  rather  the  most  important  thing  in  it  is 
the  previously  disregarded  substance,  for  which  von  Mohl  introduced  the 
name  protoplasm.  According  to  the  newer  conception  the  cell  is  practically 
a  small  mass  of  protoplasm,  usually,  probably  always,  provided  with  one 
or  more  nuclei.  This  newer  conception  of  the  cell  has  developed  so  gradu- 
ally, and  has  so  slowly  supplanted  the  Schleiden-Schwann  view,  that  the 
old  name  has  been  retained,  although  it  no  longer  at  all  fits  the  new  con- 
ception. We  have  indeed  become  so  thoroughly  accustomed  to  the  name 
that  we  no  longer  notice  the  contradiction  of  terms  when  we  call  a  solid 
lump  without  a  membrane  a  '  cell.' 

Discovery  of  Protoplasm. — The  reformation  of  the  cell  theory  was  begun 
by  discoveries  which  were  made  in  very  different  regions  and  only  lately 
have  been  brought  to  a  focus. 

1.  At  about  the  beginning  of  the  nineteenth  century,  Bonaventura 
Oorti  and  Treviranus  had  seen  that  the  chlorophyl  bodies,  which  cause  the 
green  color  of  plants,  in  many  species  stream  around  in  a  lively  manner  in 
the  interior  of  the  cell,  but  Mohl  was  the  first  to  find  out  that  this  motion 


60  GENERAL   PRINCIPLES  OF  ZOOLOGY. 

was  not  active,  but  rather  that  they  are  moved  by  a  homogeneous  sub- 
stance in  which  they  are  embedded.  This  substance,  which  Mohl,  in 
order  to  bring  it  into  prominence,  named  protoplasma.  became  by  other 
studies  still  more  important.  In  the  reproduction  of  the  simplest  algae,  it 
was  found  that  the  protoplasm,  together  with  the  chlorophyl  bodies,  col- 
lected itself  into  an  oval  mass,  and  that  this  body  left  the  cell  membrane 
and  swam  freely  in  the  water.  Since  the  cell-wall  no  longer  showed  signs 
of  life,  while  on  the  other  hand  the  protoplasmic  body  came  to  rest  and 
formed  a  new  plant,  it  was  shown  beyond  doubt  that  this  was  the  most 
important  constituent  part  of  the  cell  (comp.  fig.  115). 

2.  In  the  study  of  animal  tissues  the  importance  of  the  peculiar  cell- 
substance,  the  protoplasm,  was  still  more  plainly  brought  out.     Here,  in 
spite  of  the  long-prevailing  preconceived  idea,  unbiassed  observation  led  to 
the  discovery  that  most  animal  cells  had  no  cell-membrane. 

3.  Very  important,  finally,  was  the  study  of  the  lowest  organisms,  the 
Protozoa.      Bujardin  sought  by  extremely  careful  observations  to  prove 
that  these  animals  had  no  organs,  but  consisted  of  a  uniform  granular  sub- 
stance, the  sarcode.    The  sarcode  alone  could  produce  all  the  vital  phe- 
nomena, such  as  movement,  sensation,  assimilation,  previously  ascribed  to 
many  organs.     Dujardin's  theory  was  stoutly  contested  by  Ehrenberg  and 
his  school,   but  finally  attained  general    acceptance  through  the  epoch- 
making  work  of  Max  Schultze  and  Haeckel. 

Schultze's  Protoplasm  Theory. — On  the  basis  of  these  three  series  of 
observations,  Max  Schultze  finally  established  the  reformation  of  the  cell 
theory  briefly  sketched  above,  when  by  accurate  study  of  the  appearance 
and  the  vital  phenomena,  and  by  means  of  numerous  experiments,  he 
proved  that  the  cell-substance  of  animals,  the  sarcode  of  Protozoa,  and 
the  protoplasm  of  plants  are  identical,  and  that  to  this  substance,  for 
which  he  retained  the  name  protoplasm,  all  the  vital  phenomena  of  animals 
and  plants  are  referable  in  the  ultimate  analysis.  The  second  important 
modification  concerns  the  changes  of  cells  into  tissues.  These  follow  not 
so  much  through  changes  of  form  and  modification  of  the  cells  into  the 
tissue  elements,  as  Schwann  thought,  but  rather  by  means  of  chemical 
changes.  By  means  of  its  formative  potentiality  the  protoplasm  gives  rise 
to  non-protoplasmic  structural  parts,  as,  for  example,  connective-tissue 
fibrils,  muscle  fibrils,  nerve  fibres,  etc.  These  give  the  various  tissues  their 
specific  character  and  perform  their  functions.  The  tissues  also  retain  as 
the  source  of  life  and  formation  the  unemployed  remnants  of  cells,  the 
connective-tissue  corpuscles,  muscle  corpuscles,  etc.  We  will  now  trace 
out  farther  these  two  fundamental  ideas  in  Max  Schultze's  '  protoplasm 
theory,'  and  thereby  briefly  sketch  the  elements  of  the  modern  theory  of 
tissues. 

Nature  of  the  Cell. — The  size  of  the  animal  cell  varies  to  a 
considerable  degree;  the  smallest  elements  are  the  male  sexual 
cells,  the  spermatozoa,  whose  bodies,  particularly  in  case  of  the 
mammals,  are  even  less  than  0. 003  mm. ;  the  largest,  on  the  other 
hand,  with  the  exception  of  the  giant  plasmodia  of  some 


GENERAL  ANATOMY.  61 

Mycetozoa,  are  the  egg  cells.  The  yolk  of  the  bird's  egg,  which 
alone  forms  the  egg  in  the  narrower  sense,  apart  from  its  coverings, 
has  for  a  time  the  morphological  value  of  a  cell,  and  in  the  case  of 
the  ostrich  egg  may  reach  a  diameter  of  several  inches.  The  form 
of  the  cell  is  likewise  variable.  Free  cells,  whose  form  is  not 
determined  by  the  environment,  are  usually  spherical  or  oval  in 
the  resting  condition,  as  the  egg  cell  shows;  united  into  tissues, 
the  cells,  on  the  contrary,  may  be  pressed  together  into  polygonal 
or  prismatic  bodies,  or  may  send  out  spindle-  or  star-shaped 
branching  processes. 

Protoplasm. — So  there  is  left  to  characterize  the  cell  only  the 
constitution  of  its  substance:  the  cell  is  a  mass  of  protoplasm  with 
one  or  more  nuclei.  It  is  not  known  whether  protoplasm  is  a 
definite  chemical  body,  which  from  its  constitution  is  capable  of 
infinite  variation,  or  whether  it  is  a  varying  mixture  of  different 
chemical  substances.  So,  also,  we  are  by  no  means  certain  whether 
or  not  these  substances  (as  one  is  inclined  to  believe)  belong  to 
those  other  enigmatical  substances,  the  proteids.  We  can  only 
say  that  the  constitution  01  protoplasm  must,  with  a  certain  degree 
of  homogeneity,  have  a  very  extraordinary  diversity.  For  if  we 
see  that  from  the  egg  of  a  dog  there  comes  always  and  only  a  dog, 
and  indeed  an  animal  with  all  his  individual  peculiarities,  that  a 
sea-urchin's  egg,  placed  under  the  most  diverse  conditions,  pro- 
duces always  a  sea-urchin,  that  a  species  of  amoeba  always  performs 
only  the  movements  characteristic  of  that  species,  we  must  assume 
that  the  functioning  constituent  part  of  this  cell,  the  protoplasm, 
has  in  each  case  its  peculiarities.  We  are  driven  to  the  assumption 
of  an  almost  unlimited  diversity  of  protoplasm,  even  if  we  concede 
an  important  share  in  the  prominent  differences  to  the  nucleus,  of 
which  we  shall  speak  later. 

General  Properties  of  Protoplasm. — The  similarity  of  proto- 
plasm, still  recognizable  through  all  its  variations,  expresses  itself 
in  its  appearance  and  in  its  vital  phenomena  Under  slight  mag- 
nification, protoplasm  appears  as  a  faintly-gray  substance,  some- 
times colored  yellowish,  reddish,  etc.,  by  pigments  taken  up  by 
imbibition,  in  which  numerous  strongly-refracting  granules  are 
embedded.  The  vital  characteristics  of  this  substance  are  move- 
ment, irritability,  power  of  assimilation  and  of  reproduction. 

By  using  higher  powers  a  finer  structure  can  be  seen  in  the 
protoplasmic  substance,  the  '  homogeneous  protoplasm  '  of  earlier 
writers.  The  nature  of  this  is  as  yet  in  question :  a  fine-meshed 
framework  (filar  substance,  spongioplasm,  cell  reticulum)  the 


62 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


interstices  of  which  are  filled  with  other  material  (interfilar  sub- 
stance, enchylema,  ground  substance).  The  dispute  lies  especially 
around  the  question  whether  this  framework  is  formed  of  threads 
and  trabeculae  or  whether  the  appearance  is  not  formed  by  small 
chambers,  bounded  by  fine  partition-walls  (foam  structure  of 
protoplasm). 

Movement  of  Protoplasm.  —  Movement  expresses  itself  first  in 
changes  of  form  of  the  whole  body  —  amoeboid  movement  —  and 

secondly  in  the  change  of  position 
of  the  small  granules  in  the  interior 
of  the  protoplasm  —  streaming  of 
granules.  Examples  of  amoeboid 
movement  (fig.  16)  are  chiefly  the 
movements  of  many  Protozoa,  and 
of  the  colorless  blood-cells  (leuco- 
cytes) of  multicellular  animals; 
here  the  protoplasmic  body  sends 
out  coarser  and  finer  processes, 
which  may  be  again  withdrawn, 
serving  for  locomotion  and  hence 
called  pseudopodia  or  false  feet. 
The  streaming  of  granules  can  be 
observed  in  the  interior  of  the  cell- 
body,  as  well  as  in  the  pseudopodia 
(After  extending  from  this.  The  pseudo- 

^fan  Tyiav  pvpri    v,p  qft  flT1p   a.  |n  up 
podia  may  evei 

at  the  limits  of  visibility  with  our 
strongest  magnifications  (fig.  17),  yet  in  them  it  can  still  be 
observed  that  the  granules  wander  hither  and  thither  like  people 
on  a  promenade,  simultaneously  centripetally  and  centrifugally, 
some  with  greater,  others  with  less  speed.  And  yet  the  granules 
are  only  passively  moved  by  the  protoplasm,  for  if  we  feed  the 
creature  with  some  pigment  granules,  like  finely-pulverized  car- 
mine, these  granules  show  the  same  remarkable  streaming.  Indeed 
nothing  better  illustrates  the  great  complexity  in  the  structure 
of  protoplasm  than  these  extremely  complicated  phenomena  of 
motion  in  such  narrow  limits  as  pseudopodia  in  general. 

Irritability  of  Protoplasm.  —  That  amoeboid  movements  and 
streaming  of  granules  can  be  induced,  brought  to  a  standstill,  and 
modified  by  mechanical,  chemical,  and  thermal  stimuli,  is  a  sure 
proof  of  the  irritability  of  protoplasm.  Most  important  are  the 
thermal  stimuli;  if  the  surrounding  medium  rise  above  the 


FIG.  16.  —  Amoeba   proteus. 

Leidy.)     efc,  ectosarc;  en,  entosarc  ; 
cv,  contractile  vacuole  ;  w,  nucleus  ; 

^  fo 


QENEHAL  AXATOMY. 


63 


n  /ft\    I  'I  II 

/  x  /  i   i    /M  A      1 


FIG.  17.— Gromfo  oviformis.    (From  Lang,  after  M.  Schultze.) 


64  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

ordinary  temperature,  the  movements  at  first  become  more  rapid 
up  to  a  maximum:  from  that  point  begins  a  slowing,  finally  coming 
to  a  standstill. — Heat-rigor.  If  the  high  temperature  continue 
much  longer,  or  if  it  rise  still  higher,  death  results.  The  fatal 
temperature  is  found  for  most  animals  between  40°  and  50°  0. 
(104°-122°  ¥.);  its  influence  explains  a  part  of  the  injurious 
-effects  which  high-fever  temperatures  have  upon  the  human 
organism.  Like  the  heat-rigor,  there  is  also  a  cold-rigor,  induced 
by  a  sharp  sinking  of  the  temperature  below  the  normal.  This 
is  accompanied  by  a  gradual  diminution  of  mobility;  it  results 
in  death  by  freezing,  which  is,  however,  not  so  easily  produced  as 
•death  by  heat.  It  is  a  remarkable  fact  that  many  animals,  conse- 
quently their  cells,  may  be  frozen;  and  in  this  condition  can 
•endure  still  severer  cold  without  dying.  (For  example :  goldfish, 
a  temperature  of  -  8°  to  -15°  C. ;  frogs,  to  -  28°;  newts,  to 
-  25°). 

Nutrition  and  Reproduction. — Irritability  and  power  of  motion 
are  the  prerequisites  of  assimilation,  the  change  of  food-substance 
into  protoplasm.  Most  animal  cells,  for  example  almost  all  tissues 
cells,  are  not  suitable  for  studying  assimilation,  because  they  live 
upon  liquid  nourishment.  But  certain  cells  of  higher  animals, 
the  colorless  blood-cells,  and  most  unicellular  animals  can  be  fed 
also  with  solid  substances;  they  take  the  food-particles  into  the 
midst  of  the  protoplasdi  by  flowing  around  them  with  the  pseudo- 
podia.  They  extract  all  the  assimilable  and  reject  the  indigestible 
portions  (fig.,  16). 

In  the  case  of  assimilation  it  is  to  be  noted  not  only  that  the 
cells  use  the  food  which  they  have  taken  for  their  own  growth  and 
for  replacing  worn-out  parts,  but  also  that  most  of  them  have  the 
power  of  producing  substances  other  than  protoplasm;  for 
example,  many  Protozoa  form  organic  shells  or  skeletons  which 
are  hardened  with  silica  or  lime.  This  formative  power,  the 
building  of  ' plasmic  products,'  is,  as  we  shall  shortly  see, 'the 
starting-point  for  tissue-formation. 

Cell  Nucleus. — The  reproduction  of  protoplasmic  bodies  is 
synonymous  with  the  division  of  the  cell:  but  to  understand  this 
we  must  first  consider  the  second  important  constituent,  the 
nucleus.  This  is  a  body  enclosed  in  the  protoplasm,  whose  form, 
though  definite  for  each  kind  of  cell,  shows  in  general  wide  varia- 
tions. Usually  it  is  a  spherical  or  oval  vesicle;  but  it  may  be 
elongated  or  club-shaped,  bent  into  a  horseshoe,  with  constrictions 
like  a  rosary,  or  even  be  branched,  treelike  (fig.  18);  in  many  cells 


GENERAL   ANATOMY.  65 

it  is  disproportionally  large,  so  that  the  protoplasm  surrounds  it 
only  with  a  thin  layer,  in  others  again  it  is  so  small  that  it  can 
scarcely  be  found  in  the  protoplasm  among  the  other  substances. 
Formerly,  on  this  account,  it  was  in  very  many  cases  overlooked, 
and  even  now  it  can  often  be  demonstrated  only  by  great  care, 


FIG.  18. — Various  forms  of  nuclei,  a,  horseshoe-shaped  nucleus  of  an  Acinete ;  ibi 
branching  nucleus  from  the  Malpighian  vessel  of  a  Sphingid  larva ;  c,  rosary- 
shaped  nucleus  of  Stentor  cceruleus. 

and  by  employment  of  a  special  technique  based  upon  the  micro- 
chemical  reaction  of  the  nuclear  substance. 

The  Nuclear  Substance. — The  nuclear  substance  is  distin- 
guished from  protoplasm,  among  other  ways,  by  its  greater 
coagulability  in  certain  acids,  e.g.,  acetic  and  chromic,  which 
therefore  are  often  used  for  demonstrating  the  nucleus.  If  in  a 
living  cell  the  nucleus  be  invisible  on  account  of  the  similarity  of 
its  refraction  to  that  of  the  protoplasm,  the  addition  of  2$  acetic 
acid  will  often  bring  it  into  sharp  contour. 

Structure  of  the  Nucleus. — In  its  minute  structure  the  nucleus 
affords  a  wonderful  variety  of  pictures  varying  according  to  the 
objects  chosen,  but  which  are  not  sufficiently  understood  to  permit 
of  a  single  description  accepted  by  all.  According  to  their  reac- 
tions to  stains  two  substances  in  particular  are  distinguished: 
chromatin  or  nuclein  (fig.  19,  cli),  which  is  easily  stained  by  certain 
staining-fluids  (carmine,  haematoxylon,  saffranin),  and  the  achroma- 
tin  or  linin,  which  stains  not  at  all  or  only  under  special  conditions. 

The  achromatin  forms  a  network  or  reticulum  (according  to 
another  view  a  honeycomb  structure)  filled  with  a  nuclear  fluid, 


66 


GENERAL  PRINCIPLES   OF  ZOOLOGY. 


bounded  externally  by  a  nuclear  membrane,  easily  isolated  in  large 
nuclei.  If  little  nuclear  fluid  be  present,  and  the  reticulum  con- 
sequently be  coarse-meshed,  the  nucleus  seems  compact.  If  the 
fluid  be  abundant,  the  nucleus  appears  vesicular.  This  is  especially 


jchp 


FIG.  19.— Vesicular  nuclei  with  achromatic  reticulum  and  different  arrangements  of 
the  chromatin  and  nucleolar  substance,  p,  plastin  (nucleolar  substance);  eft, 
chromatin;  chp,  chromatin  plus  plastin  land  2,  nuclei  of  Actinosphcerium ;  3, 
of  Ceratium  hirundella  (after  Lauterborn) ;  4,  germinal  vesicle  of  Unio  (after 
Flemming);  5,  nucleus  with  many  chromatin  nucleoli. 

the  case  when  the  lines  of  the  framework  are  separated  by  con- 
siderable amounts  of  nuclear  fluid  (fig.  19,  4). 

The  chromatin  enters  into  close  relations  with  a  less  stainable 
substance,  the  plastin  or  paranuclein  (also  sharply  distinct  from 
achromatin).  In  the  nuclei  of  Protozoa  plastin  and  chromatin 
are  usually  intimately  united,  the  first  forming  a  substratum  in 
which  the  latter  is  embedded  (chp).  The  united  substances 
are  most  frequently  closely  and  regularly  distributed  as  fine  gran- 
ules on  the  reticulum,  so  that  the  entire  nucleus  appears  uni- 
formly chromatic  (fig.  18).  More  rarely  the  mixture  collects  into 
one  or  more  special  bodies,  the  chromatic  nucleoli  (1,  2}.  The 
nucleolus  is  ordinarily  a  rounded  body,  more  rarely  branched 
(fig.  19,  1). 

In  the  nuclei  of  the  Metazoa  there  may  occur  the  same  intimate 
mixture  of  plastin  and  chromatin  (6).  As  a  rule,  however,  the 
plastin  (apparently  not  the  whole,  but  a  surplus)  is  separate  from 
the  chromatin.  Thus  there  occur  in  the  nuclei  of  many  eggs 


GENERAL  ANATOMY.  6T 

nucleoli  which  contain,  the  one  chromatin,  the  other  exclusively 
plastin  (4).  In  tissue  cells  only  the  plastin  has  the  form  of 
nucleoli  (true  or  chromatin-free  nucleoli,  5),  while  the  chromatin 
is  distributed  on  the  nuclear  reticulum  (chromatin  reticulum). 
Somewhat  the  same  may  occur  in  the  Protozoa  (fig.  19,  3). 

Significance  of  the  Cell  Nucleus — For  a  long  time  the  func- 
tional significance  of  the  nucleus  in  the  cell  was  shrouded  in 
complete  darkness,  so  that  it  began  to  be  regarded,  in  comparison 
with  the  protoplasm,  as  a  thing  of  little  importance.  The  evidence 
that  the  nucleus  plays  the  most  prominent  role  in  fertilization  has 
altered  this  conception.  Then  arose  the  view  that  the  nucleus 
determines  the  character  of  the  cell ;  that  the  potentiality  of  the 
protoplasm  is  influenced  by  the  nucleus.  If  from  the  egg  a  definite 
kind  of  animal  develop,  if  a  cell  in  the  animal's  body  assume  a 
definite  histological  character,  we  are,  at  the  present  time,  inclined! 
to  ascribe  this  to  the  nucleus.  From  this,  then,  it  follows  farther 
that  the  nucleus  is  also  the  bearer  of  heredity;  for  the  transmission 
of  the  parental  characteristics  to  the  children  (a  fact  shown  to  us 
by  our  daily  experience)  can  only  be  accomplished  through  the 
sexual  cells  of  the  parents,  the  egg  and  sperm  cells.  Again,  since 
the  character  of  the  sexual  cells  is  determined  by  the  nucleus,  the 
transmission  in  its  ultimate  analysis  is  carried  on  by  the  nucleus. 
This  idea  has  a  further  support  in  experiments  on  Protozoa.  If 
one  of  these  unicellular  animals  be  cut  into  nucleate  and  anucleate 
halves,  the  latter  sooner  or  later  degenerates,  the  former  persists 
and  regenerates  the  lost  parts.  Within  the  nucleus  it  is  probably 
the  chromatin  which  controls  the  functions  of  the  protoplasm  and 
is  accordingly  (as  observations  on  fertilization  also  seem  to  show) 
the  bearer  of  heredity,  while  the  achromatin  is  the  seat  of  contrac- 
tility, and  as  such  plays  a  part  in  cell  multiplication. 

The  Centrosome. — Besides  the  nucleus  there  frequently  occurs 
a  special  body  in  the  protoplasm,  the  centrosome,  which  on 
account  of  its  small  size  and  a  behavior  similar  to  achromatin 
with  reference  to  staining-fluids  was  long  overlooked,  and  even 
now  its  demonstration  is  difficult.  It  is  apparently  well  distributed 
among  the  Metazoa,  but  is  absent  from  most  Protozoa.  In  many 
it  appears  only  at  certain  times  and  then  disappears.  What  is 
known  of  it  makes  it  probable  that  it  is  a  derivative  of  the  nucleus, 
a  part  of  the  achromatin  which  has  left  the  nucleus;  in  other  cases 
possibly  a  second  nucleus  which  by  degeneration  has  lost  the 
chromatin  and  retained  only  the  active  nuclear  substance,  the 
achromatin.  In  its  function  the  centrosome  is  a  specific  organ  of 


68  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

cell  division  which  controls  both  the  division  of  the  nucleus  and 
that  of  the  cell  itself. 

Multiplication  of  Cells. — Increase  in  cells  occurs  exclusively 
by  division  or  by  budding  .(gemmation).  Most  common  is  binary 
division  in  which  a  circular  furrow  appears  on  the  surface  of  the 
cell,  deepens  and  cuts  the  cell  into  two  equal  parts.  Multiple 
division  is  more  rare  and  can  only  occur  in  multinucleate  cells. 
Here  the  cell  divides  simultaneously  into  as  many  (sometimes 

hundreds)  daughter-cells  as 
there  were  nuclei  present.  In 
all  forms  of  division  the  simi- 
larity of  the  products  is  char- 
acteristic, while  in  budding  the 
resulting  parts  are  unequal. 
In  budding  one  or  more 
smaller  daughter-cells,  the 
buds,  are  constricted  from  a 
large  mother-cell  (fig.  20). 

Direct     Cell     Division.  - 
Every  cell  division  is   accom- 
panied by  nuclear  division  or 
FIG.  20.-ceii  budding.    Podophrya  gemmi-  at  least  presumes  that  nuclear 
^^g^(^nl^i?~to         <TOdiviBi<>n     has     previously    oc- 
curred.    Direct   and   indirect 

division  are  recognized.  Direct  division  is  most  common  in 
Protozoa,  and  especially  in  nuclei  with  abundant  chromatin  (fig. 
20,  145).  The  nucleus  is  elongated  and  is  divided  by  constriction, 
in  the  same  way  that  the  cell  itself  constricts.  Since  the  proto- 
plasm has  no  special  arrangement  with  regard  to  the  dividing 
nucleus  (the  latter  besides  protected  by  its  membrane),  we  must 
conclude  that  the  nucleus  divides  itself  and  is  not  passively 
divided.  The  dividing  force  resides  in  the  achromatic  framework, 
which  correspondingly  often  exhibits  a  certain  arrangement,  a 
fibrous  structure  in  the  direction  of  the  elongating  nucleus. 

Indirect  Cell  Division,  Karyokinesis. — Indirect  cell  division, 
karyokinesis  or  mitosis,  is  most  beautifully  shown  in  cells,  poor  in 
chromatin,  which  possess  a  centrosome.  The  process  is  introduced 
by  a  division  of  the  centrosome  (fig.  21).  The  daughter  centro- 
somes  migrate  to  two  opposite  poles  of  the  nucleus,  which  now 
loses  its  membrane  and  becomes  the  nuclear  spindle.  The 
characteristics  of  the  spindle  are  that  it  is  drawn  out  into  points 
at  two  poles  which  are  indicated  by  the  position  of  the  centro- 


GENERAL  ANATOMY. 


69 


somes,  while  from  these  poles  fine  threads,  the  spindle-fibres, 
run  to  the  centre  or  equator  of  the 
nucleus.  These  fibres  are  in  many 
cases  certainly  derived  from  the 
achromatic  nuclear  reticulum,  while 
in  others  a  greater  or  less  part  in 
their  formation  is  taken  by  the 
protoplasm.  A  debated  point  is  the 
relations  of  the  fibres  in  the  equa- 
torial plane  of  the  spindle.  Do  all 
the  fibres  extend  from  pole  to  pole  ? 

Do  all  Of  them  end  in  the  equatorial  FIG.  21.— Spindle  formation  imd  divi- 
sion of  the  centrosomes  in  Ascaris 

plane,  SO  that  the  Spindle  Consists  of  megalocephala.  (After  Brauer.)  c, 

centrosomes ;  c?i,  chromosomes. 

two  cones  ot  fibres  separated  at  the 

equator  ?  Or,  lastly,  are  fibres  of  both  kinds  present  in  the  same 
spindle  ?  It  would  appear  that  differences  exist  in  these  respects 
in  different  objects. 

All  of  the  chromatin  of  the  nucleus  lies  in  the  equator,  united 
in  the  '  equatorial  plate/  but  by  this  must  not  be  understood  a 
connected  mass  but  a  layer  of  separate  bodies,  the  chromosomes, 
for  the  chromatin  of  the  nucleus  divides  early  into  particles  which 
are  rarely  spherical  or  rodlike,  but  usually  have  the  shape  of 
U-shaped  loops.  These  chromosomes  are  of  equal  size  in  the  same 


FIG.  22.— Cell  division  in  the  skin  of  Salamandra  maculosa.    (After  Rabl.) 

cell,  and,  what  is  of  greater  theoretical  significance,  their  number 
is  identical  in  all  the  cells  of  all  the  tissues  of  one  and  the  same 
species. 


70  GENERAL  PRINCIPLES   OF  ZOOLOGY. 

The  first  step  in  the  karyokinetic  formation  of  the  daughter 
nuclei  is  the  division  of  the  chromosomes,  which  is  usually  com- 
pleted in  the  equatorial  plate  (division  of  the  equatorial  plate),  but 
occasionally  may  be  completed  at  an  earlier  stage.  The  division 
is  an  accurate  halving  (fig.  22,  b).  The  two  halves  of  a  mother- 
chromosome,  the  daughter  chromosomes,  now  travel,  under  the 
influence  of  the  spindle-fibres,  towards  the  opposite  poles  of  the 
spindle.  In  this  way,  by  a  splitting  of  the  equatorial  plate,  the 
lateral  plates  arise,  the  elements  of  each  uniting  and  producing 
the  daughter  nuclei.  The  centrosomes  remain  separate  as  division 
organs  for  the  next  nuclear  division  (fig.  22,  c,  d,  e). 

What  further  distinguishes  the  indirect  from  the  direct  cell 
division  is  the  active  participation  of  the  protoplasm.  The 
centrosome  is  -the  centre  of  a  marked  radiation  of  the  protoplasmic 
reticulum  (fig.  21).  When  the  centrosome  divides  a  double  radia- 
tion (amphiaster)  appears.  Not  only  the  spindle-fibres  but  the 
protoplasmic  rays  extend  from  the  daughter  chromosomes.  Since 
the  arrangement  and  degree  of  development  of  the  protoplasmic 
radiations  stand  in  certain  relation  to  the  phases  of  cell  division 
we  must  recognize  in  them  the  expression  of  the  effective  forces 
(apparently  contractile)  in  the  protoplasm  which  cause  cell 
division. 

Between  these  two  extreme  cases  of  direct  and  indirect  division  are 
all  possible  transitions  which  show  how  the  mechanism  of  nuclear  divi- 
sion has  been  completed  step  by  step,  first,  by  the  fibrous  arrangement  of 
the  nuclear  reticulum  (spindle  structure) ;  second,  through  the  develop- 
ment of  the  centrosome  by  which  the  division  obtains  an  influence  on  the 
protoplasm ;  and  third,  by  the  development  of  the  chromosomes.  In 
reference  to  the  latter  the  irregular  division  of  the  chromatin  mass  in 
direct  division  is  relatively  crude  in  comparison  with  the  complicated 
processes  involved  in  the  formation  and  division  of  the  chromosomes. 
These  become  intelligible  if  we  regard  the  chromatin  as  the  controller  of 
the  cellular  processes  and  the  bearer  of  heredity  (cf.  fertilization,  infra). 
The  more  highly  organized  the  animal,  the  more  its  cells  have  to  inherit 
and  the  more  important  it  is  that  the  physical  basis  of  heredity  should  be 
accurately  divided  in  amount  and  in  quality  between  the  daughter  cells. 
This  is  accomplished  by  mitosis. 

Nuclear  Fragmentation  is  to  be  distinguished  from  direct  division  ;  by  it 
the  nucleus  becomes  broken  up  into  numerous  parts  which  alone  cannot 
live  and  as  a  rule  degenerate.  A  typical  example  is  afforded  by  the 
breaking  up  of  the  macron ucleus  during  conjugation  in  the  Infusoria 
(fig.  146). 

Multinuclearity,  Multicellularity. — Nuclear  division  and  cell 
division  commonly  constitute  a  well-arranged  mechanical  process, 


GENERAL  HISTOLOGY.  71 

the  separate  phases  of  which  follow  one  another  according  to 
a  definite  law.  The  plane  of  division  is  per- 
pendicular to  the  long  axis  uniting  the  two 
poles  of  the  spindle.  But  the  interrelation  of 
cytoplasm  and  nucleus  is  by  no  means  an  unchange- 
able and  indissoluble  one,  for  very  often  nuclear 
division  takes  place  without  participation  of  the 
cytoplasm.  If  this  process  be  repeated  several 
times,  there  results  a  mass  of  protoplasm  with  many 
nuclei  (fig.  23),  which  now  on  its  part  may  become 
many  cells,  if  subsequently  the  protoplasm  divides 
according  to  the  number  of  nuclei.  Hence  multi-  Fc?ii^ith 
nucleated  protoplasmic  masses  are  transitional  stages  nuclei- 
between  the  simple  mononucleated  cell  and  a  collection  of  several 
mononucleated  cells,  and  in  consequence  of  this  are  sometimes 
regarded  as  the  equivalent  of  one  cell,  sometimes  as  equivalent  to 
many  cells,  and  are  called  sometimes  multinucleated  giant-cells, 
sometimes  cell-complexes  or  syncytia.  In  the  following  pages  a 
multinucleated  mass  of  protoplasm  will  be  considered  as  a  single 
cell,  because  the  essential  feature  of  the  cell  is  that  it  constitutes  a 
vital  unit,  it  has  a  physiological  individuality,  and  in  this  respect 
a  multinucleated  mass  of  protoplasm  behaves  like  a  mononucleated; 
as  the  tissue  cells  and  the  Protozoa  show,  the  plane  of  organization 
is  not  raised  in  the  least  by  the  multinuclearity.  A  change  only 
begins  at  the  moment  when  many  particles  of  protoplasm  are 
separated  from  one  another,  and  many  vital  units  are  formed,  i.e., 
when  in  place  of  multinuclearity  a  true  multicellularity  appears. 

II.  THE  TISSUES  OF  THE  ANTMAL  BODY. 

Definition  of  Tissue. — In  the  formation  of  tissues  two  processes 
are  operative:  (1)  the  multiplication  of  cells  by  means  of  division 
into  cell-complexes,  and  (2)  the  histological  differentiation  of  cells. 
A  tissue,  therefore,  can  be  defined  as  a  complex  of  differentiated 
cells  Mstologically  similar. 

Nature  of  Histological  Differentiation. — The  histological  differ- 
entiation consists  chiefly  in  that  the  cells  have  definite  form  and 
definite  position  relative  to  the  neighboring  cells;  in  addition, 
there  almost  always  occurs,  as  a  second  and  more  important 
feature,  the  histological  modification  of  the  cell.  The  fact  has 
already  been  mentioned  that  the  cell  uses  its  food-material,  not 
only  for  its  own  growth,  for  increase  of  its  protoplasm,  but  also, 
in  another  manner,  for  forming  substances,  protoplasmic  products, 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


either  in  its  interior  (internal  plasmic  products),  or  more  often  on 
its  surface  (external  plasmic  products).  The  histological  change  is 
the  formation  of  specifically  functioning  pla&mic  products.  If  we 
^  take  as  an  example  the  manner  in  which  a  cell 

becomes  a  muscle  fibre  (fig.  24),  we  see  that  it 
continually  secretes  upon  its  surface  new  fibrillaa 
of  specific  muscle  substance  (in  the  case  of  the 
vertebrates,  new  cross-striated  muscle  fibrillae), 
until  finally  the  remnant  of  the  formative  cell, 
the  muscle  corpuscle,  is  contained  in  a  mantle  of 
muscle  fibrillae.  In  an  analogous  way,  each 
tissue,  upon  histological  examination,  is  seen  to 
be  composed  of  cells  and  plasmic  products.  The 
former  control  the  formation,  the  renewal,  and 
the  sustenance  of  the  tissue;  the  latter  are  the 
agents  of  its  physiological  function.  The  advan- 
tages of  tissue  formation  are  far-reaching,  since 

FIG.   24.— Formation       & 

of  muscle  fibrils  in  m  general  they  are  connected  with  division  of 

the    frog.       (Dia-          &  -L 

gram.)    a,   forma-  labor  (frequently  referred  to  later).     So  long  as 

tivecell;  b,  forma-  ..  .;,          .,         .       .,      ,,,      ,,    ,,  .,    ,    „         .. 

tive  cell  with  two  the  cell  unites  in  itself  all  the  vital  functions, 

atedSnmsciJfibrnsj  these    are    incomplete    because    they    mutually 

witn°rni?mVe r o^s  hinder  each  other  in  their  free  development;  the 

plasmic  product,  on  the  other  hand,  has  only  the 

single  function  peculiar  to  it  and  can  therefore  discharge  its  duties 

with  greater  completeness.     The  muscle  fibrillae,  the  characteristic 

elements  formed  by  the  muscle  cells,  have  preserved  of  the  various 

properties  of  protoplasm  only  the  capability  of  contraction;  but 

this  power  of  contraction  is  much  more  energetic  and  stronger  than 

the  mere  movement  of  protoplasm.     The  nerve  fibrillae  serve  only 

for  the  transmission  of  stimuli,  but  in   an  extraordinarily  more 

rapid  and  orderly  manner  than  does  simple  protoplasm. 

Classification  of  Tissues. — Since  in  every  tissue  its  function 
interests  us  most,  it  would  be  natural  to  base  the  classification  of 
tissues  upon  the  function  and  the  intimate  structure  connected 
therewith.  For  a  long  time  the  tissues  have  been  arranged  in  four 
groups:  1.  Epithelial  tissue;  2.  Supporting  tissue;  3.  Muscular 
tissue;  4.  Nervous  tissue.  Within  these,  however,  certain  con- 
stituent parts  of  the  animal  body,  to  which  indeed  the  term 
'  tissue '  is  scarcely  applicable,  find  no  shelter :  these  are  the  sexual 
cells,  the  blood,  and  the  lymph.  The  former  may  be  spoken  of 
in  connexion  with  the  epithelium,  the  latter  in  connexion  with 
the  supporting  substances. 


GENERAL  HISTOLOGY.  73 


i.  Epithelial  Tissues. 

Morphology  of  Epithelial  Tissues. — On  several  grounds  the 
epithelia  must  be  considered  first.  They  are  the  oldest  tissues; 
they  are  the  first  to  appear  in  the  animal  kingdom,  there  being" 
animals  which  consist  only  of  epithelia.  Further,  each  separate 
organism  during  the  first  stages  of  embryonic  life  consists  only  of 
epithelia]  layers,  the  germ -layers.  "With  this  is  also  connected  the 
fact  that  in  epithelial  tissues  the  cells  have  undergone  the  least 
degree  of  histological  change,  and  that  the  formation  of  plasmic 
products  is  subordinated. 

Function  of  Epithelium, — The  most  important  purpose  of  the 
epithelium  is  to  form  a  protecting  and  excluding  covering  over 
surfaces,  equally  valuable  whether  the  surfaces  are  external  (surface 
of  the  body)  or  caused  by  cavities  in  the  interior  of  the  body  (the 
body  cavity,  lumen  of  the  gut  and  blood-vessels).  The  importance 
of  the  epithelia  in  this  respect  is  shown  by  the  fact  that  if  the 
protecting  layer  be  removed,  inflammation  arises  and  continues 
until  the  epithelium  is  regenerated.  Only  exceptionally  do  areas 
occur  which  are  free  from  epithelium;  the  teeth  of  vertebrates,  the 
antlers  of  stags,  are  parts  of  the  body  which,  on  account  of  their 
hardness,  can  exist,  at  least  for  a  more  or  less  considerable  time, 
without  epithelial  covering. 

Glandular  and  Sensory  Epithelia. — By  their  superficial  posi- 
tion epithelia  are  suited  for  presiding  over  two  other  functions:  all 
substances  which  ought  to  be  removed  from  the  body — some 
because  they  have  become  useless,  and  consequently  injurious- 
(excreta),  and  others,  as,  for  example,  the  digestive  fluids,  because 
they  have  to  perform  important  functions  (secreta) — must  pass  the 
surface,  and  are  therefore  separated  by  the  epithelia ;  these  are  the 
glandular  epithelia.  Further,  all  influences  of  the  external  world 
chiefly  impress  the  surface  of  the  body,  causing  sensations;  hence 
also  certain  epithelia  are  of  the  greatest  importance  for  the  recep- 
tion of  sensory  stimuli,  and  serve  for  hearing,  seeing,  smelling, 
tasting,  and  touching.  Such  areas  of  epithelium  are  called  sensory 
epithelia. 

Covering  Epithelium. — The  covering  epithelium  consists  of 
cells  which,  in  order  to  serve  the  function  of  the  tissue,  are  united 
by  a  small  quantity  of  cementing  substance.  We  speak  of  simple 
or  of  stratified  epithelia,  according  as  we  find  in  sections  running 
perpendicularly  to  the  surface  one  or  several  superimposed  layers 
(figs.  25,  26,  27). 


74  GENERAL  PRINCIPLES  OF  ZOOLOGJ. 

Simple  Epithelium.  —  Exclusively  one-layered  epithelia  are 
found  in  all  invertebrated  animals  and  in  Amphioxus;  in  the 
vertebrates,  on  the  other  hand,  they  are  limited  to  the  internal 
cavities  of  the  body,  and  even  here  are  occasioDally,  as  always  in 
the  skin,  replaced  by  a  many-layered  epithelium.  According  to 


TIG.  25  —Various  forms  of  epithelia.  a,  flattened  epithelium  of  Sycandra  raphanw, 
a'  in  cross-section,  a"  in  surface  view;  b  and  c,  cuboidal  and  columnar  epithe- 
lium of  a  mollusc  (Haliotis  tuberculata) ;  d,  flagellated  epithelium  of  an  actinian 
(Calliactis  parasitical;  e,  ciliated  epithelium  from  the  intestine  of  the  fresh- water 
mussel;  /,  epithelium  (e)  with  cuticle  (c)  of  Cimbex  coronatus  (a  wasp). 

the  shape  of  the  cells  we  distinguish  cuboidal  or  pavement,  flat, 
and  columnar  epithelium.  In  the  case  of  pavement  epithelium 
(fig.  25,  V)  the  cells  are  all  developed  about  equally  in  all  direc- 
tions of  space,  and  because  they  have  become  compressed  by 
lateral  pressure  have  the  appearance  of  cubical  blocks  or  paving- 
stones.  In  columnar  epithelium  the  long  axis,  the  distance  from 
the  deeper  to  the  peripheral  end  of  the  cell,  is  especially  great 
(fig.  25,  c)',  finally,  in  flat  or  squamous  epithelium  this  is  greatly 


GENERAL  HISTOLOGY.  75 

shortened  (fig.  25,  a)  and  the  separate  cells  have  become  changed 
into  thin  plates. 

Flagellated  and  Ciliated  Epithelia. — Further  differences  which 
obtain  in  the  three  kinds  of  epithelium  mentioned  above  are 
caused  by  the  presence  or  absence  of  processes  (cilia,  or  flagella) 
on  the  peripheral  end  of  the  cells.  Both  are  fine  threads  which 
arise  from  the  body  of  the  cell,  extend  above  the  surface  and  here 
maintain  an  extremely  lively  motion.  In  case  of  flagellated 
epithelium  (fig.  25,  d)  each  cell  has  only  one  vibratile  projection, 
but  this  is  strongly  developed ;  in  the  case  of  ciliated  epithelium 
(fig.  24,  e),  on  the  other  hand,  the  surface  of  the  cell  is  covered 
with  a  thick  forest  of  minute  threads  moving  in  unison. 

Cuticle. — The  majority  of  the  one-layered  epithelia  are  covered 
by  a  cuticle,  a  membrane  which  is  secreted  by  the  epithelial  cells 
in  general,  and  hence  very  frequently  shows  the  impression  of  the 
cells  as  polygonal  markings.  In  many  cases  thin  and  inconspic- 
uous, it  may  in  other  instances  become  thickened  into  a  very  con- 
siderable layer,  much  thicker  than  the  matrix  layer  of  epithelium 
which  secretes  this  cuticle.  The  cuticle  is  plainly  composed  of 
layers  parallel  with  the  surface,  and  forms  a  more  effective  protec- 
tion for  the  surface  of  the  body  than  does  the  epithelium;  it 
becomes  a  protective  armor,  as  shown,  among  other  examples, 
by  the  calcareous  shells  of  molluscs  and  the  chitinous  integument 
of  insects  (fig.  25,  /). 

Stratified  Epithelia. — The  protection  furnished  by  the  cuticle 
in  the  case  of  simple  epithelium,  may  in  the  stratified  be  obtained 
immediately  through  a  chemical  change  of  a  part  of  the  cells 
themselves.  In  the  stratified  epithelia  the  cells  of  the  various 
layers  always  can  be  distinguished  by  their  form.  The  deepest 
layer  consists  of  cylindrical  cells;  the  superficial,  on  the  other 
hand,  of  more  or  less  flattened  elements;  between  lie  several  layers 
of  transitional  forms,  so  that  starting  from  the  cylindrical  cells  we 
gradually  pass  through  the  cubical  cells  to  the  flat  cells  of  the  sur- 
face. As  this  arrangement  shows,  there  exists  a  genetic  con- 
nexion between  the  cell -layers:  the  lower  cylindrical  cells  are  in  a 
state  of  active  multiplication;  their  descendants,  with  gradual 
changes  of  form,  become  the  superficial  layers,  here  to  replace  an 
equal  quantity  of  worn-out  cells  (fig.  26). 

In  the  course  of  this  change  of  position,  the  protoplasmic 
bodies  may  undergo  an  alteration;  in  the  reptiles,  birds,  and 
mammals  (fig.  27)  they  became  cornified,  first  the  margins,  then 
the  inner  part  of  the  cell,  changing  into  horn.  Of  the  living  cell 


76 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


the  nucleus  remains  for  some  time,  until  at  length  this  vanishes, 
and  then  the  cell  becomes  completely  changed  into  a  dead,  horny 
scale.  In  the  skin  of  the  higher  vertebrates  the  zones  of  the  living 
protoplasmic,  and  the  cornified  cells  no  longer  capable  of  life,  are 
sharply  marked  off  from  one  another.  In  cross-section  they  are 
readily  distinguished  as  the  stratum  corneum  (sc)  and  the  stratum 


FIG.  27.  -  Stratified  epithe- 
lium of  man.  sM,  stratum. 
Malpighi;  sc,  stratum  cor- 
neum. 


FIG.  26.— Section  through  the  skin  of  Petromj/zon 
planeri.  Ep,  the  many-layered  epithelium  of  the 
epidermis,  including  B,  goblet  cells;  Ko,  granu- 
lar cells;  Ko,  Co,  derma  (with  blood-vessels 
G),  consisting  of  bundles  of  fibrils  running  hori- 
zontally (  W)  and  vertically  (S).  (From  Wieders- 
heim.) 


FIG.  28.— Single-lay  ere  d 
epithelium  of  a  snail.  c, 
cuticle ;  d,  goblet  cells. 


Malpighi  (sM)  of  the  skin  (fig.  27).  In  the  many-layered  epithelia 
the  cuticle  has  lost  its  importance,  and  it  is  either  an  inconspicu- 
ous boundary  line  or  is  entirely  absent. 

Glandular  Epithelium. — Glandular  epithelium  is  distinguished 
physiologically  from  ordinary  pavement  epithelium  by  the  fact 
that  it  also  produces  secretions  or  excretions;  anatomically  it  is 
recognizable  by  the  presence  of  t  gland  cells/  cells  which  carry  on 
the  secretion  and,  to  a  greater  or  less  extent,  reveal  their  character 
by  their  structure.  Characteristic  glandular  cells  are,  for  example, 


GENERAL   HISTOLOGY. 


77 


the  goblet  cells;  here  the  secretion,  usually  mucus,  is  collected  as 
a  clear  mass  in  the  interior  of  the  cell,  the  cytoplasm  being  com- 
pressed into  a  thin  external  wall,  reminding  one  of  a  goblet  c6n- 
taining  the  nucleus  at  its  base  (fig.  26,  28,  d).  Other  gland  cells 
are  the  granular  cells,  swollen  bodies  completely  filled  with  secre- 
tory granules  (fig.  26,  K'6).  Naturally  all  grades  of  transition 
between  pavement  and  glandular  epithelium  occur.  Commonly 
the  latter  name  is  only  employed  when  the  gland  cells  are  especially 
numerous,  thereby  giving  to  the  epithelial  area  a  pre-eminently 
secretory  character.  This  is  especially  the  case  with  the  structures 
which  have  the  name  of  glands,  among  which  we  distinguish 
unicellular  and  multicellular  glands. 

Unicellular  Glands. — Unicellular  and  multicellular  glands 
increase  the  secretory  surface  by  invagination.  Invagination  of  a 
single  cell  produces  the  unicellular  glands  which  are  chiefly  found 
among  the  invetebrate  animals  (fig. 
29);  a  gland  cell  here  becomes  so  '' 
enormous  that  there  is  no  room  for  it  d- 
in  the  epithelium,  but  it  is  pushed 
into  the  deeper,  the  subepithelial 
layers,  the  nucleated  cell  body,  dis- 
tended by  secretion,  sending  up  a 
slender  process,  the  duct,  to  the 
epithelial  surface. 

Multicellular  Glands. — In  the  for- 
mation of  multicellular  glands  a  con- 
siderable area  of  glandular  epithelium 
grows  as  a  cylindrical  cord  or  tube 
from  the  surface  down  into  the  deeper 

tissues;      this      COrd      of     Cells      seldom  FIG.  29 -Unicellular  glands  from 

edge  of  the  mantle  of  Helix  po- 

remains   simple;   it   usually  branches    matta.  e,  epithelium;  d, uniceiiu- 

J  lar  glands;  p,  pigment  cells. 

and    forms    the     compound    glands, 

which  may  consist  of  hundreds  or  thousands  of  glandular  sacs,  all 
emptying  into  a  common  duct.  Among  the  multicellular  glands 
are  to  be  distinguished  tubular  and  acinous  (racemose)  forms.  In 
tubular  glands  (fig.  30)  the  simple  or  branched  glandular  pouches 
preserve  the  same  tubular  diameter  from  beginning  to  end;  in  the 
acinous  glands  (fig.  31),  on  the  contrary,  the  blind  end  of  the 
glandular  pouch  widens  into  a  sac  (acinus),  largely  composed  of 
secretory  cells,  and  related  to  the  outer  part  of  the  glandular 
pouch,  the  duct,  as  grapes  are  to  their  stem.  To  the  tubular 
glands  belong  the  liver,  kidney  and  sweat  glands  of  man;  to  the 


78 


GENERAL  PRINCIPLES   OF  ZOOLOGY. 


acinous  belong  the  salivary  glands,  not  only  of  the  vertebrates,  but 
also  of  the  arthropods  and  molluscs. 


FIG.  30.— Tubular  glands.  (After  Toldt.)  A,  glands  of  Lieberkuhn  from  the  human 
intestine;  A',  of  the  conjunctiva  of  the  eye;  B,  of  the  cat's  stomach;  C,  from  the 
medullary  pyramids  of  the  dog's  kidney;  U,  from  the  cortex  of  the  rabbit's 
kidney. 

Sexual  Epithelium. — The  sexual  cells  may  be  considered  in 
connexion  with  glandular  epithelium.     As  the  secretion  of  some 


FIG.  31.— Acinous  salivary  gland  of  the  aphid  Orttiezia  cataphracta.    (After  List.)    In 
some  acini  the  nuclei  and  boundaries  of  the  cells  are  shown. 

glands  must  be  expelled  from  the  body,  so  the  sexual  cells  are 
elements  which  differ  from  the  rest  of  the  organism,  and  must 
reach  the  exterior  in  order  to  perform  their  function.  Just  as  the 


GENERAL  HISTOLOGY. 


79' 


gland-cells  are  usually  scattered  among  ordinary  epithelial  cells, 
so  the  sexual  cells,  almost  without  exception,  lie  embedded  in 
epithelium;  it  maybe  in  the  epithelium  of  the  skin  (fig.  32),  of  the 


FIG.  32.— Germinal  epithelium  of  a  medusa,    eh,  ectoderm;  en,  entoderm ;  o,  egg; 

e,  epithelium. 

gut,  of  the  body  cavity,  or  of  parts  cut  off  from  this  (fig.  33). 
This  connexion  of  the  sexual  cells  with  the  epithelium  has  a 
deeper  meaning  in  the  fact  that  many  organisms,  and  particularly 
organisms  of  low  structure,  consist  exclusively  of  epithelia  and 


FIG.  33.— Section  through  the  ovary  of  a  new-born  child.  (After  Waldeyer.)  geT 
germinal  epithelium;  pe,  primitive  egg  in  the  germinal  epithelium;  p,  egg-pouch; 
g.  egg-nest  constricted  off  from  the  pouchlike  growth  (p);  /,  single  egg  with  fol- 
licle; r,  blood-vessel. 

therefore  must  necessarily  develop  their  sexual  products  in 
epithelium.  In  other  words,  sexual  and  epithelial  cells  are  the 
oldest  elements  of  the  animal  body,  and  hence  very  early  came 
into  relation  with  one  another. 

Sexual  epithelium  (or,  as  it  is  often  called,  germinal  epithe- 
lium) like  glandular  epithelium  has  a  tendency  to  grow  into  the 
subepithelial  tissues  in  the  form  of  isolated  or  branching  tubes 


80 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


(figs.  33,  p,  34),  and  thus  in  many  groups  of  animals  the  sexual 
organs  bear  the  character  of  branched 
glands;  for  this  reason  one  speaks  as  often 
of  sexual  glands  as  of  sexual  organs  (fig.  34). 
The  male  and  female  cells,  the  specific  ele- 
ments of  the  germinal  epithelia  and  of  the 
sexual  glands,  differ  in  the  fact  that  the  eggs 
are  generally  the  largest,  the  spermatczoa 
the  smallest,  cells  of  the  animal  body. 

Egg-cell. — The  egg-cell  (fig.  35)  as  it  is 
formed  in  the  ovary  varies  in  size  according 
to  the  animal  group:  in  case  of  the  micro- 
scopic Gastrotricha  it  is  less  than  0.04  mm., 
in  man  about  0.2  mm.,  in  the  frog  several 
millimetres,  and  in  the  large  birds  often 
several  inches ;  however,  only  the  yolk  of  the 
bird's  egg  is  the  egg-cell,  the  white  of  the 
egg  and  the  shell  are  structures  which  are 
formed  outside  of  the  ovary  in  the  oviduct. 
These  remarkable  differences  in  size  are 
caused  less  by  the  quantity  of  the  peculiar 
cell-substance,  protoplasm  (formative  or 
primary  yolk),  than  by  the  accumulation  of 
deutoplasm  (food  or  accessory  yolk,  also 


FIG.  34. 


FIG.  35. 

a,  formative  cell;  7),  follicular 

epithelium;  c,  nutritive  cells;  d,J5gg-cells;  /,  fibrous  covering  extending  out  into 

aid 


FIG.  34.  —  Ovarian  tube  of  an  insect,  Vanessa  urticce. 

epithelium;  c,  nutritive  cells;  d,  egg-cells; 

the  terminal  fibres  (g).    (After  Waldeyer.) 
FIG.  35.—  Egg-cell  of  Stronyylocentrotus  Uvidus. 


briefly  called  yolk).  The  function  of  the  deutoplasm  is  to  nourish 
the  embryo  during  development,  and  hence  consists  of  substances 
rich  in  fat  and  proteid,  arranged  in  spherical  oil-drops,  or  in  fine 


GENERAL  HISTOLOGY. 


81 


granules  or  polygonal  bodies,  the  yolk-granules.  Its  quantity, 
and  therefore  the  size  of  the  egg,  is  in  part  proportional  to  the 
length  of  time  which  the  egg  is  cut  off  from  any  other  supply  of 
nourishment.  In  general  we  find  the  largest  eggs  in  the  case  of 
the  highly  organized  oviparous  animals,  where  a  long-continued 
course  of  development  is  necessary  to  lay  the  foundation  of  the 
manifold  organs.  Besides  the  protoplasm  and  deutoplasm,  a  cell 
nucleus  or  germinal  vesicle  (sometimes  visible  to  the  naked  eye) 
surrounded  by  a  membrane  always  occurs  in  the  egg.  Its  contents 
are  mainly  the  nuclear  fluid,  through  which  is  distributed  an 
achromatic  network,  and  in  addition  the  nucleolus,  called  also 
the  germinal  spot.  Often  there  are  multinucleolated  germinal 
vesicles,  especially  in  eggs  which  contain  very  much  yolk. 

The   Spermatozoa,  the  morphological  elements  of  the   male 

reproductive  product,  are  so  small  that  their  finer  structure  can  be 

studied  only  with  the  strongest  powers  of  the  microscope  (fig.  36, 

a  and  /?).     Easiest  to  recognize  in  them  is  the  head,  which  from 

a         J5 


FIG.  36  —Various  spermatozoa,  a,  of  the  night-hawk;  0,  of  the  green  frog;  >,  of  tho 
crayfish ;  6,  of  a  crab ;  e,  of  the  round  worm  (Ascaris).  n,  nucleus ;  w,  middle 
piece  ;  »,  flagellum  ;  7c,  homogeneous  body. 

its  variety  of  form — spherical,  oval,  sickle-shaped,  etc. — often 
renders  possible  the  specific  determination  of  the  spermatozoa. 
The  head  is  the  closely  compacted  chromatic*  part  of  the  nucleus, 
and  hence  colors  very  deeply  in  staining  fluids.  Next  comes  an 


GENERAL  PRINCIPLES   OF  ZOOLOGY. 


unstaining  second  part,  the  middle  piece,  and  then  the  tail,  a  long 
flagellum,  which  causes  the  active  motility  of  the  ripe  sperma- 
tozoon. Cytoplasm  is  usually  present  only  in  an  extremely  thin 
layer  surrounding  the  nucleus. 

The  spermatozoa  of  nearly  all  animals,  except  the  nematodes 
and  crustaceans,  are  constructed  according  to  this  type.  In  these 
two  groups  it  is  worthy  of  notice  that  the  spermatozoa  are 
remarkably  large  and  incapable  of  motion,  and  that  they  enclose  a 
homogeneous  strongly  refractive  body  (fig.  36,  k),  previously  not 
found,  the  significance  of  which  is  not  clear.  The  spermatozoon 
of  Ascaris  (fig.  36,  e)  has  the  form  of  a  sugar-loaf  with  a  broad 
rounded  end,  containing  the  nucleus;  the  spermatozoon  of  the 
crayfish  (fig.  36,  y),  on  the  other  hand,  has  the  shape  of  a  cake- 
pan,  from  whose  periphery  springs  a  circle  of  fine,  stiff,  and 
pointed  fibres. 

The  two  kinds  of  spermatozoa  found  in  a  few  animals  are  problem- 
atical. In  the  testis  of  one  and  the  same  individual  of  Paludina  vivipara 
occur  together  hair-like  spermatozoa  with  corkscrew  heads  and  vermiform 
spermatozoa  with  a  bunch  of  cilia  on  the  hinder  end.  The  first  accomplish 
fertilization  ;  the  physiological  significance  of  the  second  is  unknown. 

The  last  modification  of  epithelium  of  which  we  have  to  speak 
is  sensory  epithelium,  characterized  by  the  connexion  of  certain 

of  its  cells,  the  sensory  cells,  with 
the  finest  twigs  of  branching  nerves 
which  arise  in  the  central  nervous 
system.  This  connexion  may  be  of 
two  kinds.  In  the  first  the  cell 
(primary  sense  cell)  is  slender  and 
filiform,  the  position  of  the  nucleus 
being  indicated  by  a  swelling.  The 
peripheral  end  is  concerned  with 
the  reception  of  sensatory  stimuli, 
while  the  deeper  end  is  continued 
directly  into  the  nerve  ends  and 
correspondingly  is  branched  into  two 
or  more  extremely  fine  processes 
FIG.  37.— Sensory  epithelium,  a,  of  an  which  take  on  the  character  of 

Act  in  ian ;  /3,  from  the  olfactory  epi-  ,*•,     •-,!          /n          ow\ 

theiium  of  man;  d,  supporting  cells;  nerve   nbrillae     (ng.    67).       In    the 

second  type  the  sensory  nerve  ends 

in  a  ganglion  cell  beneath  the  epithelium,  which  sends  processes 
into  the  latter,  the  ends  of  these  being  applied  to  the  sensory  cell 
(secondary  sense  cell),  the  connexion  being  one  of  contact,  not  of 


GENERAL  HISTOLOGY.  83 

continuity.  In  both  the  peripheral  end  of  the  cell  bears  appen- 
dages for  sense  perception;  auditory  and  tactile  hairs,  stronger 
processes  in  the  case  of  olfactory  and  taste  cells,  conspicuous  rods 
in  visual  cells.  Almost  without  exception  the  sensory  cells  are 
part  of  the  skin  (ectoderm),  or  at  least  arise  from  it  in  develop- 
ment. This  is  true  for  sense  organs  like  the  eye  and  ear  of  verte- 
brates, which  are  separated  from  the  skin  by  thick  intermediate 
tissue,  for  in  these  the  sensory  epithelium  (retina,  crista  acustica) 
is  derived  from  the  ectoderm. 

Supporting  Cells. — In  the  region  of  the  sensory  epithelium 
and  between  the  sensory  cells  are  found  still  other  epithelial  cells, 
which  are  not  connected  with  nerves,  but  have  accessory  functions: 
they  serve  as  supports  for  the  sensory  cells;  in  the  eyes  they  con- 
tain pigment;  in  the  auditory  organs  they  often  bear  the  otoliths, 
etc.  They  have  the  general  name  of  supporting  or  sustentative 
cells. 

2.  Connective  Tissues. 

Contrast  of  Epithelium  with  Connective  Tissue. — From  a  his- 
tological  point  of  view  there  can  be  found  no  greater  difference 
than  exists  between  epithelium  and  connective  tissue;  the  former 
belongs  to  the  surface,  the  latter  to  the  interior  of  the  body;  in 
the  former  the  cells  play  the  chief  role,  in  the  latter,  on  the  con- 
trary, their  importance  is  subordinate  to  the  plasmic  products,  the 
'  intercellular  substances '  which  chiefly  determine  the  character 
of  the  various  kinds  of  connective  tissue. 

In  spite  of  this  contrast  the  connective  tissues  are  genetically 
connected  with  epithelium.  In  embryos  which  at  first  consist 
only  of  epithelia  the  connexion  can  be  directly  seen.  The 
epithelia  secrete  a  gelatinous  substance  from  their  deeper  surfaces 
into  which  separate  cells  migrate.  Thus  arises  the  embryonic 
connective  tissue,  the  mesenchyme  (fig.  107). 

Function  of  Connective  Tissue. — The  primary  function  of  con- 
nective tissue  is  to  fill  the  spaces  between  the  various  organs  in 
the  interior  of  the  body,  thus  connecting  not  only  the  single  parts 
of  the  organs,  but  also  the  various  organs  themselves.  In  conse- 
quence of  this  the  connective  tissues  contribute  to  the  firmness  of 
body,  and  are  frequently  employed  in  building  up  a  skeleton.  To 
accomplish  this,  substances  which  are  usually  firmer  than  proto- 
plasm are  formed  on  the  surface  of  the  cells,  and,  since  they  lie 
between  the  cells,  these  are  called  intercellular  substances.  In 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


proportion  as  the  intercellular  substance  increases  in  volume  the 
cells  themselves  diminish  and  become  inconspicuous  corpuscles, 
the  connective-tissue  corpuscles,  or,  as  seldom  happens,  entirely 
disappear.  Since,  in  the  connective  tissues,  the  intercellular  sub- 
stances are  most  important,  it  is  readily  understood  that  the  dis- 
tinctions between  the  various  kinds  of  connective  tissue  rest  chiefly 
upon  the  differences  of  this  intercellular  substance.  The  following 
forms  are  to  be  distinguished:  (1)  cellular  connective  tissue;  (2) 
homogeneous  connective  tissue;  (3)  fibrous  connective  tissue;  (4) 
cartilage;  (5)  bone. 

Cellular  Connective  Tissue  shows  the  characteristics  of  the 
group  least  distinctly.  It  owes  its  name  to  the  fact  that  the  cells 
make  up  the  chief  mass,  while  the  cell-products  are  inconsiderable. 
The  cells  are  large  and  vesicular  bodies  which,  like  plant  cells,  are 
closely  pressed  together  and  are  consequently  polygonal  (fig.  38). 
They  have  secreted  between  them  a  firm  but  thin  layer  of  inter- 
cellular substance. 


FIG.  38.— Cellular  connective  substance. 
Cross-section  through  the  notochord 
of  a  newly  hatched  Trout. 


FIG.  39.— Homogeneous  connective  sub- 
stance of  Sycandra  raphantu.  (After 
F.  E.  Schulze.) 


Homogeneous  Connective  Tissue. — In  the  case  of  homogeneous 
connective  substance  the  intercellular  substance  (or  matrix)  is 
usually  present  in  considerable  quantity  as  a  transparent  mass, 
nearly  invisible  under  the  microscope,  sometimes  soft  like  jelly, 
often  firmer  (fig.  39).  The  gelatinous  cells  lying  in  it  are  either 
spherical  or  send  branching  processes  into  the  matrix.  Such 
processes  may  unite  to  form  meshes  which,  like  a  pseudopodial 
network,  unite  cell  to  cell.  Frequently  the  matrix  contains,  in 
addition,  isolated  firm  fibres  or  threads,  which,  on  account  of 


GENERAL  HISTOLOGY. 


85 


their  physical  characteristics,  are  called  elastic  fibres,  and  consist 
of  a  substance  (elastin)  exceedingly  resistant  to  all  reagents. 
Finally,  in  the  matrix  there  may  develop  the  finer  connective- 
tissue  fibrils,  the  characteristic  element  of  the  next  group;  they 
may  become  so  prominent  by  increase  in  number  as  to  determine 
the  character  of  the  tissue. 

Fibrous  Connective  Tissue  is  characterized  by  the  rich  supply 
of  connective-tissue  fibrillae;  these  are  fibres  of  extraordinary  fine- 
ness, lying  in  a  homogeneous  basal  substance,  which  is  the  less 
evident  the  richer  it  is  in  fibres.  The  fibres  may  be  either  con- 
fusedly arranged,  crossing  in  all  directions,  or  may  run  essentially 
parallel  and  in  a  definite  direction.  Between  them  are  found  the 
rounded,  spindle-shaped  or  branched  connective-tissue  corpuscles 
(fig.  40).  It  is  characteristic  of  vertebrates  that  the  fibres  are 
grouped  into  bundles.  Each  bundle  is  generally  surrounded  by 
connective-tissue  corpuscles,  metamorphosed  into  flat  cells.  The 


FIG.  40.— Fibrous  connective  tissue  of  an 
Actinian. 


FIG.    41.  —  Areolar    fibrous    connective 
tissue.    (After  Gegenbaur.) 


bundles,  loosely  interwoven,  run  in  all  direction  (areolar  connec- 
tive tissue,  < cellular  tissue'  of  the  earlier  authors)  (fig.  41),  or 
they  may  be  almost  parallel,  forming  a  compact  mass  of  fibres 
(tendinous  tissue)  (fig.  42).  Since  the  fibrils  of  the  fibrous  con- 
nective tissue  of  the  vertebrates  have  another  peculiarity  not  met 
with  elsewhere,  in  that  they  are  composed  of  glutin,  and  upon 
boiling  become  gelatine  or  glue,  it  is  well  to  reserve  for  these 
forms  of  tissue  the  special  name  connective  tissue. 

Elastic   Tissue.— In  all  fibrous  connective  tissue   there  may 
appear,  as  a  further  constituent,  elastic  fibres;  they  may  indeed 


86 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


supplant  the  ordinary  connective-tissue  fibrils  and  become  the 
predominant  element  of  the  connective  tissue,  which  is  then, 
spoken  of  as  elastic  tissue. 


// 


1 


FIG.  43. 


FIG.  42. 

FIG.  42.—  Tendinous  tissue.    (After  Gegenbaur.) 

FIG.  43.—  Cartilage.  (After  Gegenbaur.)   c,  perichondrium  ;  b,  transition  into  typical 
cartilage  (a). 

Cartilage.  —  Cartilage  and  bone  are  likewise  tissues  which  find 
their  characteristic  development  only  in  the  vertebrates.  In  its 
appearance  cartilage  is  similar  to  the  homogeneous  connective 
substance  of  many  invertebrated  animals;  the  matrix  is  homo- 
geneous and,  at  first  glance,  appears  quite  structureless  (fig.  43), 
but,  under  the  action  of  certain  reagents,  assumes  a  fibrous  condi- 
tion. This  conduct,  as  well  as  the  fact  that  the  cartilage  grows 
through  changes  of  the  perichondrium,  —  a  thin,  fibrillar  skin 
covering  its  surface,  —  makes  it  more  certainly  evident  that  it  is 
homogeneously  fibrillar;  and  it  is  thereby  distinguished  from 
homogeneous  connective  substance  since  it  is  not,  like  the  latter, 
.a  lower  but  a  higher  stage  of  tissue  formation.  It  is  worthy  of 
note  that  the  matrix  of  cartilage  (chondrin)  by  cooking  produces 
a  kind  of  glue  which  differs  from  true  or  glutin  glue  in  that  it  is 
precipitated  by  acetic  acid.  In  the  matrix  the  cartilage  cells  lie 
united  in  groups  and  nests,  a  mode  of  grouping  pointing  to  their 
origin,  since  each  group  of  cells  has  arisen  from  a  single  mother- 
cell  by  successive  divisions.  In  cartilage  also,  elastic  fibres  are 
found;  if  present  in  great  number,  these  change  the  bluish  shiny, 
hyaline  cartilage  into  the  yellow-colored  elastic  cartilage. 

Bone  is  the  most  complicated  structure  in  the  series  of  connec- 
tive tissues.  It  consists  of  a  matrix  (ossein),  closely  allied  to 


GENERAL  HISTOLOGY. 


87 


glutin,  so  intimately  combined  with  inorganic  constituents  that  it 
appears  under  the  microscope  as 
a  homogeneous  mass.  The  propor- 
tion of  organic  and  inorganic  sub- 
stances varies  according  to  the  age 
and  species  of  animal :  in  man,  for 
example,  there  is  65$  inorganic  to 
35$  organic  substance;  in  the 
turtle,  63$  to  37$.  Of  the  in- 
organic constituents,  the  most  im- 
portant is  calcic  phosphate,  84$; 
in  smaller  quantities,  combinations 
of  fluoric,  chloric,  carbonic  acids 
and  magnesia.  Morphologically 
the  matrix  is  composed  of  the  bone 
lamellae  (fig.  44),  whose  arrange- 
ment is  determined  by  the  surfaces 
present  in  and  upon  the  bone.  In 
a  hollow  bone  (like  that  of  the 
upper  arm  or  of  the  hand)  there  is 
an  outer  surface  to  which  a  fibrous 
skin,  the  bone-skin  or  periosteum, 
is  closely  applied;  the  presence  of 
the  marrow-cavity  necessitates  a 
second  surface.  Finally,  the  solid 
mass  of  the  bone  is  permeated  by 
the  Haversian  canals,  which  run 
chiefly  in  a  longitudinal  direction, 
united  into  a  network  by  cross  or 
oblique  canals,  and  serve  for  the 
passage  of  blood-vessels.  Since  the 
bone  lamellae  arrange  themselves 
parallel  to  the  surfaces  mentioned,  two  systems  may  be  distin- 
guished in  cross-section,  the  fundamental  lamellae  and  the 
Haversian  lamellae.  The  former  are  arranged  parallel  to  the  sur- 
face of  the  periosteum  and  of  the  marrow-cavity  and  form  a 
mantle  of  concentric  layers  around  the  marrow-cavity.  Into  this 
groundwork  the  Haversian  canals  with  their  lamellae  enter, 
destroying  and  superseding  the  fundamental  lamellae  coming  in 
their  way.  The  Haversian  lamellae  are  concentrically  arranged 
around  the  lumen  of  the  Haversian  canals  just  as  the  fundamental 
lamellae  are  around  the  marrow-cavity. 


FIG.  44.  —  Cross-section  through  the 
human  metacarpus.  (After  Frey.)  a, 
surface  of  the  periosteum;  7>,  surface 
of  the  marrow-cavity;  c,  cross-sec- 
tions of  the  Haversian  canals  and 
their  system  of  lamellae;  d,  funda- 
mental lamellae;  e,  bone  corpuscles. 


88 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


Formation  of  Bone. — The  stratification  of  bone  is  caused  by  its 
mode  of  origin.  Where  the  bone  borders  upon  the  Haversian 
canals,  the  marrow-cavity,  and  the  periosteum,  there  is  transiently 
or  permanently  an  epithelial-like  layer  of  cells,  osteoblasts,  which 
secrete  the  bone-substance  on  their  surface.  Certain  cells  in  the 
matrix  participate  in  this  secretion,  and  here  give  rise  to  the 
bone-corpuscles,  which  are  distinguished  from  the  cartilage -cells 
by  their  numerous  processes  ramifying  through  the  matrix.  The 
processes  of  a  bone-corpuscle  branch,  and  unite  with  the  neighbor- 
ing cells  through  fusion  of  the  processes,  an  arrangement  most 
beautifully  seen  in  dried  bone,  because  here  the  cavities  and  the 
canals  of  the  matrix  are  filled  with  air.  Special  modification  of 
bony  tissue,  the  substance  of  fish-scales  and  of  the  teeth,  called 
also  ivory  or  dentine,  should  be  mentioned. 

Blood  and  Lymph,  here  treated  in  connexion  with  the  connec- 
tive substances,  are  in  reality  not  tissues  at  all,  but  nutritive 
fluids.  Two  kinds  of  nutritive  fluids  occur  in  the  vertebrates,  red 
blood  and  the  colorless,  weakly  opalescent,  or  cloudy  white  lymph. 
The  blood  of  man  and  other  vertebrates,  consists  of  a  fluid  and 
the  organized  constituents.  The  fluid  or  blood-plasma  is,  apart 
from  inorganic  constituents,  specially  rich  in  proteids;  after  the 
removal  of  the  blood  from  the  blood-vessels  a  part  of  these  separate 
by  coagulation  and  form  the  blood-clot,  made  up  of  fibrin,  leaving 
a  fluid  poor  in  proteids,  the  blood-serum.  The  organized  con- 
stituents, the  blood-cells,  are  distin- 
guished as  red  and  white  blood-cor- 
puscles. The  latter,  the  leucocytes, 
are  present  in  smaller  numbers  and 
have  great  similarity  to  the  amoebae 
found  in  water;  they  are  particles 
of  protoplasm,  contain  a  nucleus, 
devour  foreign  bodies  (for  example, 
carmine  granules  injected  into  the 
blood),  and  move  in  the  '  amoeboid ' 
manner  by  putting  out  pseudopodia 
(fig.  45). 

Red  Blood-corpuscles. — In  the 
mature  condition  >  the  red  blood- 
corpuscles  of  vertebrates  (fig.  46) 
are  circular  or  oval  discs,  which  by  external  influences  (e.g.,  by 
pressure)  may  temporarily  be  bent,  incised,  or  otherwise  modified 
in  form,  but  cannot  actively  change  their  shape,  because  they  no 


FIG.  45  —White  blood-corpuscles,  o, 
of  man;  b,  of  the  crab  (n,  the  nu- 
cleus). 


GENERAL  HISTOLOGY,  89 

longer  consist  of  protoplasm.     Embryologically  they  arise  from 
true,  nucleated,  protoplasmic  cells;  whether  these  cells  are  iden- 


Fio.  46.— Red  blood-corpuscles,  a,  of  man;  />,  of  the  camel;  c,  of  the  adder;  d',  of 
Proteus  (seen  from  the  edge);  tl",  surface  view;  e,  of  a  ray;  /,  of  Petromyzon;  n, 
nucleus  (all  the  blood-corpuscles  are  magnified  700  times,  except  d,  which  is  mag- 
nified 350  times). 

tical  with  the  leucocytes  or  are  special  <  erythroblasts '  is  still 
undertermined ;  but  gradually  the  protoplasmic  cell-body  changes 
completely  into  a  plasmic  product,  the  stroma  of  the  blood- 
corpuscle.  If  the  nucleus  be  retained  in  this  metamorphosis,  there 
is  a  slight  swelling  in  the  centre  of  the  disc;  if,  however,  the 
nucleus  degenerate,  the  bilateral  convexity  is  replaced  by  a  shallow 
concavity.  In  the  latter  case,  one  has,  in  reality,  no  right  longer 
to  speak  of  blood-cells,  since  all  the  characteristic  constituents  of 
the  cell — nucleus  and  protoplasm — have  disappeared.  Systemati- 
cally the  red  blood-corpuscles  are  of  interest,  since  non-nucleate 
forms  are  found  only  in  the  mammals  (fig.  46,  a,  b),  nucleated 
ones  in  all  the  other  vertebrates  (c,  d).  The  mammals  also  have 
circular,  the  other  vertebrates  oval,  discs.  To  this,  however, 
exceptions  occur,  since  among  the  mammals  the  Typloda  (camel, 
llama)  have  oval,  the  Cyclostomes  have  circular,  blood-corpuscles. 

Haemoglobin. — The  red  blood-corpuscles  are  the  cause  of  the 
color  of  the  blood,  as  well  as  the  agents  of  one  of  its  most  impor- 
tant functions,  the  interchange  of  gases;  both  are  connected  with 
the  fact  that  the  stroma  contains  the  coloring  matter  of  the  blood 
or  licemoglobin.  Haemoglobin  belongs  to  the  few  crystallizable 
proteids  and  is  remarkable  for  the  presence  of  a  small,  though 
extremely  important,  quantity  of  iron,  and  also  for  its  affinity  for 
oxygen.  Haemoglobin  containing  oxygen,  oxy-haemoglobin,  causes 
the  carmine-like  color  of  the  so-called  arterial  blood;  oxygen-free, 
'  reduced '  haemoglobin  causes  the  dark  red,  faintly  bluish  color  of 
venous  blood. 


90  GENERAL  PRINCIPLES   OF  ZOOLOGY. 

Lymph  is  distinguished  from  blood  by  the  entire  lack  of  red 
blood-corpuscles  and  the  slight  coagulability  of  its  plasma. 
Lymph  is  accordingly  a  proteid-containing  fluid  with  leucocytes, 
which  are  here  called  lymph-corpuscles. 

In  the  majority  of  invertebrated  animals  there  is  present  only 
one  kind  of  nutritive  fluid,  and  not  even  this  in  every  class;  the 
fluid  is  called  blood,  although  it  is  usually  colorless.  Where 
color  is  present,  it  is  generally,  if  not  always,  a  yellowish  red  or 
an  intense  red;  this  may,  even  as  in  the  vertebrates,  be  caused  by 
haemoglobin  (among  the  molluscs  in  Planorbis,  Area  tetragona, 
A.  now,  Solen  legumen,  Tellina  planata,  Pectunculus  glycimeris, 
and  others;  among  the  annelids  in  the  Capitellidae,  Glycera, 
Poly  cirrus,  Leprcea,  leeches,  and  earthworms;  among  insects  in 
Chironomus).  Often  other  coloring  matter  occurs  instead  of 
haemoglobin :  in  the  cuttlefish,  many  snails,  and  in  the  lobster  and 
Limulus,  the  oxygen  is  taken  up  by  the  bluish  haemocyanin,  which 
contains  a  trace  of  copper;  in  the  Sipunculids  by  haemoerythrin, 
etc.  The  blood-plasma,  as  a  rule,  is  the  seat  of  the  color  (Chiro- 
nomus, Hirudinea,  earthworms,  and  most  other  annelids);  only 
exceptionally  do  colored  blood-corpuscles  occur,  as  in  the  case  of 
Area,  Solen,  and  the  other  mussels  mentioned  above,  and  also  in 
the  genus  Phoronis.  Colored  elements  containing  haemoglobin, 
identical  with  blood-corpuscles,  are  found  besides  in  the  ccelomic 
fluid  of  many  annelids  (Capitellidae,  Glycera,  Leprea,  Polycirrus), 
and  in  the  ambulacral  vessels  of  echinoderms  (Ophiactis  virens, 
some  Holothurians).  Most  widely  distributed  in  the  invertebrate 
animals  are  the  leucocytes,  which  are  distinguished  by  their  active 
amoeboid  movements;  still  even  these  may  be  absent,  and  then  the 
blood  is  a  fluid  without  any  organized  corpuscles. 

3.  Muscular  Tissue. 

Characteristics  of  Muscular  Tissue. — Most  sharply  character- 
ized functionally  is  the  muscle- tissue,  inasmuch  as  it  is  the  agent 
of  active  movements  in  the  animal  body.  Since  active  mobility 
occurs  in  protoplasm,  it  is  important  to  notice  the  differences 
between  the  two  kinds  of  movement.  The  distinctions  lie  in 
the  direction  and  in  the  intensity  of  the  movement.  A  mass  of 
protoplasm  has  the  capacity  to  move  hither  and  thither  in  all 
directions,  because  in  it  there  is  a  high  degree  of  mobility  be- 
tween the  smallest  particles.  Muscles  and  hence  their  separate 


GENERAL  HISTOLOGY.  91 

elements,  the   muscle-fibres  and   muscle-fibrils,  on  the  contrary, 

can  shorten  only  by  correspondingly  increasing  in 

diameter  (fig.  47);   they  can   therefore   accomplish 

motion  only  in  a  definite  direction,  that  of  the  axis 

of  the  muscle.     The  muscle-substance  consequently 

is  more  limited  in  its  movement  than  is  protoplasm, 

but   on   the  other   hand  it   has   the   advantages  of 

greater  energy  and  greater  rapidity.      An  observer 

conversant  with  the   different  kinds   of   motion   is 

able  to  decide  with  considerable  accuracy,  from  the 

intensity   and   rapidity,  whether  in  a  given  case  a 

movement  has  been  brought  about  by  the  agency  of 

protoplasm  or  by  the  contractile   substance  in  the 

narrower  sense  (muscle-substance).  tracted  state. 

Formation  of  Muscle-substance. — These  physiological  con- 
siderations show  that  protoplasm  and  the  contractile  substance  are 
morphologically  different,  and  that  therefore  one  must  distinguish 
sharply  between  formative  cells,  or  muscle-corpuscles,  and  the 
product  of  these  cells,  the  contractile  substance,  just  as  in  the 
case  of  connective  tissue,  between  the  connective-tissue  corpuscles 
and  the  connective-tissue  fibrils.  This  distinction  actually  occurs, 
but  optically  it  is  not  equally  demonstrable,  for  the  reason  that  it 
is  not  prominent  histologically.  In  animal  histology  there  are 
recognized  two  kinds,  it  might  even  be  said  two  stages,  in  the 
formation  of  muscle-substance,  the  homogeneous,  or  smooth,  and 
the  cross-striated.  Since  the  former  looks  very  similar  to  non- 
granular  protoplasm,  the  boundary-line  between  it  and  the 
muscle-corpuscle  is  more  difficult  to  recognize  than  in  the  case  of 
the  cross-striated  muscle-substance,  which  in  its  minute  structure 
is  quite  different  in  appearance  from  protoplasm.  In  cross-striated 
muscles  the  contractile  portion  consists  of  two  substances  regularly 
alternating  with  one  another  in  the  direction  of  the  contraction  of 
the  muscle,  of  which  the  one  is  doubly,  the  other  singly,  refractive 
(figs.  24,  47,  50). 

Smooth  and  Cross-striated  Muscle-fibres. — The  smooth  muscle- 
substance  represents  a  lower  stage  of  development  than  the  cross- 
striated,  since  it  chiefly  occurs  in  the  less  highly  organized  and 
more  inactive  animals.  Interesting  in  this  respect  is  the  fact  that 
in  the  two  stages  of  development  of  one  and  the  same  animal  the 
simple  and  inert  polyp  has  smooth  muscles,  while  the  more  highly 
organized  and  actively  motile  medusa  has  cross-striated  muscles 
(fig.  48).  The  difference  in  their  action  has  led  in  the  vertebrates 


92  GENERAL  PRINCIPLES   OF  ZOOLOGY. 

to  a  peculiar  distribution  of  the  muscle-substance,  the  smooth 
musculature  being  chiefly  distributed  to  the  internal  organs, 
which  are  not  under  control  of  the  will  (involuntary  muscles), 
while  the  musculature  of  the  body,  subject  to  the  will  and  hence 
demanding  more  rapid  action,  is  cross-striated  (voluntary  muscles). 
We  must  not  conclude  that  the  difference  between  smooth  and 
cross-striated  musculature  coincides  with  the  distinction  between 
visceral  and  body  musculature;  it  should  be  noticed  that  the  body 
musculature  of  all  molluscs  is  smooth,  the  visceral  as  well  as  the 


FIG.  48.— Epithelial  muscle-cells,    o,  of  a  medusa;  b,  of  an  actinian. 

body  muscles  of  many  insects  and  Crustacea,  and  the  muscles  of 
the  heart  of  vertebrates  are  cross-striated. 

It  was  pointed  out  above,  in  connexion  with  epithelia  and 
connective  tissue,  that  these  tissues  differed  fundamentally.  This 
contrast  has  its  bearing  in  dealing  with  the  muscles,  for  both 
epithelial  and  mesenchymatous  cells  may  form  contractile  sub- 
stances and  therefore  there  are  two  genetically  different  kinds  of 
muscles,  the  epithelial  and  the  mesenchymatous  (contractile  fibre- 
cell).  Both  kinds  of  muscle-cells  can  a  priori  form  smooth  as 
well  as  cross-striated  muscle-substance;  but  the  collection  of  con- 
nective (mesenchymatous)  tissue  around  inner  organs  has  caused 
most  contractile  fibre-cells  to  be  smooth,  while  most  of  the 
epithelial  muscle-cells  are  cross-striated. 

Epithelial  muscle-cells  are  cells  of  which  one  end  extends  to  the 
surface  of  the  body  or  the  surface  of  an  internal  cavity  (body 
cavity,  lumen  of  the  gut,  etc.),  and  may  here  have  a  cuticle,  cilia, 
or  flagella,  while  at  the  opposite  end  it  has  secreted  contractile 
substance  in  the  form  of  muscle-fibrils  (fig.  48).  They  combine 
the  double  function  of  epithelial  and  muscle  cells. 

Contractile  fibre-cells,  on  the  other  hand,  are  connective-tissue 
cells,  which  usually  have  surrounded  themselves  with  a  layer  of 
contractile  substance;  corresponding  to  their  origin,  they  have  the 
form  of  connective-tissue  cells,  and  are  spindle-formed  or 
branched;  the  branches  arising  from  the  ends  of  the  cells  (fig.  49). 
The  similarity  of  form  renders  the  distinction  between  ordinary 
connective-tissue  cells  and  fibre-cells  difficult;  if  the  contractile 


GENERAL  HISTOLOGY. 


93 


layer  on  the  surface  be  slightly  developed,  the  distinction  is  im- 
possible. To  recognize  the  character  of  the  elements,  therefore, 
we  must  choose  well-defined  examples,  in  which  the  uninucleated 
or  the  multinucleated  mass,  the  <  axial  substance/  is  sharply 
marked  off  from  the  muscle-mass,  the  ' cortical  layer'  (fig.  49, 
c,  d,  e). 


FIG.  49. 


FIG.  50. 


FIG.  49.— Contractile  fibre-cells,  a,  of  man;  1)-e,  of  Beroe  (a  Ctenophore);  ft,  young 
fibres ;  c,  branched  ends  ;  d,  middle  portion  of  a  fibre;  e,  cross- section. 

FIG.  50.— Cross-striated  primary  bundle.  (After  Gegenbaur.)  w,  nuclei ;  s,  a  point 
where  the  sarcolemma  is  plainly  shown  on  account  of  the  tearing  of  the  fibres. 

In  vertebrates  and  arthropods  the  contractile  fibre-cells  occur 
in  the  vegetative  organs  as  elements  of  the  '  organic  musculature ' ; 
on  the  other  hand  we  find  here  the  epithelial  musculature  in  the 
cross-striated  primary  bundles,  separated  from  the  epithelium, 
and  only  developmentally  referable  to  the  epithelium  of  the  body 
cavity  (fig.  50).  A  primary  bundle  is  a  cylindrical  mass,  bounded 
externally  by  a  structureless  envelope,  the  sarcolemma.  Its  con- 
tents consist  of  fine  fibrils,  which,  closely  parallel  to  one  another 
and  pressed  closely  together,  run  from  one  end  of  the  mass  to  the 


94:  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

other.  Each  fibril  is  formed  of  singly  and  doubly  refractive 
parts,  which  alternate  with  one  another  in  more  or  less  compli- 
cated arrangement.  Since  now  the  doubly  refracting  parts  of  the 
fibrils  within  a  bundle  lie  at  about  the  same  level,  there  is  caused 
a  cross-striation  extending  through  the  whole  bundle.  Finally, 
scattered  here  and  there  between  the  muscle-fibrils  are  the  muscle- 
corpuscles,  spindle-shaped  protoplasmic  bodies  with  a  nucleus,  the 
remnants  of  the  cells  which  have  formed  the  musculature. 

4.  Nervous  Tissue. 

Function  of  Nervous  Tissue. — As  the  muscular  tissue  brings 
about  motion,  so  the  nervous  tissue  serves  for  the  transmission  of 
stimuli.  It  communicates  the  stimulations  of  the  sense-organs  at 
the  periphery  to  the  central  nervous  system,  the  seat  of  conscious- 
ness, and  here  brings  about  perception  (centripetal  nerve  tracts); 
further,  it  transmits  the  voluntary  impulses  to  the  periphery,  par- 
ticularly to  the  musculature  (centrifugal  nerve  tracts).  By  the 
nervous  system,  finally,  the  stimuli  arising  in  various  places  are 
co-ordinated,  thus  furnishing  the  elements  for  that  which  we  call 
independent  psychic  activity. 

Elements  of  Nervous  Tissue. — The  agent  of  the  transmission 
of  stimuli  is  undoubtedly  a  specific  nerve-substance  different  from 
protoplasm.  Hence  we  speak  of  nerve  fibrillae  as  of  muscle  fibrillae, 
the  product  of  the  special  nerve-cells,  but  the  relations  involved 
are  not  sufficiently  understood. 

The  elements  of  the  nervous  system  are  divided  into  ganglion 
cells  and  nerve-fibres,  but  it  must  be  remembered  that  these  are 
not  independent  of  each  other,  but  that  the  fibres  are  enormously 
elongated  processes  of  the  ganglion  cells.  In  the  vertebrates  the 
ganglion  cells  vary  greatly  in  size;  besides  small  elements  there 
are  large  cells,  only  exceeded  by  the  eggs  in  size,  which  correspond- 
ingly have  large  nuclei  recalling  the  germinal  vesicles.  Unipolar, 
bipolar,  and  multipolar  ganglion  cells  are  recognized,  the  differ- 
ences depending  upon  the  number  of  processes  (nerve-fibres)  which 
arise.  In  multipolar  cells  the  number  is  very  large  (fig.  51)  and 
are  of  two  kinds,  dendrites  and  axons  or  neurites.  Dendrites  are 
so  called  because  they  branch  again  and  again,  not  far  from  their 
origin  from  the  cell.  The  axons  (of  which  there  is  usually  but 
one  to  a  ganglion  cell)  can  be  followed  to  a  long  distance  with- 
out giving  off  branches,  except  here  and  there  lateral  side  twigs 
(collaterals)  which  arise  at  right  angles  to  the  main  fibre;  they 


GENERAL  HISTOLOGY. 


often  pass  over  into  peripheral  nerves, 
so  the  morphological  distinc- 
tion from  dendrites  lies  in  the 
greater  distance  of  the  region 
of  branching  from  the  body 
of  the  ganglion  cell.  In  bi- 
polar ganglion  cells  both  pro- 
cesses are  neu  rites,  the  cell 
itself  thus  being  an  element 
intercalated  in  the  course  of 
a  nerve-fibre,  as  also  is  a  uni- 
polar ganglion  cell.  The  single 
process  of  this  divides  near 
the  cell  in  a  T-shaped  man- 
ner, so  that  the  unipolar  cell 
is  to  be  regarded  as  a  bipolar 
ganglion  cell  in  which  the  two 
neurites  are  united  for  a  short 
distance. 

This  conception  is  intel- 
ligible in  the  light  of  recent 
researches  on  the  structure  of 
the  ganglion  cell  and  its  pro- 
cesses (fig.  52).  Both  consist 


They  branch  at  their  tips, 


FIG.  51.— Multipolar  ganglion  cell  of  man, 
(After  Gegenbaur.)    a,  axon. 


FIG.  52.— Motor  ganglion  cell  from  the  thoracic  region  of  the  spinal  cord  of  a  dog, 
(After  Bethe.)    n,  nucleus. 

of  extremely  fine  fibrillae,  and  inter-  and  perifibrillar  substances 
cementing  them    together.      Each   process   brings   a   bundle  of 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


fibrillae  to  the  ganglion  cell,  in  which  they  spread  out  and  pass 
over  into  other  processes.  The  branching  of  neurites  and 
dendrites  is  a  separation  of  the  contained  fibrillse;  the  ganglion 
cell,  the  place  of  exchange  of  fibrillae  between  the  various  processes. 
Hence  the  ganglion  cell  is  not  a  simple  cell,  but  a  cell  plus  plasma 
products. 

The  similar  fibrillar  structure  of  nerve-fibres  has  long  been 
known.  In  the  central  nervous  system  of  vertebrates  the  most 
minute  elements  are  the  nerve  fibrillae,  distinguished  from  muscle 

fibrillae.  by  the  absence  of  cross- 
striation ;  from  connective  -  tissue 
fibrillae  by  the  ease  with  which  they 
are  injured;  in  preserved  material 
they  frequently  swell  and  show  vari- 
cosities  (fig.  53).  Many  fibrillae 
united  in  a  bundle  form  a  nerve- 
fibre  (fig.  54,  A)  which  is  called  a 
gray  nerve-fibre  in  distinction  from 
the  white  or  medullated  fibres.  In 
the  latter  the  fibre  or  axis-cylinder 
is  surrounded  by  a  medullary  sheath 
(fig.  54,  B)  composed  of  my  elm,  a  fat- 
like  substance,  blackened  by  osmic 
acid  and  separated  into  variously 
shaped  ( myelin  drops. '  The  medul- 
lary sheath  appears  to  act  as  an 

FIG.  53.        FIG.  64.  F.G.  55.      insulator. 

FIG.  53.— Nerve  flbriiise  with  varicosi-        Both  medullated  and  non-med- 

ullated  fibres  can  be  enclosed  in  a 
'sheath  of  Schwann.'  This  is  a 
feature  of  the  fibres  composing  the 
peripheral  nervous  system  and  is  lacking  in  brain  and  spinal  cord. 
It  is  a  delicate  envelope  with  nuclei  here  and  there  (fig.  55).  At 
times  it  forms  constructions  which  cut  through  the  medullary 
sheath  to  the  axis-cylinder  (nodes  of  Ranvier). 

Multipolar  and  bipolar  ganglion  cells  also  occur  in  the  inverte- 
brates, most  commonly  in  the  coelenterates  (fig.  56),  more  rarely  in 
worms  (e.g.,  Lumbricus),  arthropods,  and  molluscs,  and  then 
chiefly  in  the  peripheral  nervous  system.  In  the  ganglia  (the 
nervous  centres  of  the  last  three  groups)  the  ganglion  cell  usually 
gives  rise  to  a  single  strong  process,  which,  however,  is  richly  pro- 
Tided  with  lateral  branches  or  dendrites  (fig.  74).  The  medullary 


B 


ties.    (From  Hatsohek.) 
FiG.54.— Non-medullated  (_Q_..Q  fiv,^ 
FIG.  55.-Medullated          j- nerve-fibres, 
A,     without,     B,    with     sheath     of 
Schwann.    (From  Hatschek.) 


GENERAL  HISTOLOGY.    .  97 

sheath  and  sheath  of  Schwann  are  usually  absent  in  invertebrates 
even  in  the  peripheral  nerves.  A  thin  myelin  layer  has  been  rarely 
observed  in  arthropods  and  annelids.  On  the  other  hand  the  true 
conducting  elements,  the  nerve  fibrillae,  have  been  seen  in  inverte- 


FIG.  56. — Ganglion  cells  of  an  actinian. 

brate  nerve-fibres,  and  these  have  been  followed  into  the  ganglion 
cell  in  which  the  afferent  and  efferent  fibrillae  are  united  in  a 
lattice-like  manner. 


SUMMARY    OF    HISTOLOGICAL    FACTS. 

Cells. — 1.  The  most  important  morphological  element  of  all 
tissues  is  the  cell. 

2.  The  cell  is  a  mass  of  protoplasm  which  contains  one   or 
several  nuclei  (uninucleated,  multinucleated  cells). 

3.  The  nucleus  probably  determines  the  specific  character  of 
the  cell,  since  it  influences  its  functions;  accordingly  it  is  also  the 
bearer  of  heredity. 

4.  Cells  and  nuclei  increase  exclusively  by  division  or  budding. 
Tissues. — 5.   Tissues  are  complexes  of  numerous  similar  his- 

tologically  differentiated  cells. 

6.  Histological  differentiation  rests  in  part  upon  the  fact  that 
the  cells  take  on  a  definite  form  and  arrangement,  in  part  upon  the 
formation  of  plasmic  products,  which  determine  the  character  of 
the  tissue  (muscle-fibres,  connective-tissue  fibrils). 


98  GENERAL  PRINCIPLES   OF  ZOOLOGY. 

Classification  of  Tissues. — 7.  According  to  function  and  struc- 
ture (1)  epithelia,  (2)  connective  tissue,  (3)  muscular  tissue,  (4) 
nervous  tissue  are  distinguished. 

8.  The  physiological  character  of  epithelia  is  determined  by  the 
fact  that  they  cover  the  surfaces  of  the  body,  their  morphological 
character  in  that  they  consist  of  closely  compressed  cells  united 
only  by  a  cementing  substance. 

9.  According  to  their  further  functional  character   epithelia 
are  divided  into  glandular  epithelia  (unicellular  and  multicellular 
glands),  sensory,  germinal,  and  protective  epithelia. 

10.  According  to  the  structure  are  distinguished  simple  (cubi- 
cal,   cylindrical,    squamous    epithelia)    and    stratified    epithelia, 
ciliated  and  flagellated  epithelia,  epithelia  with  or  without  cuticle. 

11.  The  physiological  characteristic  of  the  connective  tissues  is 
that  they  fill  up  spaces  between  other  tissues  in  the  interior  of  the 
body. 

12.  The  morphological  distinction  depends  upon  the  presence 
of  the  intercellular  substance. 

13.  According  to  the  quantity  and  the  structure  of  the  inter- 
cellular substance  the  connective  substances  are  divided  into  (1) 
cellular   (scanty  intercellular   substance);    (2)  homogeneous;    (3) 
fibrous  connective  tissue;  (4)  cartilage;  (5)  bone. 

14.  The   physiological    character   of    muscular  tissue    is  its 
increased  capacity  for  contraction. 

15.  The  morphological  characteristic  is  the  fact  that  the  cells 
have  secreted  muscle-substance. 

16.  According  to  the  nature  of  the  muscle-substance  are  dis- 
tinguished smooth  and  cross-striated  muscle-fibres. 

17.  According  to  the  character  and  origin  of  the  cells  (muscle- 
corpuscles)   the   muscles   are   divided   into   epithelial    (epithelial 
muscle-cells,  primary  bundles)   and  connective-tisue  muscle-cells 
(contractile  fibre-cells). 

18.  The  physiological  distinction  of  nervous  tissue  rests  upon 
the  transmission  of  sensory  stimuli  and  voluntary  impulses,  and 
upon  the  co-ordination  of  these  into  unified  psychic  activity. 

19.  The  conduction  takes  place  by  means  of  nerve-fibres  (non- 
medullated   and  medullated  fibrils  and   bundles  of  fibrils);   the 
co-ordination   of   stimuli   by   means    of   ganglion-cells    (bipolar, 
multipolar  ganglion-cells). 

20.  Blood  and    lymph   are   proteid-containing   fluids;    rarely 
without  cells,  they  may  contain  only  colorless  amoeboid  cells  (white 


GENERAL   ORGANOLOG7.  99 

blood-corpuscles,  leucocytes),  or   in   addition   to   these   also   red 
blood-corpuscles. 

21.  Red  blood-corpuscles  occur,  in  the  main,  only  in  verte- 
brates and  cause  the  redness  of  the  blood ;  they  are  absent  in  most 
invertebrate  animals. 

22.  When    invertebrate   animals    have    colored    blood    (red, 
yellow),  this  is  usually  due  to  the  color  of  the  blood-plasma. 

23.  The  red  blood-corpuscles  are  nonnucleated  in  mammals, 
nucleated  in  all  the  other  vertebrates. 


III.  THE  COMBINATION  OF  TISSUES  INTO  ORGANS. 

An  Organ  Defined. — Organs  are  formed  from  the  tissues.  An 
organ  is  a  tissue  complex,  marked  off  from  the  other  tissues,  which 
has  taken  on  a  definite  form  for  carrying  on  a  special  function. 
Thus  a  single  muscle  is  an  organ  which  consists  of  a  certain 
amount  of  muscular  tissue;  with  scalpel  and  scissors  it  can  be 
removed  from  its  environment  as  a  connected  whole  and  can  still 
accomplish  a  definite  movement. 

Principal  and  Accessory  Tissues. — In  each  organ  there  is  a 
tissue  which  determines  the  function  of  the  organ,  and  therefore 
its  physiological  character.  This  may  be  called  the  principal 
tissue,  for  there  may  be  other  accessory  tissues  present,  which 
merely  support  or  render  possible  the  function  of  the  principal 
tissue.  In  the  muscle  of  the  vertebrates  we  find,  besides  the 
muscle-fibres,  connective  tissue  which,  like  a  kind  of  cement, 
unites  the  bundles  of  muscle;  blood-vessels  which  provide  nourish- 
ment; finally,  nerves  by  which  the  muscles  are  aroused  to  action. 
In  the  human  liver  also,  besides  the  functionally  most  important 
part,  the  liver-cells,  blood-vessels,  nervous  and  connective  tissues 
are  present.  These  accessory  tissues  are  usually  found  only  in  the 
highly  developed  organs;  in  the  case  of  the  lower  animals  they 
may  be  absent ;  thus  the  digestive  tract  of  ccelenterates  has  only 
an  epithelial  lining;  their  nervous  system  consists  merely  of  a 
cord  of  nerve-fibres  and  ganglion-cells. 

Effect  of  Use  and  Disuse. — It  is  of  the  greatest  importance  for 
the  permanency  of  an  organ  that  it  be  constantly  in  function. 
Living  substance  is  distinguished  from  the  non-living  by  the  fact 
that,  if  it  be  destroyed  by  use,  it  is  immediately  replaced,  often  by 
more  than  sufficient  to  make  good  the  loss.  Functioning  tissues 
and  organs  under  favorable  conditions  increase  in  volume;  on  the 


100  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

other  hand,  functionless  parts  undergo  a  gradual  decrease,  which 
finally  leads  to  their  disappearance. 

Change  of  Function  of  Organs. — The  two  factors  mentioned, 
that  the  permanence  of  the  tissues  depends  upon  continued  use, 
and  that  usually  several  tissues  enter  into  the  structure  of  an 
organ,  are  important  for  the  understanding  of  the  principle  of 
change  of  function  which  plays  a  prominent  role  in  the  meta- 
morphosis of  animal  form.  It  may  happen  that  an  organ  is 
brought  under  changed  conditions  and  no  longer  has  an  oppor- 
tunity to  function  as  before.  In  that  case  the  functioning  tissue, 
from  lack  of  use,  gradually  degenerates,  but  the  organ  may  persist 
by  means  of  its  accessory  tissues  if  the  new  conditions  make  it 
possible  for  one  of  them  to  attain  to  functional  activity,  and  to 
give  the  organ  a  new  physiological  character. 

Examples  of  Change  of  Function. — A  muscle,  for  example,  may 
become  functionless  from  many  causes.  Should  the  muscle-tissue 
disappear  there  are  still  left  the  accessory  tissues,  particularly 
connective  tissue  permeated  by  blood-vessels;  this  may  remain  in- 
tact and  form  a  protecting  band,  a  tendon,  or  fascia.  We  have 
then,  morphologically,  the  same  organ,  changed  in  its  physio- 
logical character;  the  muscle  has  undergone  a  change  of  function, 
and  has  become  a  ligamentous  band.  The  visceral  arches  of  fishes 
afford  another  example;  these  primarily  are  supports  for  the  gills; 
if  now  by  the  acquirement  of  terrestrial  habits  the  gills  be  lost, 
the  visceral  arches  become  functionless  and  correspondingly  under- 
go a  partial  degeneration;  but  a  part  persists  by  assuming  a  new 
function,  and  forms  the  jaws,  the  hyoid  bone,  and  the  small  bones 
of  the  ear,  'which,  in  spite-  of  their  quite  different  functions,  are 
morphologically  the  same  structures  as  the  gill-arches. 

Homology  and  Analogy. — In  the  History  of  Zoology  (page  14) 
it  was  shown  that  comparative  anatomy  has  caused  a  discrimina- 
tion between  homology  or  morphological  equivalence,  and  analogy 
or  physiological  equivalence,  i.e.,  between  organs  which  appear  in 
the  same  relative  positions  and  relations,  and  organs  which  have 
the  same  function.  What  we  have  here  learned  of  the  structure 
of  organs  makes  it  evident  that  morphological  and  physiological 
characters  do  not  necessarily  coincide,  that  morphologically  similar 
organs  may  have  different  functions,  morphologically  different 
organs  the  same  functions. 

Systems  of  Organs. — Organs  wholly  identical,  or,  at  least, 
functioning  in  an  equivalent  manner,  may  occur  in  considerable 
numbers  in  the  same  body.  A  man  has  many  muscles,  and  many 


GENERAL   ORGANOLOGT.  101 

organs  which  carry  on  digestion.  Hence  we  may  group  all  organs 
which  in  the  body  have  equivalent  or  similar  functions,  and  speak 
of  systems  of  organs.  In  all  we  recognize  nine  such  systems:  (1) 
skeletal,  (2)  digestive,  (3)  respiratory,  (4)  circulatory,  (5)  excre- 
tory, (6)  genital,  (7)  muscular,  (8)  nervous,  and  (9)  sensory 
systems.  Not  all  are  necessarily  present;  a  skeleton,  for  instance, 
is  frequently  lacking.  The  most  different  functions  which  in  man 
are  divided  among  different  complicated  and  specialized  systems 
may  be  performed  in  a  lower  animal  by  one  and  the  same 
apparatus.  Yet  according  to  the  fundamental  functions  the  fol- 
lowing groups  of  organs  may  be  recognized :  I.  Organs  of  assimila- 
tion (2-5);  II.  Organs  of  reproduction  (6);  III.  Organs  of  motion 
(7) ;  IV.  Organs  of  perception  (8  and  9). 

Vegetative  and  Animal  Organs.— The  organs  of  assimilation  and  of  repro- 
duction (I  and  II)  are  grouped  together  as  vegetative,  the  others  (III  and 
IV)  as  animal  organs.  The  older  zoologists  used  to  say  that  assimilation 
and  reproduction  are  functions  which  are  common  to  animals  and  plants  ; 
that,  on  the  contrary,  sensation  and  motion  are  lacking  in  plants,  and  are 
exclusively  characteristic  of  animals.  The  atom  of  truth  in  the  funda- 
mental idea  needs  reconsideration  in  the  light  of  our  present  knowledge. 
We  have  seen  that  the  protoplasm  of  plants  and  animals  has  not  only  the 
power  of  assimilation  and  reproduction,  but  also  power  of  motion  and  of 
irritability.  The  latter  characteristics  consequently  cannot  be  entirely 
lacking  in  all  the  plants,  for  they  are  found  in  the  most  important.  In 
fact  many  plants,  as  the  mimosas,  the  compass-plants,  insectivorous  plants 
show  great  irritability  ;  many  low  plants,  the  reproductive  states  of  algae, 
move  quite  as  actively  as,  or  even  more  actively  than,  many  of  the  lower 
animals.  On  the  other  hand,  there  are  many  animals  which  in  the  mature 
condition  are  fixed  in  position  like  plants.  Many  Protozoa  and  worms, 
most  of  the  zoophytes,  some  echinoderms  like  the  Crinoids,  even  many 
Crustacea,  the  cirripedes  (barnacles),  can  change  their  location  only  during 
the  earlier  stages  of  development,  in  later  life  being  limited  to  movements 
of  single  parts  of  the  body,  the  arms,  tentacles,  etc.  In  the  sponges 
motions  are  so  insignificant  that  they  cannot  be  seen  at  all  by  the  naked 
eye,  and  scarcely  even  with  the  aid  of  the  microscope. 

Nevertheless  the  two  terms,  animal  and  vegetative,  must  be  retained. 
For  although  motion  and  sensation  occur  in  the  vegetable  kingdom,  still 
they  reach  no  high  development ;  indeed  we  may  say  they  become  more 
and  more  inconspicuous  the  higher  the  plants  ;  in  the  animal  kingdom,  on 
the  contrary,  they  are  unfolded  in  extraordinary  perfection  and  lie  at  the 
basis  of  its  most  characteristic  features. 


102  GENERAL  PRINCIPLES   OF  ZOOLOGY. 


Vegetative  Organs. 
A.    Organs  of  Assimilation. 

Assimilation  Defined. — If  the  term  assimilation  be  used  in  its 
widest  sense,  one  must  speak  in  this  connection  of  all  the  con- 
trivances in  the  animal  body  which  render  growth  possible  during 
the  period  of  progressive  development,  and,  during  mature  life, 
compensate  for  the  loss  of  energy  connected  with  each  period  of 
functional  activity,  in  order  to  preserve  to  the  body  its  functional 
powers.  In  each  period  of  functional  activity  organic  compounds 
are  oxidized.  Compounds  which  are  especially  rich  in  carbon  and 
hydrogen  (as  well  as  some  nitrogen  and  sulphur)  and  are  poor  in 
oxygen  are  changed  by  oxidation  into  carbon  dioxide,  water,  and 
various  nitrogenous  products,  like  urea,  uric  acid,  etc.  A  com- 
pensation takes  place,  for  not  only  is  the  useless  substance 
removed,  but  also  compounds  of  oxygen  and  materials  rich  in 
carbon  are  furnished  to  the  tissues  to  replace  the  material  oxidized. 

Assimilation  in  Animals. — In  lowly  organized  animals  all  the 
processes  connected  with  compensative  assimilative  changes  take 
place  through  the  agency  of  one  and  the  same  organ,  the  digestive 
tract;  but  in  the  higher  animals,  through  specialization,  normal 
assimilation  is  a  definite  series  of  separate  phenomena.  Between 
the  lower  and  the  higher  animals  there  are  evidently  intermediate 
conditions  where  specialization  has  halted  at  an  earlier  or  a  later 
stage. 

Different  Organs  of  Assimilation. — Assimilation  begins  with 
the  presence  of  suitable  food;  the  solid  and  liquid  constituent 
parts  of  the  body  must  digest  and  incorporate  this,  i.e.,  it  must 
be  altered  so  that  it  can  be  absorbed  and  distributed  to  the  tissues. 
All  this  takes  place  through  the  agency  of  the  digestive  tract, 
which  is  provided  with  accessory  organs,  the  digestive  glands;  the 
digestive  tract  likewise  removes  all  matter  remaining  undigested 
(the  faeces).  The  necessary  oxygen,  gaseous  food,  so  to  speak,  is 
usually  taken,  however,  by  particular  parts  of  the  body,  the 
respiratory  organs,  the  gills  or  lungs.  The  oxygen  and  the 
digested  (consequently  liquefied)  organic  and  inorganic  compounds 
must  further  be  distributed  in  the  body  to  the  organs  and  tissues 
according  to  their  needs.  Therefore  there  are  usually  blood- 
vessels or  circulatory  organs,  which  permeate  the  body  in  all 
directions.  But  the  tissues  need  not  only  a  means  of  obtaining 
but  also  of  getting  rid  of  certain  compounds.  The  accumulation 


GENERAL   ORGANOLOGY.  103 

of  the  oxidation  products  arising  from  functional  activity  is 
injurious,  to  some  extent  even  poisonous,  to  the  organism;  conse- 
quently they  must  be  removed,  and  in  a  dissolved  state  they  are 
taken  up  by  the  blood-vascular  apparatus,  and  are  brought  to 
definite  places  for  expulsion  or  excretion.  Fluid  wastes  are 
expelled  by  the  kidneys  of  vertebrates,  the  Malpighian  vessels  of 
insects,  the  water-vascular  system  of  worms;  these,  together  with 
their  accessory  apparatus,  are  embraced  under  the  name  '  excretory 
organs/  Excreta  are  to  be  distinguished  from  fceces;  excreta  are 
substances  which  have  been  a  part  of  the  tissues  of  the  body  itself, 
and,  through  oxidation,  have  become  useless;  while  those  sub- 
stances which  constitute  the  faeces  were  useless  from  the  beginning, 
and  have  never  belonged  to  the  body,  but  have  remained  separated 
from  the  tissues  by  the  boundary  of  the  epithelium  of  the  digestive 
tract.  The  gaseous  oxidation  product  of  the  animal  body,  carbon 
dioxide,  is  removed  by  the  blood-vascular  apparatus  through  the 
agency  of  the  respiratory  organs.  Since  in  the  respiratory  organs 
there  takes  place  an  exchange  of  the  useless  carbon  dioxide  for 
the  oxygen  necessary  to  life,  these  organs  have  a  double  function, 
being,  at  the  same  time,  excretory  organs  and  organs  for  taking 
up  food. 

After  this  general  survey,  we  must  enter  somewhat  mor& 
minutely  into  a  discussion  of  the  various  systems  of  organs. 

I.  The  Digestive  Tract. 

Archenteron  or  Primitive  Digestive  Tract. — Since  the  taking 
in  of  food  and  its  assimilation  are  functions  most  important  for 
the  well-being  of  the  animal,  it  is  to  be  expected  that  of  all  the 
organs  in  the  animal  series  the  digestive  tract  should  be  formed 
first,  and  also  in  almost  every  case  should  be  earliest  established  in 
the  embryo.  The  fact  that  many  worms  (cestodes)  and  Crustacea 
(Rhizocephala)  have  no  digestive  tract  does  not  alter  this  state- 
ment; for  it  can  be  definitely  affirmed  that,  in  adaptation  to 
special  conditions  of  life,  particularly  parasitism,  the  digestive 
tract  has  degenerated.  The  simplest  multicellular,  free-living 
animals  are  merely  simple  or  branched  digestive  pouches  which 
have  only  a  single  opening,  functioning  both  as  mouth  and  anus 
(fig.  57).  Such  an  animal  has  necessarily  two  epithelial  layers, 
one  of  which  lines  the  digestive  tract,  the  other  covers  the  surface 
of  the  body.  These  two  fundamental  cell-layers  are  called  ento- 
derm  and  ectoderm.  In  many  coelenterates  they  are  the  only 


104 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


layers  of  the  body.  In  most  animals  they  are  separated  by  inter- 
mediate tissues,  called  collectively  mesoderm.  The  higher  the 
animal,  the  more  differentiated  is  the  mesodermal  layer.  The 
primitive  digestive  cavity  lined  by  entoderm  is  called  the  archen- 


FIG.  57. 


FIG.  58. 


FIG.  57.— Longitudinal  section  through  the  nutritive  polyp  of  a  siphonophore.  (After 

Haeckel.)    o,  mouth-opening;  en,  entoderm  ;  efc,  ectoderm. 
FIG.  58.— Stenostoma  leucops,  in  division,    a,  ectodermal  fore-gut,  at  a'  forming  anew 

for  the  hinder  animal;  rn,  the  blindly  ending  entodermal  mid-gut;  e,  ectodermal 

ciliated  epithelium ;  0,  ganglion  with  ciliated  pit ;  to,  water-vascular  canal ;  y', 

ganglion  of  the  hinder  animal. 

teron.  In  the  case  of  medusae  and  polyps  it  forms  the  entire  diges- 
tive tract,  but  in  most  animals  this  is  not  sufficient  for  the  needs 
of  digestion  and  the  alimentary  tract  is  increased  by  invaginations 
of  parts  of  the  surface  of  the  body. 

Stomodaeum  and  Proctodaeum. — Even  in  many  coelenterates 
and  lower  worms  an  invagination  arises  at  the  anterior  end  of  the 
digestive  tract,  forming  the  ectodermal  fore-gut  or  stomodmim 
(fig.  58).  From  the  higher  worms  onwards,  it  is  accompanied  by 
a  second  invagination  at  the  hinder  end,  the  ectodermal  hind-gut, 
or  proctodceum  (fig.  59) ;  embryologically,  this  is  formed  as  a  blind 


GENERAL   ORGANOLOGT. 


105 


sac  whose  closed  end  unites  with  the  likewise  closed  posterior  part 
of  the  archenteron  (now  called  also  mesenteron  or  mid-gut)  until 
the  separating  wall  disappears,  whereupon  mid-  and  end-gut  com- 
municate with  each  other,  and  the  digestive  tract  becomes  a  canal 
extending  through  the  entire  body. 

Divisions  and  Appendages  of  the  Digestive  Tract. — The  part 
which  the  archenteron  takes  in  comparison  with  the  ectodermal 


FIG.  59. 


FIG.  60. 


FIG.  59.— Bee-larva  just  after  hatching :  seen  from  the  ventral  surface.  The  diges- 
tive tract  consists  of  three  portions;  a,  fore-gut;  m,  mid-gut;  e,  hind-gut  (not  yet 
connected  with  the  mid-gut) ;  sg,  limits  of  segments ;  st,  stigma ;  t,  trachea ;  n, 
ventral  nerve-cord.  (After  Butschli.) 

FIG.  60.— Digestive  tract  of  the  domestic  fowl,  a,  oesophagus ;  b,  crop;  c,  glandular 
stomach;  d,  gizzard  ;  e,  liver ;  f,  gall-bladder ;  gr,  pancreas ;  ft,  t,  small  intestine; 
fc,  caeca ;  J,  large  intestine  ;  m,  ureters ;  n,  oviduct ;  o,  cloaca. 

proctodseum  and  stomodaeum  in  making  up  the  completed  diges- 
tive tract  is  very  different  in  the  various  groups.  On  one  side  the 
Crustacea,  on  the  other  side  the  vertebrates,  offer  the  strongest 


106  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

contrast;  the  Crustacea  have  a  very  short  mid-gut  and  consequently 
a  long  extent  of  fore-  and  hind-gut  formed  from  the  ectoderm;  in 
vertebrates,  on  the  contrary,  the  ectodermal  portions  are  extremely 
short. 

The  width  of  the  lumen  varies  in  the  course  of  the  alimentary 
canal  and  renders  possible  the  distinction  of  different  divisions, 
which,  so  far  as  possible,  have  been  provided  with  uniform  names. 
Fig.  60,  drawn  from  a  domestic  fowl,  illustrates  the  usual  terms. 
The  mouth-opening  leads  into  a  wider  cavity,  which  is  usually 
divided  into  an  anterior  division,  the  buccal  cavity,  and  a  posterior 
one,  the  pharynx.  The  narrow  tube  leading  from  this  is  the 
oesophagus  (a) ;  here  and  there  it  may  widen,  or  bear  a  pouchlike 
pagination,  the  crop  or  ingluvies  (#),  for  the  temporary  reception 
of  food.  From  the  oasophagus  the  food  passes  into  a  considerable 
enlargement,  the  stomach.  Birds,  like  many  other  animals,  have 
a  double  stomach,  a  thin-walled  portion  rich  in  glands,  and  a 
second  part,  the  walls  of  which  are  remarkable  for  the  thick 
masses  of  muscle;  the  former  is  the  glandular  stomach  (c],  the 
latter  is  the  grinding  stomach  or  gizzard  (d),  serving  for  comminu- 
tion of  the  food.  Behind  the  stomach  the  digestive  tube  narrows 
into  the  small  intestine  (h),  the  hinder  widened  part  of  which  is 
the  large  intestine  (/),  terminating  in  the  anus.  The  limit  of  the 
small  and  large  intestine  is  usually  marked  by  blind  pouches,  the 
cceca  (&).  Connected  with  the  anal  gut  also  are  the  outlets  of  the 
kidneys  (m)  and  of  the  sexual  apparatus  (n) ;  hence  the  terminal 
portion,  serving  as  the  outlet  for  the  urine  and  faeces,  and  also  for 
the  sexual  products,  is  called  the  cloaca  (o). 

In  animals  which  require  abundant  food  the  area  of  the 
alimentary  tract  is  not  sufficient  to  furnish  the  digestive  fluids,  so 
that  evaginations  of  the  wall  (glands)  serve  to  increase  this.  Into 
the  mouth  empty  the  salivary  glands;  into  the  first  part  of  the 
small  intestine,  close  behind  the  stomach,  the  liver  (e)  and  the 
pancreas  (g)  (or  a  single  glandular  apparatus,  whose  secretion 
combines  the  characters  of  gall  and  of  pancreatic  juice,  the  hepato- 
pancreas).  Finally,  in  the  hind-gut  there  sometimes  occur  glands 
which  form  a  fetid  secretion — the  anal  glands.  The  length  of  the 
digestive  tract  is  chiefly  influenced  by  the  kind  of  food.  In  many 
groups  of  animals  there  is  found  a  difference  between  herbivores 
and  carnivores,  the  former  having  a  very  long  and  consequently 
convoluted  digestive  tract.  That  of  a  carnivore  is  about  four  or 
five  times  the  length  of  the  body,  while  in  an  herbivorous  ungulate, 
on  the  other  hand,  it  is  twenty  to  twenty-eight  times.  Similar, 


GENERAL   ORGANOLOQY.  107 

though  -not  so  great,  are  the  differences  between  carnivorous  and 
plant-eating  beetles. 

II.  Respiratory  Organs. 

Sources  of  the  Oxygen  used  in  Breathing. — The  oxygen  which 
each  animal  must  obtain  in  exchange  for  the  carbon  dioxide  formed 
in  the  tissues  is  derived  either  from  the  air  or  from  the  water, 
according  as  the  animal  is  terrestrial  or  aquatic.  Less  frequently 
it  is  the  case  that  water-dwellers  breathe  air,  and  hence  are  com- 
pelled, from  time  to  time,  to  rise  to  the  surface  of  the  water  for  a 


FIG.  61.— Left  second  foot  o£  a  crayfish  with  attached  gill  (br).  (After  Huxley.)  czp, 
coxopodite;  bp,  oasipodite;  ip,  ischiopodite;  mp,  meropodite;  cp,  carpopodite;  pp, 
propodite;  dp,  dactylopodite;  cxs,  bristles  of  the  coxopodite;  e,  lamina  of  the  gill. 

supply  of  air;  this  is  true  for  the  great  marine  mammals,  and  for 
many  insects,  spiders,  and  snails  which  are  found  in  fresh  water. 
Air-  and  water-breathing  takes  place  exclusively  through  the  skin, 
so  long  as  this  is  delicate  and  readily  permeable,  and  so  long  as 
no  higher  development  of  organization  necessitates  a  more  active 
interchange  of  material.  If,  on  the  other  hand,  the  demand  for 
oxygen  be  greater,  other  more  special  breathing-organs  are  found 
— gills  for  water-breathing,  lungs  and  tracheae  for  air-breathing,  in 


108 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


addition  to  which  the  skin  functions  as  an  accessory  organ  of  more 
or  less  importance. 

Gills. — The  gills  are  usually  thin-walled  areas  of  the  skin 
which  are  abundantly  supplied  with  blood-vessels,  and  where  richly 
branched  tuftlike  projections  or  broad  leaves  have  grown  out,  thus 
furnishing  the  largest  possible  surface  for  the  interchange  of  gases; 
these  occur  in  such  a  position  as  to  be  most  exposed  to  fresh  water ; 
in  the  crayfish,  for  example,  they  are  on  the  legs,  where  the  motion 
drives  fresh  water  constantly  through  them  (fig.  61);  in  the 
swimming  worms,  on  the  back;  in  the  tube-dwelling  worms,  at 
the  anterior  end,  projecting  out  of  the  tube  (fig.  62);  in  most 


FIG.  62.— Anterior  end  of  Terebella  nebulosa.     (After  Milne  Edwards.)  ph,  pharynx ; 
-yd,  dorsal,  vv,  ventral,  blood-vessel ;  br,  gills ;  t,  tentacles. 


amphibians  (fig.  4),  on  each  side  of  the  neck.  More  rarely  the 
digestive  tract  functions  for  water-breathing;  in  the  fishes, 
Enteropneusta,  and  tunicates  gills  have  been  formed  in  connection 
with  the  pharynx,  its  lateral  walls  being  pierced  by  the  gill-slits, 
which  open  to  the  exterior  on  the  surface  of  the  body.  The  water 
containing  oxygen  in  solution  passes  out  through  the  gill-slits,  and 
bathes  the  gill-filaments,  which  are  richly  provided  with  blood- 
vessels. The  hind-gut  also  in  many  fishes,  insects,  and  worms 


GENERAL   ORGANOLOGY.  109 

may  become  an  accessory  respiratory  organ,  being  filled  from  time 
to  time  with  fresh  water. 

Aerial  Respiration. — In  the  air-breathing  animals  the  respira- 
tory apparatus  is  derived  either  from  the  digestive  canal  or  from 
the  skin.  With  the  vertebrates  the  former  is  the  case,  since  the 
lungs,  either  directly  or  by  the  mediation  of  the  trachea  and  bronchi, 
are  in  connexion  with  the  lumen  of  the  digestive  tract.  On  the 
contrary,  in  the  case  of  invertebrate  animals  (snails  and  spiders) 
when  the  term  '  lung '  is  used,  it  refers  always  to  an  invaginatioii 
or  sac  of  the  skin;  of  such  a  nature  are  the  tracheae  of  insects, 
tubes  containing  air,  beginning  at  the  surface  of  the  body  with  a 
hole  or  stigma,  and  branching  internally  (fig.  59,  st). 

Distinctions  between  the  Respiratory  Systems  of  Chordates 
and  Invertebrates. — In  general,  then,  a  distinction  can  be  drawn 
between  the  respiratory  systems  of  vertebrate  and  invertebrate 
animals :  in  the  former,  the  digestive  tract,  or  derivatives  from  it, 
are  respiratory;  in  the  latter,  on  the  contrary,  it  is  the  skin.  On 
the  side  of  the  vertebrates  the  only  exceptions  are  most  amphibians 
and  a  few  fishes  (Protopterus),  in  which  the  gills  are  tuf tlike  pro- 
jections of  the  skin  (figs.  4  and  5);  while  among  the  invertebrates 
some  aquatic  insects  respire  by  the  hinder  end  of  the  digestive 
tract. 

III.  Circulatory  Apparatus. 

In  order  that  the  oxygen,  taken  up  by  the  respiratory  organs? 
and  the  constituents  of  the  food  digested  in  the  alimentary  canal 
may  reach  the  tissues,  there  is  no  need  of  special  organs,  so  long 
as  the  body  consists  of  only  two  thin  epithelial  layers,  the  ectoderm 
and  entoderm.  When,  however,  a  third,  a  mesodermal,  layer  is 
interpolated  between  these,  and  the  body  consequently  becomes 
more  bulky,  there  is  usually  some  apparatus  for  distributing  .the 
food.  The  simplest  is  when  the  digestive  tract  departs  from  the 
character  of  a  straight  tube  and  branches,  and  by  means  of  these 
branches  extends  into  the  various  parts  of  the  body.  We  speak 
then  of  a  g astro-vascular  system,  because  the  alimentary  canal 
itself  takes  on  the  function  and  the  branching  arrangement 
generally  characteristic  of  the  vessels  or  '  vascula'  (fig.  63). 

Coelom. — The  coslom  or  enterocoele  is  apparently  derived  from 
a  pair  of  gastric  diverticula  which  have  become  completely  cut  off 
from  the  archenteron  (compare  development  of  mesoderm,  infra). 
It  is  a  cavity  pushed  in  between  the  intestinal  tract  and  the  body- 
wall,  is  lined  by  a  special  epithelium,  the  peritoneum,  and  encloses 


110 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


most  of  the  vegetative  organs.  If  the  two  halves  of  the  coelom 
approach,  without  uniting,  dorsal  and  ventral  to  the  gut,  the 
result  is  dorsal  and  ventral  membranes,  the  mesenteries,  which 
support  the  alimentary  canal.  Of  these  the  ventral  is  most  fre- 
quently, the  dorsal  least  often,  degenerate.  In  many  invertebrates 
the  coelom  plays  an  important  role  in  nutrition  since  it  contains  a 
lymphoid  fluid,  rich  in  proteids  and  containing  cellular  corpuscles. 
It  loses  this  significance  the  more  the  blood  system  is  developed, 


FIG.  63. 


FIG.  64. 


FIG.  (&.—Leptoplana  tremellaris.  a,  mouth ;  b,  buccal  cavity ;  c,  opening  of  the  head 
of  the  pharynx  into  the  buccal  cavity ;  d,  central  stomach ;  e,  branched  ento- 
dermal  gut;  /,  ganglion;  0,  testicle;  ft,  seminal  vesicle;  /c,  uterus;  /,  receptaculum 
seminis  ;  m,  female  sexual  opening. 

FIG.  64.— Schema  of  circulation  of  the  blood,  a,  arteries  ;  c,  capillaries ;  7i,  auricle; 
/c,  ventricle  ;  fel,  valves ;  p,  pericardium  ;  v,  veins. 

and  in  the  vertebrates,  so  far  as  nutrition  is  concerned,  it  is  a 
rudimentary  organ. 

A  sharp  distinction  should  be  drawn  between  the  ccelom  and  other 
cavities  in  the  body.  Not  every  '  body  cavity '  is  a  coelom,  but  frequently 
there  occur  large  spaces  which  are  entirely  different  in  origin  and  in 
relations.  Frequently,  as  in  arthropods,  these  'body  cavities'  contain 
blood  and  are  in  reality  but  expansions  of  the  vascular  system.  To  such 
cavities  the  term  hcemocode  has  been  given. 

Heart,  Arteries,  Veins,  Capillaries. — The  most  complete 
method  of  food  distribution  is  accomplished  by  the  Uood-vessels, 


GENERAL   OROANOLOGY.  Ill 

which,  therefore,  belong  generally  to  the  higher  animals,  and 
function  whether  a  body  cavity  is  present  or  not.  Blood-vessels 
are  tubes  with  fluid  contents,  the  blood,  which  transports  the 
oxygen  received  through  the  respiratory  organs,  as  well  as  the  food 
absorbed  from  the  digestive  tract,  and  later  gives  these  up  to  the 
tissues.  Since  such  an  interchange  of  substances  presupposes  that 
the  blood  circulates  in  the  vessels,  definite  parts  in  the  course  of 
the  blood-vessels  are  contractile;  they  are  covered  by  muscles  which 
by  their  contraction  narrow  the  tube  and  push  the  fluid  forwards. 
In  the  lower  forms  wide  areas  in  the  course  of  the  blood-vessels  are 
contractile;  in  higher  animals  a  greater  regularity  of  circulation 
is  reached ;  a  definite  specialized  muscular  part  of  the  course,  the 
heart,  alone  propels  the  blood. 

The  Higher  Development  of  the  Heart. — A  free  motion  of  the 
heart  is  only  possible  when  it  is  separated  from  the  contiguous 
tissues  and  enclosed  in  a  special  cavity  (fig.  64).  Hence  we  see 
that  the  heart  always  lies  either  free  in  the  body  cavity  or  enclosed 
in  a  special  pouch  ( p),  the  pericardium  (in  all  cases  a  portion  of 
the  general  body  cavity,  but  not  always  of  the  coslom,  which  has 
become  independent).  The  division  of  the  heart  into  a  part  which 
receives  the  blood,  the  atrium  or  auricle  (h),  and  a  part  which 
drives  the  blood  onward,  the  ventricle  (&),  is  of  less  functional 
importance;  hence  this  division  is  not  carried  out  in  all  cases. 
There  are  also  special  mechanisms  within  the  heart,  the  valves 
(kl),  which,  by  closing,  prevent  the  blood  from  flowing  back  when 
the  walls  relax  at  the  end  of  the  contraction. 

Blood-vessels. — In  order  that  the  blood  system  may  properly 
perform  its  function,  in  addition  to  circulation,  it  is  necessary  that 
the  nutritive  substances  be  readily  taken  up  and  given  out  again 
to  the  tissues.  The  part  of  the  course  of  circulation  concerned  in 
this  must  have  easily  permeable  walls,  must  be  widely  distributed 
in  the  body,  and  have  a  large  superficial  area.  These  demands  are 
met  by  the  capillaries  (c),  extremely  fine  and  thin-walled  tubes, 
which  surround  and  permeate  all  organs.  Through  their  walls, 
usually  formed  of  a  thin  epithelial  layer  alone,  the  proteid  substances 
for  nourishing  the  tissues  can  pass,  and  the  oxygen  can  be 
exchanged  for  carbon  dioxide.  Between  the  heart  and  the  capil- 
laries there  exists,  corresponding  to  their  different  functions,  great 
differences  in  structure;  they  must  therefore  be  united  by  special 
transitional  vessels — vessels  which  begin  large  and  thick-walled  at 
the  heart,  and  by  branching,  and  thinning  of  their  walls,  pass 
gradually  into  the  capillaries;  of  such  vessels  there  are  two  kinds, 


112 


GENERAL  PRINCIPLES   OF  ZOOLOGY, 


the  firmer  arteries  (a)  leading  to  the  capillary  region,  and  the 
thinner- walled  veins  (v)  leading  back  to  the  heart. 

Correlation  of  Respiratory  Organs  and  Blood  System, — It  is  a 
law  that  in  all  animals  the  blood-vascular  system  has  been  influ- 
enced in  its  arrangement  and  structure  more  by  respiration  than 


FIG.  65.— Scheme  of  circulation  in  a  fish,  a',  ascending  (ventral)  aorta;  a2,  descend- 
ing (dorsal)  aorta ;  c,  carotid  ,  da,  intestinal  arteries ;  dc,  intestinal  capillaries , 
dv,  intestinal  veins;  7i,  auricle;  k,  ventricle;  Tea,  afferent  gill-a.rteries;  kv,  efferent 
gill-arteries ;  Zc,  liver-capillaries ;  sc,  body-capillaries ;  i>c,  cardinal  veins  ;  vTi, 
hepatic  vein  ;  ty',  jugular  vein. 

by  nutrition  in  the  narrower  sense;  there  exists  a  correlation 
between  the  organs  of  respiration  and  of  circulation.  A  double 
capillary  region  must  be  distinguished;  besides  the  body  capillary 
system  already  mentioned  there  is  the  respiratory  capillary  region, 
whose  exclusive  office  is  to  remove  the  carbon  dioxide  from  the 


GENERAL   ORGANOLOGY.  113 

blood  and  to  furnish  oxygen  to  it  (gill  and  lung  capillaries).  A 
twofold  capillary  region  makes  necessary  also  a  twofold  system  of 
arteries  and  veins  (systemic  arteries  and  systemic  veins,  respira- 
tory arteries  and  respiratory  veins).  The  accompanying  diagram 
(fig.  G5)  of  the  blood  circulation  of  fishes  illustrates  this.  Veins 
lead  from  the  capillary  region  of  the  tissues  of  the  body  to  the 
auricle  of  the  heart;  from  the  auricle  the  blood  flows  into  the 
ventricle,  and  through  the  afferent  gill-arteries  into  the  gill-capil- 
laries. Thence  it  is  conducted  through  the  'gill-veins7  (eiferent 
arteries),  which  unite  into  a  single  large  trunk;  this  again  gives 
off  lateral  branches  passing  into  the  capillary  region  of  the  body. 
Since  the  branches  of  the  main  trunk  formed  by  the  *  gill- veins' 
lead  again  into  a  capillary  region,  they  must,  like  the  main  stem, 
be  called  arteries. 

Arterial  and  Venous  Blood. —  During  its  course  through  the 
body  the  blood  twice  changes  its  chemical  character  and  corre- 
spondingly its  color.  The  blood  which  flows  from  the  body 
capillary  region  has  given  up  its  oxygen  to  the  tissues,  receiving 
in  exchange  carbon  dioxide,  and  has  become  dark  red.  This 
character  is  maintained  until,  in  the  gill-capillaries,  it  again 
becomes  oxygenated,  giving  up  the  carbon  dioxide  and  becoming 
bright  red.  The  different  character  of  the  blood  can  be  recognized 
in  the  arteries  and  veins  of  the  systemic  circulatory  system;  the 
dark  blood  containing  carbon  dioxide  is  called  venous,  and  the 
bright  red,  containing  oxygen,  arterial  blood,  since  the  former 
flows  in  the  veins,  the  latter  in  the  arteries.  These  terms  are 
entirely  unsuitable,  as  can  readily  be  seen  from  the  above  diagram 
(fig.  65),  because  they  easily  lead  to  the  false  assumption  that  veins 
must  always  conduct  blood  containing  carbon  dioxide,  and  arteries 
always  oxygenated  blood.  In  opposition  to  this,  the  diagram 
shows  that,  in  the  respiratory  circulation  (the  shorter  course),  the 
conditions  must  be  the  reverse  of  those  in  the  systemic  circula- 
tion, since  here  the  arteries  contain  ( venous/  while  the  veins 
contain  '  arterial/  blood. 

Closed  and  Lacunar  Blood-vascular  Systems. — Such  a  blood- 
vascular  system  as  has  here  been  described  is  called  a  closed  one, 
because  the  blood  always  flows  in  special  tubes  provided  with  their 
own  walls.  Opposed  to  the  closed  stands  the  lacunar  blood-vascular 
system;  here  the  blood-vessels  lose,  after  a  time,  the  character  of 
tubes  and  become  wide  cavities,  or  sinuses,  which,  without  special 
walls,  are  enclosed  between  the  intestines  and  other  organs 
(hsemocoale,  supra). 


114 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


Example  of  Lacunar  Blood-vascular  System.  —  The  best  exam- 
ple of  a  lacnnar  blood-vascular  system  is  furnished  by  the  insects 
and  myriapods,  which  have  only  the  heart  and 
short  arterial  trunks;  from  the  ends  of  the 
arteries  the  blood  enters  the  haemocoele,  and 
from  this  through  lateral  slits  (ostia)  again 
enters  the  heart  (fig.  66).  In  the  groups  of 
arthropods  and  molluscs  are  found  all  transi- 
tions between  so  extreme  a  case  of  a  lacunar 
blood  -vascular  system  and  the  almost  com- 
pletely closed  one.  Here  appears  again  a 
close  correlation  of  the  circulatory  and  respir- 
atory organs,  the  latter  determining  the 
development  of  the  former.  If  the  respira- 
tion  be  diffusely  distributed  over  or  through 
the  body,  and  the  distribution  of  the  oxygen 
goes  on  without  special  vessels,  the  circula- 
tory apparatus  is  very  simple;  on  the  other 
hand,  if  the  respiration  be  connected  with 
definitely  restricted  areas,  and  a  regular  dis- 

i«.  uu.  —  .fi.ii  bei-iui:  eiiu  \JL      ,      .,        .  .      "          ,,  ,  ., 

the  heart  of  Scoiopen-  tribution  of  oxygen  be  necessary,  the  appara- 
tus  is  differentiated  into  heart,  arteries,  veins, 
n!'  alary  and  capillaries.     Details  may  be  found  in  the 
f  heart  j   sections  on  crustaceans,  spiders,  and  insects, 

' 


FIG.  66.— Anterior  end  of 


o,  ostia.  f 

Lymph-  vessels.  —  A  special  part  of  the  vascular  system  is  the 
lymph  system,  which  is  known  only  in  vertebrates.  In  the  capil- 
lary region  of  the  body,  it  is  true,  proteids  may  pass  over  into  the 
tissues,  but  it  is  evident  that  a  possible  overflow  cannot  re-enter 
the  blood-vessels  in  the  same  way,  on  account  of  the  higher  pres- 
sure prevailing  in  the  capillaries.  This  overflow  is  conducted  back 
to  the  veins  through  the  lymph-vessels.  The  lymph-vessels  begin 
with  lacunae  in  the  tissues,  and  gradually  pass  into  vessels  with 
definite  walls.  The  lymph-vessels  of  the  digestive  tract  are  par- 
ticularly important  since,  during  digestion,  they  become  filled  with 
the  proteid  and  fatty  constituents  of  the  digested  food;  they  are 
called  the  chyle-vessels,  because  they  contain  the  chyle,  distin- 
guished from  ordinary  lymph  by  its  milky  color. 

Cold-  and  Warm-blooded  Animals.  —  In  connexion  with  the 
blood-vascular  system,  two  expressions  are  much  used  but  not 
generally  correctly  understood  by  the  general  public,  viz.,  cold- 
blooded and  warm-blooded  —  or,  more  correctly,  animals  with 


GENERAL   ORGANOLOGT.  115 

variable  and  animals  with  definite  temperatures.  Under  the  head 
of  animals  with  varying  temperature  (poikilothermous)  or  cold 
blood  are  placed  forms  whose  temperature  is  largely  dependent 
upon  the  temperature  of  the  environment,  rising  and  falling  with 
it,  but  usually  a  few  degrees  above  it.  In  our  climate,  where  the 
atmospheric  temperature  is  considerably  lower  than  the  tempera- 
ture of  the  human  body,  such  animals,  for  example  the  frog, 
would  feel  cold  to  our  touch,  since  they,  particularly  in  the  cool 
season,  have  a  much  lower  temperature  than  we. 

Such  creatures  as,  living  under  any  thermal  condition,  maintain 
about  the  same  temperature,  are  termed  warm-blooded  or  definite- 
temperatured  (idiothermous,  homoiothermous)  animals.  Man  in 
summer  and  winter,  under  the  equator  and  at  the  north  pole,  has 
approximately  a  temperature  of  36°  C.  (98|°  F.),  showing  higher 
temperatures  only  in  fever.  In  order  to  maintain  a  constant  tem- 
perature during  the  varying  external  conditions,  the  animal  must 
have  a  heat-regulator;  it  must  have  the  power  to  regulate  the 
warmth  of  its  body,  on  the  one  hand  by  limiting  the  production  of . 
heat,  on  the  other  by  controlling  its  loss.  If  the  environment  be 
warmer  than  is  suitable  for  the  body  temperature,  then  the  pro- 
duction of  heat  must  be  limited  to  the  smallest  quantity  com- 
patible with  the  vital  processes;  but,  if  this  does  not  suffice,  the 
loss  of  heat  must  be  increased  by  evaporation  from  the  surface, 
usually  accomplished  by  active  perspiration.  If,  on  the  contrary, 
the  environment  be  cold,  then,  conversely,  every  unnecessary  loss 
of  heat  must  be  avoided,  while  the  production  of  heat  must  be 
increased.  It  is  clear  that  idiothermy,  since  it  requires  compli- 
cated apparatus,  can  occur  only  in  the  highly  organized  animals. 

IV.  Excretory  Organs. 

Nature  of  the  Organs  of  Excretion. — The  excretory  organs  are 
tubes  or  glandular  canals  which  open  upon  the  surface  of  the  body, 
either  directly  or  by  way  of  an  end-gut  (cloaca),  and  conduct  sub- 
stances which  have  become  useless  to  the  body  to  the  exterior. 

The  presence  of  a  blood-vascular  system  or  a  coelom  or  both 
together  exercises  an  important  influence  on  their  structure. 
When  neither  are  developed  the  excretory  tubules  in  order  to 
remove  the  excreta  from  the  tissues  must  branch  and  penetrate 
the  body  in  all  directions  like  a  drainage  system,  being  frequently 
connected  in  a  network  recalling  the  blood-capillaries  (proto- 
nephridia  or  water-vascular  st/stem  of  parenchymatous  worms, 


116 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


fig.  67). 


The  canals  begin  with  closed  tubes,  which  are  provided 
internally  at  the  end  with  a  bundle  of 
actively  vibrating  cilia,  the  '  flame '  (fig. 
68).  One  or  more  main  trunks  lead  from 
the  canal  system  to  the  exterior.  A  little 
before  the  external  opening  (excretory 
pore)  there  is  frequently  a  contractile 
enlargement,  the  urinary  bladder. 

With  the  appearance  of  a  coelom  there 
is  a  central  place  for  the  collection  of 
excreta.  The  nephridia  or  segmental 
organs  —  usually  simple  tubes  (rarely 
branched)  open  at  both  ends — lead  from 
this  to  the  exterior.  One  opening  is  ex- 
ternal (fig.  69),  the  other  communicates 
with  the  co3lom  by  means  of  a  ciliated 
n 


FIG.  67. 


FIG.  68. 


FIG.  67.— Distomum    hepaticum   with   water-vascular    system.      (From    Hatschek.) 

jp,  porus  excretorius ;  o,  mouth. 
FIG.  68.— Blind  end  of  one  of  the  finest  water-vascular  canals  (k)  of  a  Turbellarian. 

(From  Lang.)    n,  nucleus ;  /,  processes  of  the  terminal  cell ;  it1/,  '  flame '  of  the 

terminal  cell ;  u,  vacuole. 

funnel,  the  nepJirostome,  a  wide  mouth  with  active  cilia  which 
connects  with  the  canal  of  the  tube.  Through  this  the  ex- 
cretion (in  annelids  peritoneal  cells  laden  with  guanin — the  dis- 
integrated '  chloragogue '  cells)  is  carried  to  the  outside. 

The  excretory  organs  (kidneys)  of  vertebrates  are  derived  from 
such  nephridia.  The  fact  that  in  the  embryos  (and  frequently  in 
the  adults)  these  open  into  the  coelom  by  nephrostomes  makes  it 
probable  that  also  in  the  vertebrates  the  coelom  was  once  important 
in  excretion  (fig.  70).  The  increasing  importance  of  the  blood- 
vessels which  envelop  the  nephridial  canals  and  bring  to  them 
the  waste  matter  taken  from  the  tissues  is  probably  the  cause  of 
the  loss  of  connexion  of  the  kidneys  with  the  coelom  by  degenera- 


GENERAL   ORGANOLOGY. 


117 


tion  of  the  nephrostomata.  The  relation  of  the  blood-vessels  to 
the  nephridial  tubes  becomes  specially  close  by  the  development 
of  the  glomeruli  (Malpighian  corpuscles);  bundles  of  capillaries 


»fe 


N*  M 


FIG.  69. 

FIG.  69. — Segmental  organ  of  an  Oligochaete.  (From  Lang.)  fz,  ciliated  funnel ;  dis, 
septum  ;  ngl,  non-glandular,  tig'1,  glandular,  part  of  the  canal ;  eh,  terminal  ves- 
icle ;  In,  body- wall. 

FIG.  70.— Diagram  of  the  primitive  kidney  of  a  vertebrate.  (From  Hatschek.)  Dotted 
lines  mark  the  limits  of  the  segments.  A,  anal  opening;  P,  mouth  of  the  duct  of 
the  primitive  kidney  (W);  Ns,  nephrostome ;  M,  Malpighian  bodies  of  the  seg- 
mental  tubules  (S). 

carrying  the  walls  of  the  canal  before  them  and  so  projecting  into 
the  lumen  of  the  tube. 


B.   Sexual  Organs. 

Sexual  Glands  and  Ducts. — In  the  sexual  apparatus  of  animals 
are  distinguished  the  areas  where  the  germinal  cells  are  produced, 
the  sexual  glands  or  gonads,  and  the  ducts  for  these.  The  former 
are  present,  temporarily  or  permanently,  in  all  multicellular 
animals;  the  latter,  on  the  contrary,  may  be  completely  absent. 
If  the  sexual  products  arise  in  the  skin  or  in  the  walls  of  the 
digestive  tract,  as  is  usually  the  case  in  the  coelenterates,  then 


118 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


special  outlets  are  superfluous,  since  the  ripe  elements  can  reach 
the  exterior  directly  by  rupture  of  their  covering  or  by  means  of 
the  digestive  tract. 

Germinal  Epithelium  and  Germinal  Glands. — -Male  and  female 
sexual  cells,  as  we  have  seen,  originate  from  an  undifferentiated 
incipient  organ,  or  anlage,  which  is  called  the  germinal  epithelium. 
Usually  it  forms  a  part  of  the  epithelial  lining  of  the  body  cavity, 
in  many  animals  permanently,  in  others  only  temporarily;  in  the 
latter  case  it  separates,  usually  by  constriction,  and  forms  gland- 
like  bodies,  the  gonads  or  sexual  glands. 

Gonochorism  and  Hermaphroditism. — In  most  animals  the 
germinal  epithelium  produces  either  only  female  or  only  male 
sexual  cells;  such  animals  are  called  separate-sexed,  dio&cioiis  or 


di 


FIG.  71.— Sexual  organs  of  Lumbricm  agricola.  (From  Lang,  after  Vogt  and  Yung.) 
The  seminal  vesicles  of  the  right  side  are  removed,  Zwi,  ventral  nerve- cord;  IIP 
and  of,  ventral  and  lateral  rows  of  setae;  st1,  st2,  receptacula  seminis;  sb1,  sb2,  TO", 
the  three  seminal  vesicles  of  the  left  side,  which  are  connected  with  a  median 
unpaired  seminal  capsule  (sbw).  Enclosed  in  the  latter  are  the  anterior  and  pos- 
terior testes  (Ti1,  ft2),  and  the  anterior  and  posterior  seminal  funnels  (t1,  t2),  which 
lead  into  the  vas  deferens  (vd).  o,  ovaries;  to,  ciliated  funnels  leading  into  the 
oviducts  (ov);  di,  dissepiments;  VIII-XV,  eighth  to  fifteenth  segments. 


gonochoristic,  in  opposition  to  the  hermaphroditic  forms,  in  which 
both  kinds  of  sexual  glands  are  contained  in  one  and  the  same 
individual.  Different  degrees  of  hermaphroditism  can  be  distin- 
guished; commonly  testes  and  ovary  are  contained  in  the  same 


GENERAL  ORGANOLOGT.  119 

animal,  some  distance  apart,  as  in  the  earthworm,  in  which  two 
segments  are  male,  while  a  third  segment  is  female  (fig.  71). 
More  rarely  there  is  a  union  of  testes  and  ovary  into  a  single 
glandular  body  or  hermaphroditic  gland;  our  land-snails  have  an 
hermaphroditic  gland,  which  produces  spermatozoa  and  eggs  in 
the  same  follicle. 

Occurrence  of  Hermaphroditism.  —  Hermaphroditism  is,  in 
general,  of  more  frequent  occurrence  in  the  lower  than  in  the 
higher  animals.  Insects  and  vertebrates  are,  almost  without 
exception,  dioecious;  only  two  cases  of  normal  hermaphroditism 
are  known  among  them,  a  sea-perch,  Serranus  scriba,  a  bony  fish, 
and  Myxine  glutinosa,  the  hagfish.  More  commonly  hermaph- 
roditism occurs  as  an  abnormality;  a  striking  form  is  lateral 
hermaphroditism,  in  which  one  half  of  the  animal  has  only  male, 
the  other  half  only  female,  gonads.  If  the  males  and  females  of 
a  species  be  distinguishable  by  their  appearance,  then  lateral 
hermaphroditism  is  expressed  in  their  external  form,  since  one  half 


FIG.  72.— Lateral  hermaphroditism  of  a  gipsy  moth  (Ocneria  dispar).    Left  female, 
right  male.    (After  Taschenherg.) 

of  the  animal  has  the  characteristic  marks  of  the  male,  the  other 
half  those  of  the  female.  Hermaphroditic  lepidoptera  and  bees 
are  known  in  which  the  male  half  bears  the  special  form  of  the 
male  antennas,  eyes,  and  wings,  and  thus  is  essentially  different  from 
the  female  half  (fig.  72).  Still  it  must  be  noted  here  that,  in 
many  instances  where  the  external  appearance  pointed  towards 
hermaphroditism,  anatomical  investigation  has  disclosed  either 
only  male  or  only  female  sexual  glands  in  a  rudimentary  condition 
(gynandromorphism).  True  hermaphroditism  (the  presence  of 
both  kinds  of  sexual  glands  in  the  same  animal)  is  extremely  rare 
in  mammals  and  in  man.  What  is  described  as  hermaphroditism 
does  not  in  the  majority  of  cases  deserve  the  name. 


120 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


Genital  Ducts.  —  Very  frequently  in  the  animal  kingdom  the 
excretory  apparatus  furnishes  outlets  for  the  sexual  products.  In 
the  annelids  and  in  the  vertebrates  portions 
of  the  nephridial  system,  either  exclusively 
or  in  addition  to  their  excretory  function, 
become  accessory  sexual  organs.  Hence  we 
speak  of  a  urogenital  system.  This  remark- 
able connexion  of  genital  and  excretory 
organs  has  a  double  cause,  a  physiological  and 
an  anatomical.  Physiologically  important  is 
the  fact  that  eggs  and  spermatozoa  behave 
like  excreta;  being  substances  which  are  no 
longer  destined  for  the  use  of  the  individual, 
but  must  reach  the  exterior  in  order  ta 
become  efficient.  The  morphological  cause 
is  the  relation  to  the  co3lom.  A  iirogenital 
system  is  formed  only  in  animals  in  which  the 
germinal  epithelium  arises  from  the  epithe- 
lium of  the  ccelom,  and  in  which  the  kidneys 

FIG.  73.  —  Sexual   appara-  ,-\     •  j-  j  j_i        • 

tus  of   Vortex  vtridis.  or    their    rudiments    stand   permanently   in 


glands- 


connexion  with  the  body  cavity  and  thus 
form  the  natural  outlet  for  its  products. 
Whether  the  accessory  sexual  parts  are  por- 
tions  of  the  excrefcorJ  organs  or  are  inde- 
pendent  structures,  they  have  in  the  animal 
series  a  definite  arrangement  adapted  to  their  function  (fig.  73). 
Canals  lead  from  the  sexual  glands  to  the  exterior,  the  oviducts  in 
the  female,  the  vasa  defer  entia  in  the  male  (and  the  herma- 
phroditic duct  from  the  hermaphroditic  gland). 

Accessory  Sexual  Apparatus.  —  The  terminal  portion  of  the  vas 
deferens  is  often  very  muscular  and  is  called  the  ductus  ejacula- 
torius;  it  may  be  evaginated  as  a  penis  or  cirrus,  or  project 
permanently  beyond  the  surface  of  the  body.  The  terminal 
portion  of  the  oviduct  is  often  widened  so  that  two  portions  may 
be  distinguished,  the  uterus,  which  harbors  the  eggs  during  their 
development,  and  the  vagina,  which  serves  for  copulation.  In 
addition  there  may  occur  in  both  sexes  other  accessory  glands  of 
the  most  diverse  character.  Oviduct  and  vas  deferens  may  be 
provided  with  sac-like  evaginations  which  serve  for  the  reception 
of  the  sperm.  In  the  female  these  are  called  receptacula  seminis, 
in  the  male  vesiculce  seminales;  the  former  give  lodgment  to  sperm 
which  enters  the  female  sexual  passages  during  coition,  the  latter 


GENERAL   ORGANOLOGY.  121 

to  sperm  which  has  been  formed  in  the  testes  of  the  same  indi- 
vidual. 


Animal  Organs. 

I.  Organs  of  Locomotion. 

Voluntary  Locomotion. — The  power  to  change  their  location 
voluntarily  is  a  peculiarity  so  prominent  in  animals  that  to  the 
general  public  it  is  sufficient  for  deciding  whether  an  organism 
belongs  to  the  vegetable  or  to  the  animal  kingdom.  On  this 
account  it  is  necessary  to  call  attention  to  the  fact  that  numerous 
animals  lose  the  power  of  locomotion,  becoming  fixed  to  the 
ground,  to  plants,  or  to  other  animals.  All  sponges  and  corals, 
most  hydroid  polyps,  and  the  crinoids  among  the  echinoderms, 
have  actively  swimming  larvae,  but  become  fixed  in  the  adult  and 
thus  obtain  such  a  marked  similarity  to  plants  that,  although  true 
animals,  they  were  long  regarded  as  plants.  Further,  many  mol- 
luscs and  worms  are  firmly  fixed  by  their  shells;  indeed,  many 
crustacean  forms,  the  cirripeds,  have  completely  lost  their  free 
motility.  But  a  more  careful  investigation  in  all  these  cases  will 
show  that  the  power  of  moving  the  separate  parts  exists,  for  the 
corals  can  retract  their  tentacles,  the  cirripeds  their  featherlike 
feet,  and  the  clam  can  close  its  shell. 

Locomotion  among  Lower  Animals. — The  lowest  forms,  the 
Protozoa,  progress  almost  exclusively  by  processes  of  the  cell: 
pseuclopodia,  cilia,  or  flagella.  In  the  metazoa  this  is  rarely  the 
case.  Amoeboid  movements  of  the  epithelial  cells,  indeed,  occur 
in  the  coelenterates  and  also  in  many  worms,  but  do  not  suffice 
for  change  of  position.  More  effective  is  the  ciliated  or  flagellated 
epithelium,  by  which  ctenophores,  turbellarians,  and  rotifers 
swim;  this  occurs,  besides,  in  many  larvae  of  animals  which,  in 
the  mature  state,  are  unable  to  change  their  location  or  do  so  only 
by  the  aid  of  muscles.  Nearly  all  coelenterates,  echinoderms, 
molluscs,  and  the  majority  of  the  worms  leave  the  egg-membranes 
in  the  form  of  the  planula,  i.e.,  as  a  larva  swimming  by  means  of 
cilia. 

Locomotion  among  Higher  Animals. — The  musculature  is  alone 
adapted  for  energetic  motions.  The  arrangement  of  this  varies 
with  and  depends  upon  the  constitution  of  the  skeleton.  Forms 
without  a  skeleton  have  commonly  the  i  dermo-muscular  tunic/  a 
sac  of  circular  and  longitudinal  muscle  fibres  which  is  firmly  united 


122  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

with  the  skin.  If  a  skeleton  be  formed  by  the  skin,  as  in  the 
arthropods,  then  the  sac  breaks  up  into  groups  of  muscles,  which 
find  points  of  attachment  upon  the  dermal  skeleton;  if,  on  the 
other  hand,  as  in  the  vertebrates,  an  axial  skeleton  be  formed,  a 
fixed  point  is  furnished  for  muscular  action,  so  that  the  muscula- 
ture obtains  a  quite  new  character,  in  particular  a  deeper  position. 
A  locomotor  apparatus  quite  unique  is  the  ambulacra!  system  of 
the  echinoderms,  a  system  of  delicate  little  tubes  with  protrusible 
portions  which  function  as  feet,  described  in  connexion  with  that 
group. 

II.  Nervous  System. 

Scarcely  a  system  of  organs  in  the  animal  series  shows  such  a 
regular  development  as  the  nervous  system.  The  different  stages 
which  can  be  grouped  may  be  termed  the  diffuse,  the  linear,  the 
ganglionic,  and  the  tubular  types. 

Diffuse  Nervous  System. — The  diffuse  type  is  certainly  the 
most  ancestral;  it  shows  the  two  elements,  nerve  fibres  and 
ganglion  cells,  regularly  distributed  through  the  whole  body,  or, 
at  least,  through  certain  layers  of  the  body.  The  skin  of  the 
body,  the  ectoderm,  is  to  be  looked  upon  as  one  of  the  fundamental 
elements  in  the  nervous  system,  since  it  is  related  to  the  external 
world,  and  hence  receives  the  sensory  impressions,  so  important 
for  the  development  of  nervous  tissue.  The  corals  and  hydroid 
polyps  are  examples,  since  in  them  the  ectoderm  is  permeated  in 
all  directions  by  a  delicate,  subepithelial  spider-weblike  network 
of  nerve  fibres  and  ganglion  cells,  which  encroach  even  upon  the 
entoderm. 

Linear  Nervous  System. — From  the  diffuse  type  the  other 
chief  types  can  be  derived  through  concentration,  which  is  chiefly 
conditioned  by  the  fact  that  there  are  a  few  points  which  are  most 
advantageously  located  for  the  reception  of  sensory  stimuli,  and 
hence  for  the  development  of  nervous  elements.  In  the  medusae 
such  a  place  is  the  rim  of  the  bell;  consequently  a  stronger  nerve- 
cord  remarkably  rich  in  ganglion  cells  is  found  here.  This,  as 
well  as  the  nerve-ring  and  the  five  ambulacral  nerves  of  echino- 
derms, may  be  called  a  central  system,  thereby  distinguishing  the 
rest  of  the  nervous  network  as  the  peripheral  nervous  system. 

Ganglionic  Central  Nervous  System. — Numerous  transitional 
forms  lead  to  the  ganglionic  central  nervous  system  of  the  worms, 
molluscs,  and  arthropods  (fig.  74).  The  central  nervous  system 
here  consists  of  two  or  more  ganglia;  each  ganglion  being  a 


GENERAL   OROANOLOOT. 


123 


rounded  bunch  of  regularly  arranged  nerve-fibres  and  ganglion- 
cells.  The  former  constitute  the  centre  of  the  mass,  and,  since 
they  cross  in  all  directions,  give  the  appearance  of  fine  granula- 
tions; this  fact  has  led  to  the  unsuitable,  because  misleading,  name 
of  <  Leydig's  dotted  substance/  The  ganglion-cells,  on  the  other 


FIG.  74.— Third  abdominal  ganglion  of  a  crayfish.  (After  Retzius.)  C,  connective  or 
longitudinal  commissure;  G,  ganglion  cell  layer;  g'  ganglion  cell  whose  neurites 
enter  the  connective;  02,  ganglion  cell  whose  neurites  enter  the  peripheral  nerve ; 
_L,  Leydig's  dotted  substance;  JV,  peripheral  nerve. 

hand,  collect  in  a  thick  layer  around  the  dotted  substance.  The 
peripheral  nerves,  and  also  the  commissures,  the  cords  connecting 
similar  ganglionic  masses,  extend  outwards  from  the  ganglia. 

Supracesophageal  (or  Cerebral)  Ganglia. — Since  most  animals 
are  symmetrical,  the  ganglia  occur  in  pairs;  left  and  right  ganglia 
correspond  to  one  another  and  are  connected  simply  by  a  cord  of 
nerve-fibres,  the  transverse  commissure.  Of  most  constant  occur- 
rence are  two  ganglia,  which  lie  dorsally  above  the  pharynx,  and 
hence  are  called  the  supracesophageal  or  cerebral  ganglia.  If  other 
ganglia  occur,  they  lie  ventrally  and  below  the  digestive  tract 
(ventral  nerve-cord). 


124 


GENERAL  PRINCIPLES  OF  ZOO  LOOT. 


Ladder  Nervous  System.  —  A  widely  recurring  arrangement  is 
that  termed  the  ladder  nervous  system  (of  annelids  and  arthropods) 
(fig.  75).  Numerous  pairs  of  ganglia  (in 
the  example  before  us,  nine)  lie  in  serial 
order  on  the  ventral  side  of  the  animal,  and 
are  connected  by  longitudinal  commissures 
(connectives),  and  also  by  transverse  com- 
missures connecting  the  left  and  right 
ganglia.  The  first  pair  of  the  series  is. 
formed  by  the  infra-oesophageal  ganglion, 
which  sends  out  commissures  right  and  left, 
surrounding  the  pharynx,  to  the  supra- 
O3sophageal  ganglion.  The  supra-  and  infra- 
oesophageal  ganglia  together  with  the 
oesophageal  commmissures  form  the  cesopha- 
geal  ring,  a  nerve-ring  surrounding  the 
oesophagus. 

Tubular  System.  —  The  tubular  type  of 
nervous  system  is  found  only  in  the  chordates 
(fig.  76).  The  vertebrate  brain  and  spinal 
cord  may  be  regarded  as  parts  of  a  tube  with 
greatly  thickened  walls,  developed  in  differ- 
ent ways.  In  the  centre  lies  the  extremely 
narrow  central  canal,  which  widens  anteriorly 
into  the  several  ventricles  of  the  brain.  In 
a  transverse  section  the  nervous  elements 

FIG.  75.  —  Ladder   nervous  -i  T   ,  -,  ->  ^  . 

system  of  Porceiuo  scatter  are  seen  grouped  around  the  central  canal  m 


a  manner  almost  the  reverse  of  that  of  the 
ionic  type.  On  the  periphery  lies  a 
°f  nerve-fibres  (the  <  white  matter'  of 
human  anatomy)  ;  next  is  a  central  portion 
formed  of  ganglion-cells  and  nerve-fibres  (the  'gray  matter'), 
which  is  marked  oil  from  the  central  canal  by  a  special  epithelium 
(ependyma). 

Relations  between  the  Nervous  System  and  the  Skin.  —  For 
almost  all  animals  it  has  been  ascertained  that  the  nervous  system 
arises  from  the  ectoderm.  Therefore,  in  many  animals,  the  nerve- 
cords  and  the  ganglionic  masses  lie  pemanently  in  the  skin:  in 
others  only  during  the  development,  later  becoming  separated  by 
splitting  off  or  by  infolding,  and  thus  coming  to  lie  in  the  deeper 
layers  of  tlje  body  (fig.  9). 


GENERAL   ORGANOLOGT.  125 

III.  Sensory  Organs. 

Sensations  of  the  Lower  Animals. — What  we  know  of  the 
•character  of  the  external  world  is  founded  upon  experiences  gained 
through  our  sensory  organs.  We  thus  know  the  external  world 
only  in  so  far  as  it  is  accessible  to  the  senses,  controlled  by  the 
judgment.  If  things  exist  outside  of  ourselves  which  have  no 
influence  upon  our  senses,  we  can  form  no  conception  of  them. 
It  follows  from  this  proposition  that  we  can  gain  knowledge  of  the 


.ju 


w 

,VH 


*  A* 

FIG.  76.— Cross-section  of  the  human  spinal  cord.  (From  Wiedersheim.)  Black  repre- 
sents the  gray,  white  the  white  substance  of  the  cord  ;  Cc,  central  canal,  sur- 
rounded by  the  anterior  and  posterior  commissures  (C  and  C');  <Sa,  Sp,  anterior 
and  posterior  fissures  ;  V\V,  JfW,  anterior  and  posterior  nerve-roots  ;  VH,  HH, 
anterior  and  posterior  horns  of  gray  matter;  V,  S,  If,  anterior,  lateral,  and  pos- 
terior columns  of  white  matter. 

natural  capacity  of  the  sensory  organs  of  animals  only  by  analogy 
with  our  own  experiences.  Hence  the  distinction  of  five  senses, 
touch,  taste,  smell,  hearing,  and  sight,  based  upon  human 
physiology  has  been  extended  to  the  whole  animal  kingdom.  A 
priori,  however,  it  cannot  be  denied  that  sensations  may  occur  in 
animals  which  we  do  not  experience;  following  out  this  course  of 
thought  has  led  to  the  idea  of  a  '  sixth  sense/  which,  however, 
must  remain  to  us  a  meaningless  abstraction,  since  it  is  impossible 
for  us  to  conceive  of  the  character  of  a  sense  which  we  lack. 

Anatomy  gives  Insufficient  Knowledge  of  Sensory  Organs.— 
A  further,  and  still  more  important  reason  for  our  very  fragmen- 
tary knowledge  of  animal  sensations  is  the  fact  that,  in  regard  to 
the  physiological  meaning  of  the  sensory  apparatus,  it  is  seldom 
that  we  can  depend  upon  experiments,  and  consequently  we  must 
base  our  conclusions  upon  structure.  But  the  anatomy  of  many 
sensory  organs,  like  those  of  smell  and  taste,  is  by  no  means  so 
characteristic  that  it  alone  is  sufficient  to  determine  the  physio- 
logical significance. 

Tactile  Organs. — The  skin  of  animals  functions  as  a  tactile 
organ,  usually  over  the  whole  area,  although  not  everywhere  with 


126 


GENEEAL  PRINCIPLES  OF  ZOOLOGY. 


equal  intensity.  Prominent  parts,  like  the  tentacles  of  polyps 
and  of  many  worms,  the  antennae  of  arthropods  and  the  snails, 
need  only  mention.  Special  epithelial  cells  with  stiff  hairs  pro- 
jecting above  the  surface,  the  tactile  bristles  or  tactile  hairs,  are 


-      71 


FIG.  77.  FIG.  78. 

FIG.  77.— Skin  of  an  insect  with  an  ordinary  hair  (h)  and  a  tactile  hair  (();  n,  nerve; 

*,  sensory  cell;  e,  epithelium;  c,  cuticle.    (After  vom  Rath.) 
FIG.  78. — Vater-Pacinian  corpuscle  of  the  mesentery  of  a  cat.   a,  axis  cylinder;  /,  fat; 

0,  blood-vessel;  i,  inner  bulb;  /c,  capsule  with  nuclei;  n,  medullated  nerve-fibre. 

tactile  (fig.  77).  Only  in  the  vertebrates  do  the  nerves  of  touch 
terminate  in  specially  modified  end  organs  (Vater-Pacinian  cor- 
puscles, corpuscles  of  Meissner,  etc.,  fig.  78);  these  usually  lie 
under  the  epithelium. 

Organs  of  Smell  and  of  Taste  are  accurately  known  only  in 
vertebrates.  The  olfactory  organ  of  fishes  consists  of  two  small 
pits  in  the  skin,  above  or  in  front  of  the  mouth. 

In  the  air-breathing  vertebrates  this  pair  of  pits  which  here 
also  arise  from  the  skin  are  taken  into  the  dorsal  wall  of  the  two 
respiratory  canals  leading  from  the  outside  to  the  pharynx.  Now 
since  the  olfactory  cells  distributed  in  these  pits  (fig.  37,  /?)  are 
frequently  characterized  by  bundles  of  olfactory  hairs,  while  the 
surrounding  epithelium  is  often  ciliated,  one  is  inclined  to  regard 
as  organs  of  smell  sensory  organs  of  invertebrates  (e.g.,  medusas, 
cephalopods),  which  have  the  form  of  ciliated  pits  and  lie  near  the 
respiratory  apparatus  (e.g.,  the  osphradium  of  molluscs).  Yet 
there  are  exceptions.  Experiments  seem  to  show  that  in  the 
arthropods  the  antennae  probably  serve  for  smelling.  Here  the 
sensory  perception  can  be  connected  only  with  certain  modified 


GENERAL   ORGANOLOGT.  127 

hairs,  the  olfactory  tubules  of  the  Crustacea  and  the  olfactory 
cones  of  insects.  In  a  similar  way  certain  nerve  end  organs  in 
the  region  of  the  mouth  are  considered  as  organs  of  taste,  since  the 
taste  organs  of  vertebrates,  the  so-called  taste  buds,  are  abundant 
in  the  mouth  cavity,  especially  on  the  tongue. 

Organs  of  Hearing  and  of  Sight  are  called  the  higher  sense- 
organs,  because  they  are  of  much  greater  importance  for  the 
totality  of  our  perceptions  than  the  other  organs,  since  they  fur- 
nish sensations  which  are  quantitatively  and  qualitatively  much 
more  definite.  Ears  and  eyes  have  therefore  a  complicated  and 
characteristic  structure,  which  renders  them  easily  recognizable 
by  the  almost  invariable  presence  of  certain  structures  accessory  to 
their  functions. 

History  of  the  Auditory  Organs. — The  auditory  organs  of 
vertebrates  and  of  most  of  the  other  animal  groups  can  be  traced 
back  to  a  simple  fundamental  form,  the  auditory  vesicle  (fig.  79). 


FIG.  79.— Auditory  vesicle  of  a  mollusc  (Pterotr ached).    JV,  auditory  nerve  :  Jfz,  audi- 
tory cells  with  the  central  cell,  Cz  ;  Wz,  ciliated  cells  ;  Oi,  otolitn.    (After  Claus.) 

This  has  an  epithelial  wall,  a  fluid  contents,  the  endolymph,  and 
an  auditory  ossicle  or  otolitli,  formed  from  a  single  or  from  several 
fused  auditory  concretions.  In  some  instances  the  otoliths,  to  the 
number  of  thousands,  may  remain  separate.  In  a  definite  region 
of  the  epithelial  wall  the  cells  are  developed  into  the  crista 
acustica,  the  auditory  ridge;  they  are  in  connexion  with  the 
auditory  nerve  and  bear  the  auditory  hairs  projecting  into  the 
endolymph.  The  otoliths  themselves  are  concretions  of  carbonate 
or  of  phosphate  of  lime  (exceptionally  in  My  sis  of  fluoride  of 
calcium).  They  usually  float  free  in  the  centre  of  the  vesicle,  and 
are  often  held  in  place  by  bundles  of  cilia  which  project  from  the 
non-sensitive  epithelial  cells. 


128  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

Auditory  Pit. — Every  auditory  vesicle  develops  from  a  pitlike 
invagination  of  the  skin,  and  consequently  is  for  a  time  an  auditory 
pit.  Therefore  it  is  not  surprising  that  in  many  animals  the  organ 
has  stopped  at  the  lower  stage  of  development;  for 'example,  the 
crayfish  has  an  open  auditory  pit.  On  the  other  hand,  the 
auditory  vesicle  may  develop  a  complicated  system  of  cavities  as 
in  mammals  (fig.  80),  where  it  is  divided  by  a  constriction  into 


O 


V 

FIG.  80.— Diagram  of  the  human  labyrinth.  17,  titriculus  with  the  semicircular 
canals;  S,  sacculus  connected  with  the  cochlea  (G)  by  the  canalis  reuniens;  It, 
recessus  labyrinth!;  V,  blind  sac  of  the  cochlea;  K,  apex  of  the  cochlea. 

the  sacculus  and  the  utriculus.  The  sacculus  is  provided  with  a 
spirally-wound  blind  sac,  the  cochlea,  the  utriculus  with  the  three 
semicircular  canals.  In  addition  there  is  formed  in  the  mammals, 
as  also  in  most  vertebrates,  a  sound-conducting  apparatus,  so  that 
the  auditory  organ  acquires  an  extremely  complicated  structure. 

Other  Forms  of  Auditory  Organs. — Since  there  are  animals 
without  auditory  vesicles  which  hear  well,  like  the  spiders  and 
insects,  we  must  assume  that  there  are  auditory  organs  which  are 
formed  after  another  type.  Still  we  have  no  certain  knowledge  of 
these  except  in  the  case  of  the  tympanal  auditory  organs  of  the 
grasshoppers  (which  compare). 

Function  of  the  Semicircular  Canals.  —  Experiments  upon 
representatives  of  the  most  diverse  classes  of  vertebrates  have  led 
to  the  conclusion  that  the  three  semicircular  canals,  standing  at 
right  angles  to  each  other,  are  organs  of  equilibrium,  for,  after 
these  canals  are  destroyed,  the  animals  begin  to  stagger  and  lose 
their  balance.  It  is  probable  that  in  fishes  this  is  the  sole  function 
of  the  labyrinth;  for  it  has  not  been  determined  that  fishes  hear. 
Starting  from  this  assumption,  recent  investigators  have  attempted 
to  prove  that  the  auditory  vesicles  of  invertebrated  animals  are 
exclusively,  or  at  least  largely,  organs  of  equilibration.  This  would 
explain  the  otoliths,  for  these  bodies,  of  relatively  large  specific 
•gravity,  would  affect  the  crista  in  different  ways  according  to  the 
position  of  equilibrium  of  the  body.  Statoliths  would  thus  be  a 
better  name. 


GENERAL   ORGANOLOGT. 


129 


The  Eye  is  in  all  animals  recognized  by  the  character  of  the 
sensory  epithelium,  the  retina.  This  always  has  a  large  amount 
of  pigment  which  lies  either  in  the  sensory  cells  or  in  special  cells 
arranged  between  or  behind  them.  The  simplest-formed  eye, 
therefore,  appears  as  a  sharply  circumscribed  pigment-spot  in  the 
epithelium  of  the  skin,  provided  with  nerves,  commonly  also  with 
a  lens  (fig.  81). 

Rods  and  Cones. — The  sensory  cell  itself  bears  usually  at  its 
peripheral  end  a  process,  the  rhabdom.  This  is  a  cuticular  struc- 
ture, probably  serving  to  collect  the 
rays  of  light  and  thus  to  stimulate  the 
cell,  and  has,  particularly  in  the  verte- 
brates, a  complicated  structure,  each 
rhabdom  consisting  of  an  inner  and  an 
outer  portion.  Here  can  be  frequently 
distinguished  two  kinds  of  rhabdoms, 
rods  and  cones  (fig.  82). 

The  Optic  Ganglion. — Before  the 
optic  nerve  divides  into  the  separate 
visual  cells  it  forms  a  swelling,  the 


V 

FIG.  81.  Flo  82> 

FIG.  81.—  Ocellus  (oc)  of  a  medusa  (Lizzia  Koellikeri)  with  lens  (0. 

FlGn!!fcI£lman-retinaV  (After  Gegenbaur.)    P,  pigment-layer;  E,  layer  of  sensory 

;  G,  optic  ganglion;  1,  limitans  mterna;  2,  nerve-fibre  layers;  3,  ganglion- 

cells;  4,  inner  reticular  layer;  5,  inner  granular  layer;  6,  outer  reticular  layer-  7. 

V  Muller^^bres761'5  8'  limitans  externa?  9i  r°ds  and  cones;  10,  tapetum  nigrum; 


optic  ganglion,  which  either  lies  as  a  detached  body  outside  of  the 
eye,  or  is  united  with  the  retina  into  a  connected  whole.     The 


130 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


considerable  thickness  of  the  vertebrate  retina  is  due  to  the  fact 
that  it  includes  the  optic  ganglion.  The  parts  (fig.  82)  called 
reticular  layers,  inner  granular  layer,  ganglion  cells,  and  nerve- 
fibre  layer,  constitute  the  optic  ganglion;  the  layer  of  visual  cells 
consists  only  of  the  outer  granular  layer  and  the  connected  rods 
and  cones.  The  outer  granules  are  the  nuclei  of  the  visual  cells 
to  which  rods  and  cones  belong. 

Accessory  Structures. — The   eye  may  be  further  complicated 
by  special  refractive  bodies  (cornea,  lens,  vitreous  body)  which 


G  NO  vo 


FIG.  83.— Horizontal  section  through  the  human  eye.  (After  Arlt,  from  Hatschek.) 
.E,  epithelium  of  the  cornea  (conjunctiva) ;  C,  cornea ;  vA,  anterior  chamber  of 
the  eye;  I,  iris;  hA,  posterior  chamber  of  the  eye;  Z,  zonula  Zinnii;  Os,  ora  ser- 
rata  ;  Sc,  sclerotic  coat ;  Ch,  choroidea ;  U,  retina ;  p,  papilla  of  optic  nerve ;  m, 
macula  lu tea,  area  of  most  distinct  vision;  VO,  sheath  of  the  optic  nerve;  NO, 
optic  nerve;  0,  arteria  centralis;  Cc,  corpus  ciliare;  J/,  lens;  Or,  vitreous  body. 

concentrate  the  light  in  order  to  cast  an  image  upon  the  retina; 
and  an  iris  to  regulate  the  amount  of  light.  Then,  too,  means  for 
nutrition  (the  choroid  coat)  and  for  protection  (sclerotic  coat) 
must  be  provided.  If  all  these  parts  be  present,  a  structure  results 
such  as  is  found  in  the  squid  and  in  the  vertebrates  (fig.  83). 


GENERAL   OROANOLOGY.  131 

The  Eye  of  the  Vertebrates. — The  eye  of  the  vertebrates 
usually  is  an  approximately  spherical  body  whose  surface  is  formed 
by  a  firm  membrane.  Over  the  greater  part  of  the  circumference 
this  is  an  opaque,  fibrous  or  cartilaginous  covering,  called  the 
sclera,  or  sclerotica;  it  is  transparent  only  in  the  most  anterior 
part,  and  here  it  forms  by  its  greater  convexity  a  projecting  por- 
tion like  a  watch-glass,  the  cornea.  Internally  to  the  sclera  lies 
the  choroidea,  an  envelope  of  connective  tissue,  rich  in  pigment 
and  blood-vessels,  which,  at  the  junction  of  sclera  and  cornea,  is- 
changed  into  the  iris.  The  iris,  the  seat  of  the  color  of  the  eye, 
is  pierced  in  the  centre  by  the  pupil,  an  opening  the  varying  size 
of  which  regulates  the  amount  of  light.  Next  internal  to  the 
choroid  follows  a  layer  of  black  cells,  the  tapetum  nigrum  (pig- 
mented  epithelium),  and  finally  the  retina  itself,  the  expansion  of 
the  optic  nerve  which  enters  the  eye  at  the  hinder  part.  The 
tapetum  nigrum  and  the  retina  arise  together,  and  hence  both  end 
at  the  edge  of  the  pupil,  although  the  retina  loses  its  nervous 
character  at  the  ora  serrata,  some  distance  from  the  outer  edge  of 
the  iris. 

The  cavity  of  the  eye  is  completely  filled  by  the  vitreous  body, 
aqueous  humor,  and  the  lens.  For  vision  the  lens  is  the  most 
important,  since,  next  to  the  cornea,  it  influences  most  the  course 
of  the  rays  of  light.  It  lies  behind  the  iris,  fixed  to  the  anterior 
wall  of  the  choroidea,  which  here  is  changed  into  the  ciliary 
process.  In  front  of  it  is  a  serous  fluid,  the  aqueous  humor,  partly 
in  the  so-called  posterior  chamber  of  the  eye,  between  the  lens 
and  iris,  partly  in  the  anterior  chamber,  between  the  iris  and 
cornea.  The  single,  larger  cavity  behind  the  lens  is  filled  up  by 
a  jelly-like  mass  of  tissue,  the  vitreous  body.  The  image  formed 
on  the  retina  is  inverted. 

The  Various  Types  of  Eyes. — Between  the  simple  pigment- 
spot  and  the  highly  organized  vertebrate  eye  are  many  transitional 
stages:  pigment-spots  with  lens  and  vitreous  body,  with  enveloping 
and  nourishing  coverings,  etc.  The  faceted  eye  of  insects  and 
Crustacea  shows  a  special  type  of  development,  described  later 
under  the  Arthropoda. 

SUMMARY    OF   THE    MOST   IMPORTANT    POINTS   OF    ORGANOLOGY. 

1.  Organs  are  composed  of  tissues,  and  by  their  environment 
are  led  to  the  formation  of  a  body  of  definite  shape  and  to  the 
performance  of  a  single  function;  consequently  every  organ  is 


132  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

characterized  morphologically  (according  to  its  structure  and  its 
relations)  and  physiologically  (according  to  its  function). 

2.-  Organs  of  different  animals  may  be  physiologically  equiva- 
lent,  analogous  organs  (i.e.,  with  similar  functions). 

3.  Organs  of  different  animals  may  be  morphologically  equiva- 
lent, homologous  (developing  in  similar  relations). 

4.  In  the  comparison  of   the   organs   of   two   animals   three 
possibilities  become  evident. 

a.  They  may  be  at  the  same  time  homologous  and  analogous. 

b.  They  may  be 'homologous,  but  not  analogous  (swim-bladder 
of  fishes,  lungs  of  mammals). 

c.  They  may  be  analogous,  but  not  homologous  (gills  of  fishes, 
lungs  of  mammals). 

5.  Organs  are  divided  into  animal  and  vegetative. 

6.  Animal  functions  are  those  which  are  not  completely  foreign 
to  plants,  but  are  only  slightly  developed  in  them;  in  the  animal 
kingdom,  on  the  contrary,  they  undergo  an  increase  and  become 
characteristic. 

7.  Vegetative  functions  are  developed  with  equal  completeness, 
though  in  a  different  manner,  in  plants  and  animals. 

8.  To  the  animal  organs  belong  the  organs  of  motion  and 
sensation,    such   as   the   muscles,  the   sense-organs,  the   nervous 
system. 

9.  To  the  vegetative  organs  belong  the  organs  of  nutrition  and 
reproduction. 

10.  Under  nutrition,  in  the  widest  sense,  are  included  not  only 
the  taking  in  and  digestion  of  food  and  drink,  but  also  the  taking 
in  of  oxygen  (respiration),  the  distribution  of  food  to  the  parts  of 
the  body,  and  the  removal  of  matter  which  has  become  useless. 

11.  With  nutrition,   therefore,    are   concerned   not   only  the 
digestive  tract  and  its  accessory  glands,  but  also  the  organs  of 
respiration,  the  blood-vascular  system,  and  the  excretory  organs 
(kidneys). 

12.  The  male  and  female  sexual  organs  serve  for  reproduction. 

13.  The  male  and  female  organs  may  occur  in  different  indi- 
viduals (diwcious),  or   both  may  be  found  in  one  and  the  same 
animal  (hermaphroditic). 

14.  The  highest  degree  of  hermaphroditism  is  attained  when 
one  and  the  same  gland  (the  hermaphroditic  gland)  gives  rise  to 
both  eggs  and  spermatozoa. 

15.  Very  often  the   sexual  organs  and  the  ducts   from    the 
kidneys  are  closely  united;  we  then  speak  of  a  urogenital  system. 


PROMORPHOL  OOT.  133 


IV.   PROMORPHOLOGY,  OR  STUDY  OF  THE  FUNDAMENTAL  FORMS. 

Organic  and  Inorganic  Bodies. — The  structure  of  the  individual 
animal  rests  upon  the  regular  combination  of  differently-function- 
ing organs.  The  organs  thus  assume  a  relation  to  one  another 
which  is  definite  for  each  animal  group,  or  varies  only  in  subordi- 
nate ways.  If  the  various  groups  be  compared  with  reference  to 
the  principle  of  the  arrangement  of  parts,  there  appear  a  few 
fundamental  forms  which  play  a  role  in  morphology  similar  to  that 
of  the  fundamental  forms  of  crystals  in  mineralogy.  But  we  must 
not  carry  this  comparison  too  far,  and  attempt  to  compare  the 
study  of  the  fundamental  forms,  the  promorphology,  of  animals 


FIG.  84.— Spongilla  fluriatilis,  fresh-water  sponge.  (After  Huxle 
with  dermal  pores;  be,  region  of  the  ampullae;  c/, 


(After  Huxley.)    a,  superficial  layer 


osculum. 


with  crystallography  as  of  equal  value.  A  crystal  is  a  mass  made 
up  of  similar  parts;  its  form  is  the  necessary  and  immediate  result 
of  the  chemico-physical  constitution  of  its  molecules.  A  direct 
connection  of  this  kind  between  molecular  structure  and  funda- 
mental form  does  not,  and  cannot,  exist  in  the  organism,  since 
each  organ  is  composed  of  many  chemical  combinations.  Conse- 
quently there  is  lacking  also  the  mathematical  regularity  which 
occurs  in  crystals.  Even  in  the  case  of  animals  which  have  the 
greatest  regularity  in  the  arrangement  of  their  parts  there  is  not 
an  entire  conformity  to  the  demands  of  the  fundamental  form,  so> 
that  we  are  compelled  to  ignore  certain  greater  or  less  variations. 
If,  for  example,  we  call  man  bilaterally  symmetrical,  we  overlook 
not  only  the  slight  asymmetry  of  a  nose  awry,  etc.,  but  also  what 
is  more  important — that  the  liver  has  been  pushed  to  the  right, 


GENERAL  PRINCIPLES   OF  ZOOLOGY. 


the  heart  to  the  left;  and  that  the  digestive  tract  throughout  its 
entire  course  runs  asymmetrically. 


* 


FIG.  85.— Halinmma  erinaceus,  a  radiolarian.    «,  external,  i,  internal,  latticed  spheri- 
cal skeleton ;  cfr,  central  capsule;  wk •,  extra  capsular  soft  parts ;  ?»,  nucleus. 


JT 


Sfi 


FIG.  86. — Nausithoe^  an  acraspedote  medusa  (after  Lang),  seen  from  the  end  of  the 
greatly  shortened  main  axis,  pr,  perradii;  tr,  interradii;  ar,  adradii  (perradii 
and  interradii  mark  the  four  planes  of  symmetry  of  the  animal);  *«r,  subradii:  rf, 
mantle-lobes;  f,  tentacles;  s/f,  sensory  organs:  g,  sexual  organs;  gff,  gastric  fila- 
ments; rn,  subumbrellar  circular  muscle;  in  the  centre  the  cross-shaped  mouth- 
opening. 


we 


Symmetry. — Now,  according  to  the  three  dimensions  of  space, 
can  pass  three  axes,  perpendicular  to  each  other,  through  the 


PROMORPHOLOGT.  135 

body  of  an  animal,  and  up  to  a  certain  degree  may  characterize  it 
according  to  the  nature  of  these  axes;  further,  we  may  characterize 
it  according  to  the  planes  by  which  it  can  be  symmetrically  halved, 
the  planes  of  symmetry.  Thus  we  find  the  following  fundamental 
forms : 

1.  Anaxial,    asymmetrical,    irregular,    or    amorphous    funda- 
mental form  (fig.  84). 

2.  Homaxial,  symmetrical  in  all  directions,  spherical  funda- 
mental form  (fig.  85). 

3.  Monaxial,  radially  symmetrical  (fig.  86). 

4.  Simple  heteraxial,  biradially  symmetrical  (figs.  87,  88). 

5.  Double  heteraxial,  bilaterally  symmetrical  (fig.  89). 

1.  Anaxial  or  asymmetrical  animals,  so  called,  are  those  in  which  the 
arrangement  of  parts  is  not   regularly   de- 
fined in   any  direction  or  space,   and  they 

therefore  may  grow  irregularly  in  any  direc- 
tion. There  is  no  fixed  central  point,  and 
there  is  no  possibility  of  running  definite 
axes  through  the  body  or  of  dividing  it  into 
symmetrical  parts.  (Many  sponges  and 
many  Protozoa.) 

2.  Homaxial  or  spherical  animals    have 
the  sphere  as  their  fundamental  form ;  the 
parts  of  the  body   are  arranged   concentri- 
cally around  a  fixed  central  point,  so   that 
any  number  of  axes  and  planes  of  symmetry 
<?an  be  passed   through  it ;  that   is  to  say, 
all  lines  and  planes  which  run  through  the 

central  point  of  the  sphere.  (A  few  spheri-  FIG.  87.-Diagramof  anactinian 
•cal  Protozoa,  chieny  radiolarians.)  (|t^^5S^So?l£toti^ 

3.  Monaxial     or    radial    symmetry    is     much-lengthened  main  axis, 
brought   about,   if  growth  go  on  in  a  definite  direction,  and  correspond- 
ingly also  if  the  formation  of  organs  take  place  in  directions  other  than 
perpendicular  to  this.     The  line  which  marks  this  direction  of  growth  is 
the  main  axis,  in  distinction  from  the  accessory  axes  or  radii,  which  are 
all  similar  to  each  other.     The  main  axis  can  be  determined,  because  it  is 
longer  or  shorter  than  the  accessory  axes  ;  but  it  may  also  be  of  the  same 
length  and  still  be  entirely  distinct,  since  it  contains  certain  organs  (e.g., 
the  mouth-opening)  which  are  lacking  in  the  other  planes.     In  radially 
symmetrical  animals  the  same  organs  are  always  present  in  greater  num- 
ber and  are  distributed  regularly  around  the  main  axis  in  the  direction  of 
the  radii.     Through  such  an  animal  a  great  number  of  sections  can  be 
made,  which  pass  through  the  long  axis  and  halve  the  body  symmetri- 
•cally.     If  we  cut  the  animal  in  the  direction  of  all  the  possible  planes  of 
symmetry,  we  obtain  pieces  which,  in  essential  points,  are  similarly  con- 


136 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


structed.     Great  groups  of  animals,  as  most  eohinoderms  and  ccelenter- 
ates,  are  more  or  less  completely  radially  symmetrical. 

4  and  5.  The  next  two  fundamental  forms  have  in  common  the  fact 
that  three  unequal  axes  perpendicular  to  each  other  are  distinguishable  ; 
these  may  be  designated  as  the  main  axis,  the  transverse  axis,  and  the 
sagittal  axis :  this  is  the  case  if,  leaving  the  main  axis  out  of  considera- 
tion, an  arrangement  of  organs  occur  different  in  the  sagittal  direction 
from  that  in  the  transverse  direction — if  organs  lie  in  the  former  which 


B 

FIG.  88.— Cross-section  of  an  actinian  (Adamsia  diaphana).  AB,  directive  septa, 
which  are  at  the  same  time  ends  of  the  sagittal  axis,  which  marks  one  plane  or 
symmetry  of  the  body,  while  the  second  stands  perpendicular  to  it ;  I-IV,  cir- 
cles of  paired  septa  of  first  to  fourth  order. 

are  lacking  in  the  latter  or  the  reverse.  There  are  then,  if  wre  take  into 
consideration  the  dissimilarity  of  the  axes,  two  possible  planes  of  sym- 
metry :  the  animal  can  be  symmetrically  divided,  (1)  if  the  division  passes 
through  the  main  and  transverse  axes,  (2)  if  it  passes  through  the  main 
and  the  sagittal  axes.  Such  biradially  symmetrical  animals  are  the 
ctenophores,  actinians  (figs.  87,  88),  and  corals. 

Bilateral  Symmetry. — If  now  we  further  suppose  that  the  ends  of  the 
sagittal  axes  become  unlike,  that  at  one  end  are  organs  quite  different 
from  those  of  the  other,  we  then  reach  the  most  usual  form,  bilateral 
symmetry.  The  dissimilar  ends  of  the  sagittal  axes  are  called  '  dorsal r 
and  *  ventral,'  and  further  the  terms  '  right '  and  *  left '  are  given  to  the 
ends  of  the  transverse  axis  ;  a  bilaterally  symmetrical  animal  can  be 
divided  symmetrically  into  aright  and  a  left  half  by  one  plane,  the  median, 
passing  in  the  direction  of  the  longitudinal  sagittal  axis  ;  a  frontal  sec- 
tion (a  section  through  the  longitudinal  and  transverse  axes)  always  gives 
dissimilar  parts,  dorsal  and  ventral  sides. 


PROMORPHOLOGY. 
D 


137 


FlG.  89.— Cross-section  of  a  fish  passing  through  the  fore  limbs.  DP,  sagittal  axis; 
RL,  transverse  axis;  a,  dorsal  aorta;  c,  body  cavity;  d,  gut;  eft,  notochord;  #, 
shoulder-girdle ;  h,  heart ;  w,  muscles ;  ?i,  anteri9r  end  of  the  kidneys  ;  p,  peri- 
cardium ;  oo,  neural  arch ;  lib,  haemal  arch  ;  r,  spinal  cord. 

Antimeres  and  Metameres. — The  symmetrical  parts  of  an 
animal  are  called  antimeres;  each  antimere  has  organs  which  occur 
likewise  in  its  adjacent  antimere.  The  right  arm  of  man  corre- 
sponds to  the  left,  the  right  eye  to  the  left,  etc;  the  same  organs- 
are  repeated  in  the  direction  of  the  transverse  axis.  Frequently, 
however,  the  repetition  of  organs  occurs  not  only  in  the  direction 
of  the  transverse  axis,  but  also  in  the  direction  of  the  long  axis. 
Thus  the  body  is  made  up  not  only  of  symmetrical  parts,  the 
antimeres,  but  also  of  similar  parts  placed  one  behind  the  other, 
the  metameres. 

Internal  and  External  Metamerism. — Metamerism  or  segmen- 
tation is  spoken  of  when  the  body  of  an  animal  consists  of 
numerous  segments  or  metameres  (consult  fig.  59).  Very  often 
it  is  recognizable  externally — when,  for  instance,  the  limits  of  t he- 
segments  are  marked  on  the  surface  by  constrictions  (arthropods 
and  annelids).  But  this  external  metamerism  may  be  entirely 
lacking,  and  the  metamerism  find  expression  only  internally  in 
the  serial  succession  of  organs,  in  metameric  or  segmental  arrange- 
ment. Man,  for  example,  is  segmented  only  internally;  in  his- 
skeleton  there  are  numerous  similar  parts,  the  vertebrae.,  which 


138  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

follow  one  another  in  the  long  axis.  In  fishes  the  musculature  also 
is  made  up  of  numerous  muscle  segments,  as  any  one  can  readily 
•see  by  examining  a  cooked  fish.  In  the  case  of  the  externally 
segmented  earthworm  also,  the  ganglia  of  the  nervous  system,  the 
vascular  arches,  the  nephridia  or  segmental  organs,  the  setae,  and 
the  septa  of  the  body  cavity  are  repeated  metamerically. 

Homonomous  and  Heteronomous  Metamerism. — The  examples 
mentioned  are  well  adapted  for  illustrating  the  different  forms,  the 
homonomous  and  the  Jieteronomous,  of  metamerism.  The  earth- 
worm is  homonomously  metameric,  because  the  single  segments  are 
much  alike  in  structure,  and  only  slight  differences  exist  between 
the  anterior,  the  posterior,  and  the  genital  segments.  Man  and 
.all  vertebrates,  on  the  contrary,  are  heteronomously  metameric, 
because  the  successive  segments,  in  spite  of  many  points  of  agree- 
ment with  one  another,  have  become  very  unlike.  The  segments 
of  the  head  have  an  importance,  for  the  organism  as  a  whole,  quite 
different  from  those  of  the  neck,  the  thorax,  or  the  tail.  A 
division  of  labor  has  taken  place  among  the  segments  of  an 
heteronomous  animal. 

Heteronomy  and  Homonomy. — The  distinction  between  heter- 
onomy  and  homonomy  is  of  great  physiological  interest.  The 
more  different  the  segments  of  an  animal  become  the  more 
dependent  they  are  upon  one  another  in  order  to  be  able  to  func- 
tion normally;  so  much  has  the  whole  become  unified  that  the 
single  parts  can  live  only  while  the  continuity  is  maintained.  On 
the  contrary,  if  the  connexion  between  the  parts  be  less  intimate, 
they  are  more  similar,  and  the  more  able  to  exist  after  separation 
from  one  another.  This  is  most  beautifully  shown  in  instances  of 
mutilation.  It  has  been  observed  that  when  many  species  of 
Lumbricidae  are  cut  in  two  each  part  not  only  lives  on  by  itself,  but 
it  even  regenerates  the  part  which  is  lacking;  if,  on  the  other 
hand,  the  same  thing  is  done  to  a  heteronomously  segmented 
animal,  either  death  immediately  ensues,  as  in  the  case  of  the 
higher  vertebrates,  or  the  parts  live  for  a  short  time  a  hopeless 
existence,  as  can  be  seen  in  the  case  of  frogs,  snakes,  insects,  etc. 
In  metamerism  a  phenomenon  is  repeated  which  obtains  widely  in 
the  animal  kingdom?  and  contributes  towards  its  higher  develop- 
ment; first  there  is  a  reduplication  of  parts,  then  a  division  of 
labor,  so  that  the  final  result  is  a  whole  composed  of  many  parts, 
yet  uniformly  organized. 


GENERAL  EMBRYOLOGY.  139 


II.  GENERAL   EMBRYOLOGY. 

Origin  of  Organisms. — Since  the  development  of  every  indi- 
Tidual  begins  with  an  act  of  generation,  the  ways  by  which  new 
organisms  may  arise  should  be  mentioned  first  in  this  chapter. 
If  we  wish  to  limit  ourselves  to  that  which  has  been  actually 
observed,  we  must  still  cling  to  the  old  expression  of  the  renowned 
Harvey,  "  Omne  vivum  ex  ovo,"  and  modifying  it  somewhat  say, 
Omne  vivum  e  vivo :  that  every  living  organism  is  derived  from 
another  living  organism.  We  must  limit  ourselves  to  the  mode  of 
origin  which  has  been  termed  tocogony,  or  generation  by  parents. 
The  great  importance  which  the  question  of  generation  without 
parents,  or  spontaneous  generation,  has  obtained  through  the 
evolution  theory  renders  a  consideration  of  this  question  necessary 
at  this  point. 

I.  GE^ERATIO  SPONTANEA,  ARCHEGONY. 

Theory  of  Spontaneous  Generation. — The  old  zoologists,  even  Aristotle 
himself,  believed  that  many  animals,  including  even  highly  organized 
forms,  like  frogs  and  most  insects,  arose  by  spontaneous  generation  from 
the  mud.  Not  until  the  seventeenth  and  eighteenth  centuries  did  this 
doctrine  find  energetic  opponents,  in  Spallanzani,  Francesco  Redi,  Rosel 
von  Rosenhof,  Svvammerdam,  and  others,  who  endeavored  to  prove 
experimentally  that  all  animals  lay  eggs  which  must  be  fertilized  by  the 
.spermatozoon  in  order  to  develop  further.  By  their  convincing  investi- 
gations the  doctrine  of  spontaneous  generation  was  driven  into  the  realm 
of  the  lower  animals.  Here  it  found  a  new  foundation  in  the  occurrence 
of  parasites  inside  of  animals  which,  at  the  beginning  of  their  life,  without 
doubt  must  have  been  free  from  these  internal  inhabitants.  Parasitolo- 
gists maintained  that  the  parasites  arose  quite  anew  from  the  superfluous 
plastic  material  of  their  host.  At  last,  by  a  series  of  epoch-making 
researches,  the  way  was  discovered  by  which  the  young  of  the  parasite, 
developing  from  eggs,  find  their  way  into  the  body  of  their  host.  It  was 
until  recently  considered  a  proof  of  the  doctrine  of  spontaneous  generation 
that,  after  a  time,  animal  and  plant  life  (unicellular  organisms,  infusorian 
animalcules,  etc.)  appears  in  water  supposed  to  contain  no  living  thing 
whatever  ;  further,  that  organic  fluids  became  foul  by  the  development  of 
the  lowest  of  the  plants,  the  bacteria.  At  present  we  know  that  in  all 
these  cases  germs  of  organisms,  carried  about  by  the  air,  are  the  cause  of 
the  new  development  of  life.  If  the  germs  be  killed  by  heating  the 
glass  and  boiling  the  fluid,  and  if  by  suitable  means  the  entrance  of  new 
germs  be  prevented,  then  such  a  *  sterilized  fluid '  remains  permanently 
unchanged.  It  has  been  found,  indeed,  that  spores,  particularly  of  bac- 
teria, have  an  extreme  power  of  resistance,  and  in  many  cases  must  be 


140  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

boiled  more  than  ten  minutes  before  they  are  destroyed.  As  the  final 
result  of  all  the  recent  experiments  and  observations  it  can  only  be  said 
that  t\\Q  present  existence  of  spontaneous  generation  is  not  proved.  Now 
the  question  is,  With  what  right  can  one  conclude  that  spontaneous  gene- 
ration neither  occurs  nor  has  ever  occurred  ? 

First  Origin  of  Life.— Whoever,  in  agreement  with  the  teachings  of 
astronomy,  adopts  the  view  that  our  earth  was  at  one  time  in  a  molten 
condition  and  has  gradually  cooled,  must  assume  that  life  on  the  earth 
has  not  existed  from  eternity,  but  at  some  time  has  had  its  beginning. 
If  he  wish  to  base  his  explanation,  not  upon  a  supernatural  act  of  crea- 
tion, nor  upon  hypotheses,  like  that  of  the  transference  of  living  germs- 
from  other  worlds  through  the  agency  of  meteors,  there  is  left  only  the 
hypothesis  that,  according  to  the  generally  prevailing  and  still  to  be 
observed  laws  of  chemical  affinity,  compounds  of  carbon,  oxygen,  hydro- 
gen, nitrogen,  and  sulphur  have  been  brought  together  to  produce  living 
substance.  This  process  is  called  spontaneous  generation.  If  the  carbon, 
oxygen,  nitrogen,  etc.,  which  are  now  combined  in  a  stable  manner  in 
organisms  were  formerly  unstable,  the  conditions  for  the  origin  of  organic 
compounds,  through  whose  wider  combination  life  would  be  possible,  may 
have  been  more  favorable.  Thus  the  hypothesis  of  the  first  origin  of  life 
through  spontaneous  generation  is  carried  to  a  logical  postulate. 

But  the  postulate  cannot  be  extended  to  affirm  that  spontaneous  gen- 
eration must  even  now  exist.  Since  there  are  neither  observations  nor 
convincing  theoretical  considerations  for  such  a  view,  there  is  no  necessity 
to  discuss  the  objections  here. 

II.  GENERATION  BY  PARENTS,  OR  TOCOGONY. 

As  mentioned  above,  we  shall  deal  here  only  with  those  methods 
of  reproduction  which  have  actually  been  observed,  i.e.,  generation 
by  parents.  These  methods  fall  mainly  into  two  great  groups, 
asexual  and  sexual  generation,  monogony  and  amphigony,  to  which 
may  be  added  a  third  group,  a  combination  of  these  two  methods 
of  reproduction. 

a.  Asexual  Reproduction.     Monogony. 

Monogony  Defined. — The  chief  characteristic  of  asexual  repro- 
duction is  the  fact  that  for  it  only  a  single  organism  is  necessary. 
But  since,  in  certain  modes  of  sexual  reproduction  (herma- 
phroditism,  parthenogenesis),  this  also  holds  true,  further  explana- 
tion is  necessary.  Asexual  reproduction  must  be  a  result  of  the 
growth  of  the  organism.  This  growth  may  be  general  and  result 
in  an  equal  growth  of  all  parts;  or  it  may  be  local  and  consequently 
lead  to  the  formation  of  an  outgrowth  in  the  region  of  greatest 
increase.  In  the  first  case  division  takes  place,  in  the  latter 
budding. 


GENERAL  EMBRYOLOGY. 


141 


Division. — In  the  case  of  division  (cf.  figs.  119,  122,  145)  an 
animal  separates  into  two  or  more  equivalent  parts,  so  that  it  is 
not  possible  to  distinguish  the  mother  and  the  daughter  animal; 
for  the  original  animal  has  completely  disappeared  in  the  young 
generation.  The  division  is  commonly  a  transverse  one,  in  which 
the  plane  of  division  stands  perpendicular  to  the  long  axis  of  the 
animal;  less  common  is  longitudinal  division,  rarest  is  oblique 
division  (the  planes  of  division  passing  in  the  direction  of  the  long 
axis,  or  forming  an  acute  angle  with  it). 

Budding. — In  the  case  of  budding,  the  products  are  unequal. 
One  animal  maintains  the  identity  of  the  mother  animal;  on  the 
contrary,  the  bud,  the  outgrowth  caused  by  local  increase,  appears 
as  a  new  formation,  as  the  daughter  individual.  Yet  the  differ- 
ence between  division  and  budding  is  bridged  by  intermediate 
conditions;  for,  if  we  start  with  binary  division,  this  will  approach 
budding  in  the  same  degree  as  the  division  products  become 
unlike,  so  that  the  one  takes  on  more  and  more  the  character  of  a 


FIG.  90.— A,  Hydra  grisea  in  optical  section  with  a  bud  ;  also  B,  first  stage  of  a  bud. 
i,  entoderm;  ek,  ectoderm;  s,  supporting  lamella;  t',  tent 


VIV)    0UWUV*4U)     C/rif,   Cl^  UV^VACI.  iAA  ,     O,    QU.pJJUL  Llllft    ICUUOUO.     ^ 

mal;  £",  tentacle  of  the  bud;  TU,  stomach;  o,  mouth. 


tentacle  of  the  mother  ani- 


bud,  the  other  retaining  the  character  of  the  mother  organism. 
Such  transitions  are  chiefly  possible  in  the  case  of  terminal 
budding,  where  the  buds  appear  at  one.  end  of  the  main  axis  of  the 
maternal  organism.  The  character  of  budding  is,  on  the  contrary, 
unmistakable  if  the  buds  arise  as  lateral  outgrowths  of  the  mother 


142  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

(fig.  90),  or  if  from  the  same  mother  numerous  buds  are  simul- 
taneously cut  off  (lateral  and  multiple  budding)  (compare  fig.  20). 

1}.  Sexual  Reproduction :  Amphigony. 

Amphigony  Defined. — For  sexual  reproduction  two  animals  are 
commonly  necessary,  a  female  and  a  male;  the  reproductive  cells 
— the  eggs — of  one  must  be  fertilized  by  the  reproductive  cells — 
the  spermatozoa — of  the  other,  and  thus  acquire  the  capacity  of 
giving  rise  to  a  new  organism.  Now,  since  there  are  hermaph- 
roditic animals  which  produce  simultaneously  eggs  and  sperma- 
tozoa, and  since  with  many  of  them  at  least  the  possibility  of 
self-fertilization  has  been  demonstrated,  it  becomes  clear  that  the 
emphasis  in  the  definition  of  sexual  reproduction  must  be  laid,  not 
upon  the  individual,  but  upon  the  sexual  products.  Consequently 
the  essential  point  of  sexual  reproduction  is  to  be  sought  in  the 
union  of  male  and  female  sexual  cells. 

Parthenogenesis  and  Paedogenesis. — This  explanation  is  appli- 
cable to  by  far  the  greater  majority  of  cases,  namely,  to  all  cases 
where  the  term  sexual  reproduction  can  be  applied.  Still,  in  the 
course  of  the  last  thirty  years  it  has  been  demonstrated  in  many 
instances  that  two  modes  of  reproduction  formerly  considered  as 
monogony,  parthenogenesis  and  psedogenesis,  must  be  regarded  as 
special  modifications  of  sexual  reproduction,  although  the  above- 
mentioned  conditions  are  not  strictly  satisfied.  In  both  cases  the 
eggs  develop  on  account  of  some  peculiar  internal  stimulus, 
without  the  occurrence  of  fertilization  ~by  spermatozoa.  In  case  of 
pcedogenesis  there  is  the  additional  circumstance  that  reproduction 
is  accomplished  by  animals  which  have  not  completed  their  normal 
development;  for  example,  the  larvae  of  certain  flies  reproduce 
before  they  have  passed  through  the  pupal  stage  and  become  flies. 
Paedogenesis  consequently  is  parthenogenesis  in  an  immature 
organism. 

Parthenogenesis  and  Typical  Amphigony. — Some  have  at- 
tempted to  exclude  parthenogenesis  from  sexual  reproduction  by 
claiming  that  those  eggs  which  develop  parthenogenetically  are 
pseudova,  structures  which  are  not  actual  eggs.  This  view  is 
absolutely  untenable  in  view  of  the  proof  that  the  '  pseud  ova' 
arise  just  like  ordinary  eggs  and  develop  like  them,  since  they 
cleave  and  form  germ-layers.  The  equivalence  of  parthenogenetic 
eggs  to  those  which  are  fertilized  is  best  shown  in  the  case  of  the 
bee,  where  similar  cells  give  rise  to  a  female  or  a  male  insect 


GENERAL  EMBRYOLOGY. 

according  as  they  are  or  are  not  furnished  by  the  queen  during 
oviposition  with  a  spermatozoon.  Parthenogenesis  is,  therefore, 
not  an  asexual  reproduction  which  was  antecedent  to  sexual  repro- 
duction, but  rather  a  reproduction  which  must  have  been  derived 
from  the  sexual ;  it  is  a  sexual  reproduction  in  which  a  degeneration 
of  fertilization  has  taken  place.  Such  facts  show  that,  for  the 
essential  point  of  sexual  reproduction,  fertilization  (the  entrance 
of  the  spermatozoon)  forms  indeed  an  extremely  important,  but  a 
by  no  means  indispensable,  characteristic.  To  all  cases  comprised 
under  amphigony  this  definition  alone  applies :  sexual  reproduction 
is  a  reproduction  by  means  of  sexual  cells. 

Sexual  and  Somatic  Cells. — The  distinction  of  sexual  cells  from  the 
asexual  reproductive  bodies,  the  parts  arising  by  division  and  budding,  is 
shown  by  their  relations  to  the  vital  processes  of  animals.  The  cells  of  a 
bud  have  had  a  share  in  the  vital  processes  of  the  animal  before  the  begin- 
ning of  reproduction  ;  they  were  functional  or  '  somatic '  cells.  In  the 
fresh- water  polyp  (fig.  90),  when  a  bud  arises,  the  cellular  material  em- 
ployed is  that  which  was  previously  related  to  the  mother  animal  in 
exactly  the  same  manner  as  the  other  parts  of  the  body  wall.  The  sexual 
cells  of  an  animal,  on  the  contrary,  are  permanently,  or  at  least  for  a  long 
time,  excluded  from  the  vital  processes,  remaining  in  a  resting  condition, 
and  conserving  their  vital  energies.  Therefore  there  are  also  lacking  in 
sexual  reproduction  the  relations  to  growth  which  are  so  remarkable  in 
asexual  reproduction.  For,  although  very  often  sexual  reproduction  does 
not  begin  until  the  bodily  growth  is  completed,  yet  it  is  found  repeatedly 
that  animals,  as  for  example  all  fishes,  continue  to  grow  after  the  begin- 
ning of  sexual  maturity,  until  they  are  double  or  many  times  their  size  at 
that  time.  Sexual  reproduction  is  not  even  a  special  form  of  growth,  but 
a  complete  renewal  of  the  organism,  a  rejuvenescence  of  it.  This  explains 
the  important  fact  that  asexual  reproduction  is  most  common  in  the  lower 
animals  (ccelenterates,  worms),  but  is  lacking  from  vertebrates,  molluscs, 
and  arthropods.  The  higher  the  organization  of  the  animal  the  more 
the  vital  energies  of  its  cells  must  be  employed  to  meet  the  increasing 
demands  upon  their  functional  capacity,  and  so  the  more  necessary  is 
sexual  reproduction. 

c.   Combined  Modes  of  Reproduction. 

Occurrence  in  the  Same  Species. — Very  often  two  modes  of 
reproduction  occur  in  one  and  the  same  species  of  animal  side  by 
side.  Many  corals  and  worms  have  the  power  of  multiplying  by 
division  or  budding,  and  also  of  forming  eggs  and  spermatozoa; 
again,  others  have  no  asexual  reproduction,  but  their  eggs  develop 
according  to  circumstances,  either  parthenogenetically  or  after 
fertilization.  The  appearance  of  two  kinds  of  reproduction  is  very 
often  governed  by  the  fact  that  individuals  with  different  modes  of 


144 


GENERAL  PRINCIPLES   OF  ZOOLOGY. 


reproduction  alternate  in  a  quite  definite  rhythm  with  each  other. 
Such  a  development  is  called  alternation  of  generations  in  the  wider 
sense,  and  of  this  two  special  forms  are  distinguished:  meta- 
genesis, or  alternation  of  generations  in  the  narrower  sense  (pro- 
gressive alternation  of  generations),  and  heterogony  (regressive 
alternation  of  generations). 

Progressive  Alternation  of  Generations.  Metagenesis. — Alter- 
nation of  generations  in  the  narrower  sense,  or  metagenesis,  is  the 
alternation  of  at  least  two  generations,  of  which  one  reproduces 
only  asexually,  by  division  or  budding,  the  other  either  exclusively, 
or  at  least  to  a  great  extent  sexually.  The  first  generation  is  called 
the  nurse,  the  second  the  sexual  animal.  The  reproduction  of 
hydromedusae  furnishes  the  best  example  (fig.  91).  The  nurses 


FIG.  91 .— Bougainvillea  ramnsa.    (From  Lang.)   ft,  hydranths  (nurse)  which  have  given 
rise  to  medusa-buds  (m/c) ;  m,  separated  medusa,  Margelis  ramosa  (sexual  animal). 

here  are  the  polyps,  which,  united  with  one  another  usually  in 
numbers  into  a  colony,  never  produce  sexual  organs,  but  bud  sexual 
animals,  the  medusce.  The  medusae  are  altogether  unlike  the 
polyps,  being  much  more  highly  organized,  and  freely  motile;  only 
very  rarely  have  they  preserved  the  asexual  mode  of  reproduction; 
on  the  other  hand,  they  develop  eggs  and  spermatozoa,  from  which 
the  non-motile  nurses,  the  polyps,  develop.  This  example  shows 


GENERAL  EMBRYOLOGY.  145 

that,  in  alternation  of  generations,  there  is  not  only  a  difference  in 
the  mode  of  reproduction,  but  usually,  in  addition,  a  difference  in 
form  and  organization.  Between  polyp  and  medusa  the  difference 
is  so  great  that  for  a  long  time  these  two,  though  representatives 
of  the  same  species,  were  referred  to  quite  different  classes  of  the 
animal  kingdom.  In  many  cases  the  alternation  of  generations 
may  be  still  further  complicated  by  two  asexual  generations  fol- 
lowing each  other,  before  the  return  to  the  sexual  generation  takes 
place;  one  speaks  then  of  grand-nurse,  nurse,  and  sexual  animal. 
Heterogony  is  distinguished  from  metagenesis  by  the  fact  that 
the  asexual  generation  is  replaced  by  parthenogenesis.  Conse- 
quently there  alternate  animals  of  sometimes  quite  different  struc- 
ture, of  which  the  one  arises  from  fertilized,  the  other  from 
unfertilized,  eggs.  Certain  Crustacea,  the  JJaphnidae,  show  heter- 
ogony  in  a  typical  manner.  During  a  large  part  of  the  year  only 
females  are  found;  these  increase  parthenogenetically  by  '  summer 
eggs';  then  males  appear  for  a  short  time;  they  fertilize  the 
'  winter  eggs/  which  now  are  formed,  from  which  again  partheno- 
genetic  generations  arise.  Very  often  heterogony  has  been  insuffi- 
ciently distinguished  from  metagenesis,  chiefly  for  the  reason  that 
parthenogenetic  reproduction  was  regarded  as  an  asexual  mode,  as 
was  the  case  in  the  trematodes.  The  sexually  ripe  Distomum 
produces  very  peculiar  sporocysts;  these  again  give  rise  partheno- 
genetically to  the  larvae  of  Distomum,  the  cercariae.  For  a  long 
time  the  erroneous  view  was  held  that  the  cells  from  which  the 
cercariaa  arose  were  not  eggs,  but  l internal  buds/  'germinal 
granules/  On  the  other  hand  there  have  been  included  under 
heterogony  modes  of  reproduction  in  which  no  parthenogenesis 
whatever  occurs.  Cases  have  been  called  heterogony  when  two 
generations  which  have  only  different  forms  and  organization 
alternate.  Ascaris  nigrovenosa,  an  hermaphroditic  worm,  lives  in 
the  frog's  lungs;  it  produces  the  separate-sexed  Rliabdonema 
nigrovenosum  living  in  mud,  from  whose  eggs  the  ascarid  of  the 
frog  is  again  produced. 

GENERAL    PHENOMENA    OF    SEXUAL    REPRODUCTION. 

In  sexual  reproduction  a  series  of  developmental  processes  is 
observed  which  is  repeated  in  an  essentially  similar  manner  in  all 
multicellular  animals,  and  hence  these  should  be  spoken  of  here 
together.  They  are:  (1)  the  maturation  of  the  egg;  (2)  the 
process  of  fertilization;  (3)  the  process  of  cleavage;  (4)  the  forma- 
tion of  the  three  germ-layers. 


146 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


1.   Maturation. 

The  egg  with  the  large  vesicular  nucleus  (germinal  vesicle) 
cannot  yet  be  fertilized;  to  render  it  capable  of  fertilization  it 
must  undergo  a  series  of  changes — the  process  of  maturation, 
which  consists  in  the  replacement  of  the  germinal  vesicle  by  a 
much  smaller  egg-nucleus,  and  simultaneously  the  formation  at 
one  pple  of  the  egg  of  the  ' directive  corpuscles'  or  ' polar  bodies/ 

Formation  of  the  Polar  Bodies. — The  germinal  vesicle  initiates 
the  changes,  its  walls  disappearing,  its  contents  in  part  mingling 
with  the  cytoplasm  of  the  egg,  in  part  being  employed  for  the 
formation  of  a  nuclear  spindle  (directive  spindle).  The  latter 
places  itself  with  its  axis  in  a  radius  of  the  egg  so  that  one  pole 
is  turned  towards  the  centre,  the  other  being  in  the  superficial 
layer  of  the  egg  (fig.  92,  a).  Now  begins  a  regular  cell-division,. 


FIG.  92.— Successive  stages  in  the  formation  of  the  polar  bodies  of  Ast erias  glacialis. 
sp,  directive  spindle;  rfc1,  first  polar  body;  r/c2,  second  polar  body;  e/c,  egg-nucleuss 
in  process  of  formation. 

but  the  products  of  the  division  are  of  very  unequal  size;  the  larger 
part  is  the  egg,  the  smaller  quite  insignificant  part  is  the  polar 
body  (fig.  92,  b,  c).  The  latter  projects  above  the  surface  carrying 
with  it  one  half  of  the  spindle,  and  when  the  globule  is  cut  off  half 
of  the  spindle  is  included  in  it. 

Tlie  Second  Polar  Body. — The  part  of  the  directive  spindle 
remaining  in  the  egg  immediately  forms  a  new  spindle;  the  cell- 
budding  is  repeated  and  leads  to  the  formation  of  the  second  polar 
body.  As  a  result  two  small  cells  (fig.  92,  d,  e,  f)  lie  at  one  pole 
of  the  egg,  in  many  cases  even  three,  since  during  the  formation 
of  the  second  polar  body  the  first  may  have  again  divided.  ^  The 
part  of  the  directive  spindle  still  remaining  after  the  second  divi- 
sion becomes  a  vesicular  resting  nucleus,  the  egg-nucleus,  the 


GENERAL   EMBRYOLOGY.  147 

characteristic  feature  of  the  ripe  egg  capable  of  fertilization.  In 
other  words,  by  a  double  division  there  have  been  formed  from  the 
immature  egg  four  (sometimes  three)  cells,  of  which  one  has 
received  by  far  the  greatest  part  of  the  original  mass  of  the  cell 
and  constitutes  the  ripe  egg,  while  the  others  are  small  bodies  like- 
rudimentary  eggs.  The  name  directive  corpuscles  was  given  to. 
them  because  in  very  many  cases  their  position  renders  possible  & 
definite  orientation  of  the  egg;  i.e.,  a  diameter,  the  long  axis,  can 
be  passed  through  the  egg,  one  end  of  which  is  marked  by  the 
directive  corpuscles.  With  reference  to  later  processes  of  develop- 
ment this  end  is  called  the  animal  pole  of  the  egg,  the  opposite 
end  the  vegetative  pole. 

Relation  between  Maturation  and  Fertilization. — In  many  cases  the- 
maturation  takes  place  before  the  entrance  of  the  sperm,  either  in  the- 
ovary  or  at  the  beginning  of  the  oviduct ;  in  many  animals,  on  the  con- 
trary, there  ensues  a  pause  after  the  first  polar  body  has  been  formed  ;  the- 
egg  then  requires  the  penetration  of  a  spermatozoon  in  order  to  complete 
the  further  changes,  i.e.,  the  formation  of  the  second  polar  body  and 
reconstruction  of  the  egg-nucleus.  This  dependence  of  the  last  phenomena 
of  maturation  upon  the  beginning  of  fertilization  led  for  a  long  time  to- 
the  error  that  the  formation  of  the  polar  bodies  was  a  part  of  the  fertiliza- 
tion process  itself. 

2.  Fertilization. 

Copulation  and  Artificial  Fecundation. — The  term  '  fertiliza- 
tion '  in  the  scientific  sense  refers  to  the  internal  processes  which, 
after  the  meeting  of  the  egg  and  spermatozoon,  go  on  in  the 
interior  of  the  former  and  end  with  a  complete  fusion  of  the  two 
sexual  cells;  on  the  other  hand,  special  expressions  are  necessary 
for  those  preparatory  processes  whose  purpose  is  to  render  fertiliza- 
tion possible.  Very  often,  but  not  in  all  cases,  there  is  necessary 
an  active  transfer  of  the  sperm  from  the  male  to  the  female,  a 
copulation.  In  case  of  many  marine  animals,  particularly  most 
fishes,  echinoderms,  ccelenterates,  the  eggs  and  the  spermatozoa 
are  discharged  into  the  water,  and  the  union  of  these  (impregna- 
tion or  fecundation)  depends  upon  chance.  One  can  bring  about 
then  artificially  what  is  accomplished  by  nature,  by  obtaining  from 
the  sexual  organs  the  ripe  products  and  bringing  them  together. 
For  example,  by  suitable  pressure  upon  the  body  of  sexually  ripe 
fishes  the  eggs  may  be  collected  in  one  dish,  the  sperm  in  another, 
and  the  contents  of  the  latter  poured  over  the  former,  and  thus 
in  many  cases  an  entirely  normal  development  may  be  obtained. 
Such  a  proceeding  is  called  artificial  impregnation. 


148 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


Fertilization. — The  process  of  fertilization  in  the  narrower 
sense  begins  with  the  entrance  of  the  spermatozoon  into  the  egg. 
Usually  the  egg  is  surrounded  by  a  gelatinous  envelope,  the 
chorion,  to  the  surface  of  which  the  spermatozoa  adhere,  and 
through  which  they  bore  until  they  reach  the  surface  of  the  egg 
(fig.  93).  But  since  the  chorion,  particularly  in  eggs  which"  are 
laid  in  the  air,  may  be  hard  and  resisting,  there  exists  in  it  very 
often  a  special  arrangement,  the  micropylar  apparatus,  rendering 
possible  the  entrance  of  the  spermatozoon;  this  may  be  a  single 
canal  extending  through  the  chorion,  as  in  the  eggs  of  fishes,  or  a 
group  of  such  canals,  as  in  those  of  almost  all  insects. 

Monospermy  and  Polyspermy, — Many  spermatozoa  may  pass 
through  the  gelatinous  envelope,  or  through  the  micropyle  canal, 
but  under  normal  conditions  only  one  serves  for  fertilization. 
The  spermatozoon  which  is  in  the  slightest  degree  ahead  of  the 
others  is  met  by  a  process  of  the  protoplasm  by  means  of  which  it 
enters  the  egg.  The  egg  is  now  impervious  to  all  others.  Only 
in  the  case  of  pathological  eggs  can  two  or  more  spermatozoa 
enter  and  then  multiple  impregnation  (di-  or  polyspermy)  occurs,  a 
pathological  phenomenon.  There  are  means  of  protection  against 


Fio.  93.— Egg  of  Aster  ins  ;  aclalis  during  fecundation.   (After  Fo;.)  A,  entrance  of  the 
spermatozoon;  _B,  the  s  .ermatozoon  has  entered  :  the  yolk-membrane  has  formed. 

this  abnormal  fertilization.  One,  though  by  no  means  the  only 
one,  is  the  formation  of  the  yolk-membrane,  an  impermeable 
envelope  which  is  suddenly  secreted  from  the  surface  of  the  egg, 
.as  soon  as  the  spermatozoon  has  accomplished  the  impregnation. 
Within  the  yolk-membrane  the  body  of  the  egg  contracts  into  a 
smaller  volume  by  discharging  some  of  the  more  fluid  constituents, 
so  that  between  the  yolk-membrane  and  the  surface  of  the  egg  a 
cavity  is  formed  easily  recognized  in  smaller  fertilized  eggs 
(fig.  93,  B). 

In   the   large  yolk-laden  eggs  of  many  insects  and  vertebrates  several 
spermatozoa  may  normally  enter.     But  this  does  not  alter  the  conception 


GENERAL  EMBRYOLOGY. 


149 


of  fertilization,  for  even  here  but  one  spermatozoon  fuses  with  the  egg- 
nucleus,  the  others  degenerating  sooner  or  later. 

Essential  Feature  of  Fertilization. — After  the  spermatozoon 
has  penetrated  into  the  egg,  the  head  and  the  middle  piece  which 
contains  the  centrosome  can  still  be  recognized,  as  the  chromatic 
and.  achromatic  parts  of  the  spermatozoon  or  sperm-nucleus  (male 
pronucleus),  while  the  tail  and  the  slight  amount  of  protoplasm 
disappear  in  the  yolk.  In  the  cytoplasm  of  the  egg  the  centrosome- 
of  the  sperm-nucleus  gives  rise  to  conspicuous  rays,  like  those 
observed  during  division.  Preceded  by  these  rays  the  sperm- 
nucleus  travels  towards  the  egg-nucleus  until  it  reaches  (fig.  94) ;. 
and  fuses  with  it  to  form  a  single  cleavage  nucleus.  Now  the^ 
centrosome  divides  into  two,  which  migrate  to  opposite  poles  of 
the  nucleus,  while  the  cleavage  nucleus  changes  to  a  cleavage 
spindle,  which  divides  and  thus  initiates  the  embryonic  develop- 
ment, the  successive  divisions  being  known  as  the  cleavage  or  seg- 
mentation of  the  egg.  Since  not  until  this  point  is  fertilization 
complete,  we  arrive  at  the  fundamentally  important  proposition 
that  the  essential  feature  of  fertilization  consists  in  the  union  of 
egg  and  sperm  nuclei. 


FIG.  94.— Stages  in  the  fertilization  of  the  egg  of  the  sea-urchin.  (After  O.  Hertwig.y 
The  sperm-nucleus  (x/f )  with  its  rays  is  near  the  surface  in  one  egg,  in  the  other 
near  the  egg-nucleus  (ek). 

Part  Played  by  the  Two  Nuclei  in  Fertilization.— In  many 
cases  an  abbreviation  of  development  may  take  place,  the  stage  of 
the  cleavage  nucleus  being  omitted,  and  the  egg  and  sperm  nuclei, 
without  previously  uniting,  pass  directly  into  the  cleavage  spindle. 
This  fact  in  no  wise  alters  the  above-mentioned  proposition,  but 
yet  it  is  important,  because  it  shows  more  plainly  in  what  way  the 
two  nuclei  participate  in  the  formation  of  the  cleavage  spindle. 
It  shows  that  of  the  chromosomes  which  form  the  equatorial  plate 
of  the  nucleus,  exactly  one  half  are  furnished  by  the  egg-nucleus, 


150  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

the  other  by  the  sperm-nucleus.  For,  even  before  the  spindle  has 
been  formed  and  the  contour  of  the  two  nuclei  has  disappeared, 
the  chromosomes  destined  for  the  spindle  are  completely  developed 
in  exactly  the  same  number  in  each  of  these  (fig.  95). 


FIG.  95.— Fertilization  of  Ascaris  megalocephala .  (After  Boveri.)  A,  the  ends  (centro- 
somes)of  the  spindle  formed;  B,  the  spindle  completed;  sp,  sperm-nucleus  with 
its  chromosomes;  e/,  egg-nucleus;  p,  polar  bodies. 

Heredity. — Recent  observations  have  furnished  a  certain  basis 
for  the  doctrine  of  heredity.  By  heredity  we  understand  the 
transmission  of  parental  characteristics  to  the  offspring.  This 
transmission,  on  the  whole,  takes  place  with  equal  energy  from 
the  father's  and  from  the  mother's  side;  if  we  take  the  average  of 
numerous  cases,  the  result  is  that  the  child's  peculiarities  hold  the 
mean  between  the  peculiarities  of  father  and  mother;  or,  in  other 
words,  male  and  female  individuals  in  the  average  have  an  equal 
power  of  transmitting  characteristics. 

The  Physical  Basis  of  Heredity. — Since  in  case  of  all  animals 
with  external  fertilization  a  material  connexion  between  parents 
and  offspring  can  exist  only  through  the  sexual  cells,  these  latter 
must  contain  the  substances  which  render  heredity  possible; 
further,  the  two  hereditary  substances,  in  cases  of  equal  capacity  for 
transmission,  must  be  present  in  the  egg  and  in  the  spermatozoon 
in  equal  quantity.  By  this  course  of  reasoning,  the  chromatic 
nuclear  substance  which  forms  the  chromosomes  has  come  to  be 
regarded  as  the  bearer  of  heredity;  for  we  know  that  the  egg  con- 
tains a  great  quantity  of  cytoplasm,  but  the  spermatozoon  only  the 
slightest  trace  of  it;  that,  on  the  other  hand,  egg- nucleus  and 
sperm-nucleus  furnish  equivalent  substances,  and  particularly  the 
same  quantity  of  chromosomes,  to  the  cleavage  spindles ;  hence  only 
the  chromatin  can  be  regarded  as  the  hereditary  substance  (idio- 
plasm). This  supports  the  view  expressed  before  (p.  67)  that  the 
nucleus  is  the  bearer  of  hereditary  qualities  and  determines  the 
character  of  the  cell. 


GENERAL  EMBRYOLOGY.  151 


3.   Cleavage  Process. 

Arrangement  of  the  Cleavage  Planes.  —  The  fertilized  egg-cell 
divides  in  rapid  succession  into  2,  4,  8,  16,  etc.,  cells,  which 
become  continually  smaller,  since  the  mass  of  the  egg  does  not 
increase.  The  cells  are  called  cleavage  spheres,  or  blastomeres, 
the  whole  process  the  cleavage  process,  or  segmentation, 
because,  at  each  division,  furrows  arise  on  the  surface  which 
continue  to  penetrate  more  deeply  (fig.  93).  As  a  rule  each 


FIG.  96.— The  equal  cleavage  of  Amphioxws  lanceolattis.  (After  Hatschek.)  I,  division 
into  two  (formation  of  the  first  meridional  furrow);  II,  division  into  four  (second 
meridional  furrow)  forming  four  cleavage  spheres  (fourth  is  hidden);  III,  division 
into  eight  (equatorial  furrow;  the  seventh  and  eighth  cleavage  spheres  hidden) ; 
IV,  blastula  in  optical  section.  A  single  layer  of  cells  surrounds  the  cleavage 
cavity.  In  I,  II,  III,  a  polar  body  is  shown. 

new  plane  of  cleavage  is  as  nearly  as  possible  perpendicular  to  the 
preceding.  Hence  the  first  three  cleavage  planes,  which  cause  the 
division  into  2,  4,  and  8  parts,  are  similarly  arranged  in  almost  all 
animals.  Using  the  globe  as  a  basis  for  comparison,  one  speaks 
of  a  first  and  a  second  meridional  furrow  (I,  II),  and  calls  the  third 
the  equatorial  furrow  (III).  The  intersections  of  the  two  meridi- 
onal furrows  form  the  poles  of  the  egg,  the  animal  and  the 
vegetative,  so  called  because  the  material  of  the  one  is  used  chiefly 
for  animal  organs  (nervous  system),  the  material  of  the  other  for 
vegetative  organs  (digestive  tract). 

Influence  of  the  Yolk  upon  Segmentation. — Different  kinds  of 
cleavage  processes  are  distinguished,  the  peculiarities  of  which 
depend  upon  two  factors:  (1)  upon  the  quantity  of  material,  food- 
yolk,  serving  for  nourishment  of  the  egg;  (2)  upon  the  arrange- 
ment of  this.  The  food-yolk  hinders  the  division,  since  it  is  a 
material  which  is  incapable  of  active  movement,  and  is  only 
passively  divided  through  the  activity  of  the  protoplasm  in  the 
cleavage  cells.  The  more  the  mass  of  this  increases  in  proportion 
to  the  protoplasm,  the  more  slowly  does  the  cleavage  process  pro- 
ceed. Finally  there  comes  a  point  where  the  resistance  of  the 
yolk  becomes  so  great  that  the  protoplasm  is  no  longer  able  to 
carry  out  the  work  completely;  then  only  the  protoplasmic  part 


152 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


of  the  egg  is  divided,  that  which  is  rich  in  yolk  remaining  an 
undivided  mass.  In  this  case  one  speaks  of  a  partial  cleavage  in 
comparison  with  the  ordinary  and  more  primitive  mode,  the  total 
cleavage;  further,  the  eggs  which  show  a  partial  cleavage  are  called 
meroblastic,  because  only  the  segmented  part  of  the  egg  is  directly 
employed  in  the  formation  of  the  embryo  or  bud  (fiXacrTos),  while 
the  undivided  main  mass  serves  merely  as  food-material  in  the 
course  of  growth.  Eggs  with  total  cleavage,  on  the  contrary,  are 
called  holoblastic. 

Distribution  of  the  Yolk. — The  arrangement  of  the  yolk  is 
connected  with  the  position  of  the  nucleus;  either  the  egg-nucleus 
maintains  a  central  position  and  collects  the  yolk  concentrically 
around  itself  (centrolecithal  eggs)  (fig.  97),  or  it  is  pushed,  together 


FIG.  97. 


FIG.  98. 
71,  nucleus;  p,  portion  of  the  egg 


FIG.  97.— Centrolecithal  egg.    (After  O.  Hertwig.) 

rich  in  protoplasm;  d,  portion  rich  in  yolk. 
FIG.  98.— Telolecithal  egg.    (After  O.  Hertwig.)    Letters  as  in  fig.  97. 

with  the  greater  part  of  the  protoplasm,  to  one  pole  of  the  egg, 
while  at  the  other  pole  the  yolk  predominates  (telolecithal  eggs). 
Since  the  nuclear  pole,  in  the  course  of  development,  always 
becomes  the  animal  pole,  there  can  be  distinguished  in  the  egg  an 
animal  part  rich  in  protoplasm  and  a  vegetative  part  rich  in  yolk 
(fig.  98).  In  many  telolecithal  eggs  the  two  regions  pass  gradually 
into  one  another,  but  in  others  a  distinct  boundary  separates  an 
almost  purely  protoplasmic  animal  portion  from  a  yolk-containing 
vegetative  portion.  This  condition  is  well  shown  in  the  bird's  egg 
(fig.  99).  Here  only  the  yolk  is  to  be  regarded  as  an  egg  in  the 
embryological  sense,  while  the  white,  the  egg-membrane,  and  the 
calcareous  shell  are  only  later  depositions  upon  the  surface  of  the 
egg.  The  chief  mass  of  the  yolk  is  deutoplasm,  upon  which  rests 
a  thin  layer  of  protoplasm,  the  germinal  disc,  always  uppermost 


GENERAL  EMBRYOLOGY. 


153 


whatever  the  position  of  the  egg.     The  protoplasmic  layer  contains 
the  egg-nucleus,  and,  after  fertilization,  by  progressive  develop- 


oh.l 


FIG.  99.— Diagrammatic  longitudinal  section  through  a  bird's  egg.  (After  Balfour.) 
(1)  The  egg:  b.J.,  blastoderm;  w.y.,  white  yolk:  y  y.,  yellow  yolk.  (2)  Coverings  of 
the  egg:  v.t.,  yolk  membrane  (vitelline  membrane);  x.  and  w.,  inner  and  outer 
layers  of  white;  cto.L,  chalazae;  i.s.m.  and  a.m.,  inner  and  outer  shell-membrane; 
between  them  at  the  right  end  is  the  air-chamber  (a.c./L);  s,  shell. 

ment   continually  separates   itself   (blastoderm)  more   and    more 
sharply  from  the  underlying  yolk. 

Various  Types  of  Cleavage. — After  the  foregoing  remarks  a 
brief  explanation  will  suffice  to  render  intelligible  the  following 
figures  of  the  various  modes  of  cleavage. 


a.  Holoblastic  Eggs  with  Total  Cleavage. 

1.  Equal  Cleavage. — The  yolk,  present  only  in  small  quantity, 
is  distributed  equally  through  the  egg;  upon  cleaving,   the  egg 
divides  into  parts  of  approximately  the  same  size  and  equally  rich 
in  yolk  (alecithal  eggs,  fig.  96). 

2.  Unequal  Cleavage. — The  yolk  is  abundant,  but  not  in  such 
a  quantity  as  to  prevent  complete  cleavage;  it  lies  especially  at  the 
vegetative  pole  of  the  egg,  causing  the  cleavage  in  this  region  to 
progress  more  slowly;  here  larger  cleavage  spheres  are  formed, 
because  richer  in  yolk;  hence  the  embryo,  at  the  very  first,  is 
found  to  be  composed  of  smaller  animal  cells  poor  in  yolk,  and 
larger  vegetative  cells  rich  in  yolk  (telolecithal,  holoblastic  eggs, 
figs.   100  and  101).     In  some  instances  of  unequal  cleavage  the 


154 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 
A  B 


Fio.  100.— Unequal  cleavage  of  the  egg  of  Petromyzon.  (After  Shipley,  from  Hat- 
schek.)  A,  stage  of  eight  cleavage  spheres ;  JB,  blastula  in  meridional  section. 
The  dissimilarity  of  the  cleavage  spheres  begins  with  the  equatorial  furrow. 


in 


.  101.— Unequal  cleavage  of  a  snail's  egg,  Nassa  mutabilis.  (After  Bobretzky.)  I, 
the  first  meridional  furrow  has  divided  the  egg  into  unequal  parts;  II,  the  second 
meridional  furrow  has  formed  three  smaller  and  one  larger  cleavage  sphere 
(seen  from  the  side);  III,  the  equatorial  furrow  has  formed  four  smaller  animal 
and  four  larger  but  unequal  vegetative  cells  (seen  from  the  animal  pole). 

cells  are  in  straight  lines  (fig. 
100,  A),  but  in  others  the  cells 
alternate  (fig.  102) ;  this  is  called 
spiral  cleavage  and  is  common  in 
several  groups. 

b.    Meroblastic  Eggs  with  Partial 
Cleavage. 

3.  Discoidal  Cleavage. — The 
yolk  is  so  collected  in  the  vegeta- 
tive portion  of  the  egg  that  it 
prevents  cleavage;  cleavage,  there- 
in crepiduia.  fore,  is  limited  to  the  region 
around  the  animal  pole  and  here 
forms  a  disc  of  small  cells,  the  anlage  of  the  embryo,  or  blasto- 
derm (telolecithal,  meroblastic  eggs)  (figs.  99,  103). 

4.  Superficial  Cleavage. — The  yolk  is  collected  in  the  centre  of 
the  egg  and  prevents  cleavage;  in  consequence  of  this  only  the 
outer  layer  of  the  egg  divides  into  cells,  which,  in  the  form  of  a 
continuous  superficial  layer,  enclose  the  unsegmented  central  mass 
(centrolecithal  eggs)  (fig.  104). 


FIG.  102.— Spiral  cleavage  i 
(After  Conklin.) 


GENERAL  EMBRYOLOGY. 


155 


Distribution  of  the  Types  of  Cleavage.— Of  the  four  types  of 
cleavage  mentioned  the  superficial  one  has  an  interest  from  the 
point  of  view  of  the  systematist,  since  it  occurs  exclusively  in  the 
arthropods.  The  other  modes  of  cleavage  are  distributed  as  fol- 
lows: the  discoidal  has  heen  observed  in  the  majority  of  the 
vertebrates  and  in  the  most  highly  organized  molluscs,  the 
A  0 


FIG.  103.- Discoidal  cleavage  of  the  egg  of  a  cephalopod  (Loligo  pealii). 
(After  Watase.) 


TIG.  104.— Superficial  cleavage  of  an  insect  egg  (Pieris  cratcegi).  (After  Bobretzky.) 
A,  division  of  the  cleavage  nucleus  ;  B,  movement  of  the  nuclei  to  the  periphery 
to  form  the  blastoderm;  C\  formation  of  the  blastoderm. 

cuttlefishes,  while  the  equal  and  the  unequal  cleavage  can  be  found 
in  all  the  groups  of  the  Metazoa. 

Blastula. — Sometimes  during  the  first  stages  of  segmentation, 
sometimes  later,  there  is  usually  formed  a  cavity,  the  cleavage  or 
sea  mentation  cavity,  between  the  cells  in  the  interior  of  the  egg; 
with  the  progress  of  development  this  cavity  becomes  continually 
larger  (fig.  100,  B).  Around  it  the  cells  lie  in  the  form  of  a  one- 
layered  or  of  a  many-layered  epithelium  and  form  the  blastoderm; 
hence  the  name  for  this  stage,  Nastodermic  vesicle,  or,  briefly, 
Uastula.  The  more  yolk  there  is  present,  the  smaller  is  the 
cleavage  cavity;  in  centrolecithal  eggs  with  superficial  cleavage  it 
is  entirely  absent. 


156 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


4.   Formation  of  the  Germ-layers. 

Gastrula. — Besides  the  blastula  there  is  still  a  second  stage  of 
development,  the  gastrula  or  the  two-layered  embryo,  which  is 
common  to  all  the  Metazoa.  This  stage  is 
understood  easiest  in  the  case  of  eggs  which 
have  an  equal  cleavage  (fig.  105,  Z?);  here  it 
has  the  form  of  a  double-walled  cup  with  a 
wider  or  narrower  mouth.  The  cavity  of  the 
cup  (the  primitive  digestive  tract  or  arclien- 
teroii)  is  the  beginning  of  the  most  important 
part  of  the  digestive  system;  the  opening  is 
the  primitive  mouth  or  blastopore  (iwostoma). 
Of  the  two  layers  of  cells  forming  the  wall  of 
the  cup  and  uniting  at  the  blastopore,  the 
external  is  the  ectoderm  or  outer  germ-layer, 
the  internal  the  entoderm.  or  inner  germ-layer. 
In  the  gastrula  we  meet  for  the  first  time 
the  formation  of  germ -layers,  i.e.,  the  forma- 
tion of  definite  embryonic  layers  marked  off 
from  each  other,  the  cells  not  yet  differen- 
tiated, from  which  organs  arise  through 
organological  and  histological  differentiation. 
Invagination. — The  gastrula  is  formed 
from  the  blastula  by  invagination  (fig.  105,  A}. 
The  result  is  the  same  as  when  by  pressure  of 
the  finger  upon  a  hollow  india-rubber  ball 
one  side  is  pressed  in  against  the  other;  the  layer  of  vegetative 
cells  gradually  sinks  in  and  becomes  surrounded  by  the  cells  of  the 
animal  pole  (fig.  105,  B}.  Thus  there  arises  in  the  egg,  in  addi- 
tion to  the  cleavage  cavity,  a  new  cavity,  the  anlage  of  the  lumen 
of  the  digestive  tract;  this  increases  and  finally  obliterates  the 
cleavage  cavity,  so  that  the  invaginated  part  of  the  blastoderm, 
the  entoderm,  becomes  pressed  against  the  part  which  remains 
external,  the  ectoderm. 

Modified  Modes  of  Gastrulation. — In  the  case  of  eggs  with  much  food- 
yolk  the  relation  of  the  structure  and  of  the  mode  of  formation  of  the 
gastrula  is  more  difficult  to  understand.  Here,  however,  it  is  sufficient  to 
mention  the  fact  that  the  gastrula  stage  has  fortunately  been  discovered 
for  almost  all  eggs  with  a  great  quantity  of  food-yolk,  and  that  the  yolk- 
material  finds  lodgment  principally  in  the  entodermal  cells. 

Epiblast  and  Hypoblast. — For  outer  and  inner  germ-layer  the  terms 
epiblast  and  hypoblast,  upper  and  lower  germ-layer,  have  been  much 


FIG.  105.  —  Gastrulation 
of  AmpJiioxus.  (After 
Hatscnek  )Theanimal 
pole  here  is  above,  and 
the  vegetative  pole 
below,  in  comparison 
with  fig.  93.  In  fig. 
A  the  cells  of  the 
vegetative  pole  are  be- 
ginning to  sink  in;  B, 
the  invagination  com- 
pleted, the  cleavage 
cavity  reduced  to  a 
slit  between  the  ento- 
derm (en)  and  the  ecto- 
derm (ek) ;  o,  blasto- 
pore. 


GENERAL  EMBRYOLOGY, 


157 


used  ;  these  names  are  strictly  applicable  only  to  those  eggs  with  discoidal 
cleavage.  In  the  bird's  egg,  for  example,  the  two  germ-layers  form  over 
the  unsegmented  yolk,  from  which  they  become  separated  by  the  gas- 
trular  cavity ;  thus,  then,  the  external  germ-layer  actually  lies  above,  the 
internal  below.  Other  terms  for  the  two  germ-layers  are  entoblast  and 
ectoblast. 

Delamination. — In  regard  to  the  mode  of  development  of  the  gastrula 
many  controversies  have  arisen  which  are  not  yet  finally  settled  ;  in  addi- 
tion to  invagination  there  may  exist  a  second,  but  very  much  less  frequently 

J?  ^  ******  C 


FK;.  106.— Delamination  of  the  egg  of    a  Geryonid.    (After  Fol,   from  Korschelt- 
Heider.)    ft,  cleavage  cavity  ;  0,  jelly. 

occurring  mode  of  development,  delamination.  In  delamination  the 
blastula  may  become  two-layered  by  tangential  division  of  its  cells  (fig. 
106) ;  each  single  blastoderm-cell,  or,  at  least,  the  majority  of  these  cells, 
by  this  division  falls  into  a  peripheral  ectodermic  and  a  central  ento- 
dermic  cell.  In  case  of  delamination  the  cleavage  cavity  becomes  directly 
the  cavity  of  the  digestive  track,  a  fact  which  renders  it,  difficult  to 
regard  delamination  and  invagination  as  modifications  of  one  and  the 
same  process. 

Formation  of  the  Mesoderm.  The  Mesenchyme. — Many  lower 
animals,  e.g.  most  coelenterates,  have  in  general  only  two  germ- 
layers.  When  these  are  laid  down  there  begins  immediately  the 
differentiation  of  muscle  and  nerve  fibres  and  the  other  processes 
of  histological  changes  of  the  cells,  as  well  as  a  series  of  changes 
of  form,  by  which  the  gastrula  becomes  the  adult  animal.  In 
higher  organisms,  on  the  other  hand,  before  further  differentiation 
begins,  there  arises  still  a  third  germ-layer,  which,  owing  to  its 
position  between  the  first  two,  is  called  the  mesoderm,  mesoblast, 
or  middle  germ-layer;  this  naturally  can  come  only  from  the  cell 
material  of  the  existing  germ-layers,  and  indeed  only  the  entoderm 
seems  to  participate  in  it.  Two  methods  can  be  distinguished  in 
its  formation.  In  one  the  space  between  ectoderm  and  entoderm 
becomes  widened  by  the  secretion  of  gelatinous  substance,  and 
from  the  entoderm  isolated  cells  push  into  this  jelly;  thus  there 
arises  a  middle  layer,  the  mesenchyme  (fig.  107),  somewhat  similar 


158 


GENERAL  PRINCIPLES  OF  ZOO  LOOT. 


to  gelatinous  connective  tissue,  from  which  certain  organs  either 
wholly  or  in  part  take  their  origin. 

Mesothelium. — In  the  second  case  the  mesoderm  may  preserve 
the  epithelial  character  of  the  two  primary  germ-layers,  so  that  it 


FIG.  107.— Formation  of  the  mesenchyme  and  beginning  of  gastrulation  in  Holothuria- 
tubulosa.  (After  Selenka,  from  Balfour.)  cc,  cleavage  cavity;  ep,  ectoderm;  Tiy, 
entoderm;  ms,  mesenchyme  cells;  ae,  archenteron. 

is  called  mesothelium.  The  mesothelium  is  cut  off  from  the 
entoderm,  the  mode  of  development  being  shown  in  the  embryology 
of  the  worm  Sagitta  (fig.  108). 


FIG.  108.— Formation  of  the  mesothelium  and  coelom  of  Sagitta.  A.  From  the  bottom 
of  the  gastrula  arise  two  folds,  which  divide  the  archenteron  into  the  permanent 
digestive  tract  and  the  coelomic  diverticula.  B.  The  separation  is  almost  com- 
pleted by  the  pushing  up  of  the  folds,  afc,  outer,  mk,  middle,  t/c,  inner  germ- 
layer;  wife1,  somatic  layer;  rnfc2,  splanchnic  layer;  Ih,  body-cavity. 

Coelomic  Pouches. — When  the  gastrula  of  Sagitta  has  been 
formed  two  folds  arise  from  the  archenteric  walls  opposite  the 
blastopore  (A),  thus  partially  separating  a  pair  of  lateral  chambers 
from  the  rest.  The  process  continues;  the  blastopore  closes,  while 


GENERAL  EMBRYOLOGY. 

the  entodermal  folds  extend  to  the  opposite  side,  where  they  fuse 
with  the  walls  (B).  In  this  way  a  pair  of  coelomic  pouches  are 
cut  off  from  the  rest  of  the  archenteron  which  forms  the  lumen  of 
the  digestive  tract  and  its  derivatives,  while  the  walls  of  the 
pouches  form  the  mesothelium,  that  of  the  digestive  region  the 
secondary  entoderm.  In  each  coelomic  pouch  two  walls  are  recog- 
nizable, an  inner  or  splanchnic  layer  which  unites  with  the 
entoderm  to  form  the  wall  of  the  digestive  tract,  the  splanchno- 
pleure,  while  the  somatic  layer  unites  similarly  with  ectoderm  to 
form  an  outer  body  wall,  the  somatopleure.  From  the  foregoing  it 
is  evident  that  the  mesothelium  is  strictly  not  a  single  layer,  but 
consists  of  two  layers  which  pass  into  each  other,  and  that  its 
origin  is  closely  connected  with  the  formation  of  the  body  cavity. 

Occurrence  of  Mesenchyme  and  Mesothelium — There  are  three 
possible  methods  for  the  distribution  of  mesenchyme  and  meso- 
thelium, and  these  actually  occur.  There  are  purely  mesenchy- 
matous  animals,  like  the  flat-worms,  and  purely  mesothelial,  like 
Sagitta,  many  annelids,  and  Amphioxus;  but  there  are  also- 
animals  in  which  the  mesoderm  consists  of  mesenchyme  and 
mesothelium:  either  the  mesenchyme  arises  first  and  later  the 
mesothelium,  as  in  the  echinoderms,  or  the  reverse  order  is  fol- 
lowed, as  in  most  vertebrates. 

Histological  and  Organological  Differentiation. — All  the  organs 
of  an  animal  arise  from  the  three  germ-layers  in  this  way :  first, 
embryonic  cell  material  is  marked  off  into  separate  complexes, 
usually  by  infolding  (organological  differentiation),  and  then  later 
these  become  changed  into  tissues  (Mstological  differentiation). 
The  details  differ  in  the  various  animal  groups;  the  following  is 
the  most  general :  from  the  ectoderm  arise  the  skin  with  its  glands 
and  appendages,  the  nervous  system,  and  the  sensory  epithelium^ 
the  entoderm  gives  rise  to  the  most  important  part  of  the  digestive 
tract  with  its  glands;  while  muscles,  blood,  supporting  and  con- 
nective substances,  excretory  organs,  in  whole  or  in  part,  arise  in 
the  mesoderm;  the  sexual  organs  are  also  usually  mesodermal. 

Relations  of  the  Germ-layers  in  Budding. — Of  late  the  question  has  often 
been  raised  as  to  how  far  the  germ-layer  theory  is  applicable  to  the  occur- 
rences in  asexual  reproduction.  At  first  one  would  expect  in  budding, 
and  still  more  in  the  case  of  division,  that  each  organ  of  the  daughter 
animal  would  arise  from  the  corresponding  organ  of  the  maternal  animal, 
or,  if  that  be  impossible  by  conditions  of  space,  from  a  mass  of  tissue 
belonging  to  one  of  the  same  germ-layers.  In  many  instances  this  is  cer- 
tainly the  case,  as,  for  example,  in  the  budding  of  hydroids  the  entoderm 
and  ectoderm  of  the  bud  arise  from  the  corresponding  layers  of  the  maternal 


160  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

body  (fig.  90).  But  through  recent  investigations  exceptions  to  this  rule 
have  become  known.  In  polyzoans  and  tunicates  there  are  undifferen- 
tiated  cells  which  are  employed  in  cases  of  budding ;  these  are  elements 
without  the  characteristics  of  a  definite  body  layer  which,  independently 
of  the  position  they  assume  in  the  maternal  animal,  can  be  employed, 
according  to  need,  in  the  building  up  of  organs.  In  the  regeneration  of 
lost  parts  investigations  show  that  it  is  not  necessary  that  the  missing 
structure,  in  worms  and  even  in  vertebrates,  should  be  re-formed  by  the 
same  layer  from  which  it  originally  arose.  The  lens  of  Triton  arises 
ontogenetically  from  the  epithelium  of  the  skin.  If  extirpated,  it  is  regen- 
erated from  the  pigmented  epithelium  of  the  iris. 

5.    The  Different  Forms  of  Sexual  Development. 

Embryonic  and  Postembryonic  Development.  —  While  the 
occurrences  described  (fertilization  and  cleavage  of  the  egg,  forma- 
tion of  the  germ-layers)  are  going  on  the  young  animals  are 
usually  enclosed  within  a  firm  protective  covering,  or  even  in  the 
maternal  sexual  apparatus  (uterus),  and  are  hence  called  embryos. 
Later  stages,  even  the  formation  of  the  most  important  organs, 
may  occur  during  embryonic  life,  as  we  see  in  case  of  the 
mammals,  birds,  reptiles,  many  fishes,  worms,  and  crabs,  which, 
at  the  end  of  their  embryonic  existence,-  are  complete  in  all  their 
parts,  and  need  only  the  maturity  of  the  sexual  organs,  and  growth 
of  the  body  as  a  whole,  in  order  to  reach  the  climax  of  their 
development.  On  the  other  hand,  there  are  animals,  chiefly 
aquatic,  which,  after  leaving  the  egg,  undergo  important  changes, 
like  the  coelenterates,  echinoderms,  insects,  amphibians,  etc.  The 
coelenterates,  echiuoderms,  and  many  worms  usually  escape  from 
the  egg  even  before  the  formation  of  the  germ-layers,  and,  as  free- 
swimming  ciliated  'planulae/  form  the  germ-layers  and  organs. 
Since  there  is  here  a  more  or  less  extensive  post-embryonic  develop- 
ment, it  is  a  misnomer  to  apply  the  term  '  embryology '  to  both 
stages;  it  is  necessary,  rather,  to  limit  the  name  to  the  develop- 
mental processes  inside  the  egg,  and,  on  the  other  hand,  to  speak 
generally  of  the  history  of  the  development  of  the  individual,  or 
ontogeny.  As  the  undeveloped  animal  within  its  membrane  is 
called  an  embryo,  so  the  name  larva  is  applicable  to  the  free-living 
but  not  completely  matured  animal. 

Direct  and  Indirect  Development — Metamorphosis. — Larval 
•development  may  be  either  direct  or  indirect.  In  direct  develop- 
ment, as  the  term  implies,  the  larva  pursues  the  direct  way  towards 
the  sexually  mature  animal,  the  lacking  organs  being  outlined  one 
after  another;  hence  it  is  continually  becoming  more  like  the 


GENERAL  EMBRYOLOGY.  161 

sexually  mature  animal.  In  indirect  development,  on  the  con- 
trary, organs  belonging  only  to  the  larval  life,  and  hence  called 
larval  organs,  are  formed  and  later  are  destroyed.  Therefore  in 
the  definition  of  indirect  development,  or  as  it  is  commonly  called 
metamorphosis,  special  emphasis  is  laid  upon  the  presence  of  larval 
organs.  Thus  the  caterpillars  of  butterflies  are  distinguished  not 
only  by  the  absence  of  compound  eyes  and  wings,  but  also  by  the 
presence  of  anal  feet  and  spinning-glands,  which  are  absent  in  the 
butterfly,  and  further  by  the  different  shape  of  the  jaws,  antennae, 
and  legs,  the  different  arrangement  of  the  tracheae  and  nervous 
system,  etc.  Tadpoles  are  distinguished  from  frogs  not  only  by 
the  absence  of  lungs  and  extremities,  but  also  by  the  presence  of 
gills  and  tail.  The  more  numerous  the  larval  organs,  the  more 
pronounced,  therefore,  will  be  the  metamorphosis. 

Oviparous  and  Viviparous  Animals. — The  time  at  which  the 
egg  leaves  the  mother's  body  is  independent  of  that  at  which  the 
embryo  escapes  from  the  egg  membranes.  Two  extremes  are 
known,  the  oviparous  or  egg-laying  animals,  and  the  viviparous  or 
those  which  give  birth  to  living  young.  Only  those  forms  can  be 
considered  as  strictly  oviparous  in  which  the  egg  at  the  time  of 
laying  is  a  single  cell,  in  which  case  it  is  either  not  fertilized  until 
after  extrusion,  as  in  the  case  of  most  fishes,  sea-urchins,  etc.,  or 
during  extrusion,  as  in  batrachians  and  insects.  In  viviparous 
animals,  on  the  contrary,  birth  and  the  rupture  of  the  egg  mem- 
branes occur  quite,  or  almost,  at  the  same  time,  and  from  the 
mother  there  emerges  an  animal  which  has  completed  its  develop- 
ment or,  at  least,  has  progressed  so  far  that  it  is  able  to  live  with- 
out protective  coverings. 

Ovo-viviparous  Animals. — Varying  degrees  of  ovo-viviparous  develop- 
ment connect  these  two  extremes.  What  here  appears  at  birth  at  first 
impresses  us,  on  account  of  its  covering,  as  being  an  egg  ;  but  the  first 
stages  of  development  have  already  passed,  so  that,  by  artificial  rupture 
of  the  egg  membranes,  an  embryo  more  or  less  developed,  but  usually  not 
yet  capable  of  independent  life,  is  exposed.  Birds  really  belong  in  the 
category  of  ovo-viviparous  animals,  for  their  eggs  are  fertilized  some  time 
before  they  are  laid,  and  have  already  completed  the  formation  of  the 
blastoderm.  In  the  case  of  many  worms  the  egg-shell  may  contain,  even 
at  the  time  of  laying,  an  animal  all  ready  for  hatching. 

No  Sharp  Line  between  Oviparous  and  Viviparous.— Transitional  forms 
of  this  kind  show  that  no  sharp  line  can  be  drawn  between  *  egg-laying ' 
and  '  bearing  living  young '  and  one  must  guard  against  attributing  too 
much  importance  to  the  apparent  distinctions.  Linnaeus,  following  the 
example  of  Aristotle,  was  in  error  in  regarding  the  time  of  birth  as  of 


162  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

systematic  importance.  In  many  divisions  of  animals  oviparous  as  well 
as  viviparous  forms  are  found.  The  majority  of  sharks  are  viviparous, 
but  a  few  species  lay  eggs  ;  on  the  contrary,  for  bony  fishes  the  rule  holds 
that  the  eggs  are  laid  before  fertilization.  Exceptions  are  the  viviparous 
surf  perches,  EmbiotocidaB,  of  the  Pacific  coast  and  many  Cypriuodouts  of 
fresh  water.  Most  of  the  Amphibia,  reptiles,  and  insects  are  egg-layers, 
but  not  a  few  forms  are  viviparous.  Even  among  the  mammals,  for 
which  for  a  long  time  the  '  bearing  young  alive '  was  regarded  as  diag- 
nostic, it  has  been  discovered  lately  that  the  Echidna  and  Ornitho- 
rhynchus  lay  eggs.  Finally,  exceptions  to  the  rule  occur  in  one  and  the 
same  species.  Adders  commonly  lay  eggs,  but  under  unfavorable  condi- 
tions they  retain  them  inside  their  body  until  ready  to  hatch. 

SUMMARY    OF   THE   FACTS    OF   ONTOGENY. 

1.  The  development  of  an  animal  begins  with  an  act  of  genera- 
tion; spontaneous  generation  and  generation  by  parents  are  to  be 
distinguished. 

2.  Spontaneous  generation  (generatio  aequivoca,  or  spontanea; 
abiogenesis)   is  the  origin  of  living  beings  from  lifeless  matter 
(without  pre-existing  organisms). 

3.  The  present  existence  of  spontaneous  generation  is  neither 
shown  by  observation,  nor  is  it,  on  the  whole,  probable;  yet  spon- 
taneous generation  is  a  logical  postulate,  in  order  to  explain  the 
first  origin  of  life  on  our  globe. 

4.  Generation  by  parents  (tocogony),  derivation  of  an  animal 
frem  an  animal  of  the  same  or  similar  structure,  can  take  place 
either  by  the  sexual  or  the  asexual  mode. 

5.  Asexual  generation  may  be  either  by  division  or  by  budding. 

6.  In  case  of  division  an  organism  grows  regularly  in  all  its 
parts,  and  by  constriction  falls  into  two  or  more  equivalent  new 
pieces. 

7.  According  to  the  direction  of  the  plane  of  division  in  refer- 
ence to  the  long  axis  of   the  animal  we   speak  of  longitudinal, 
transverse,  and  oblique  division. 

8.  In  case  of  budding  a  local  growth  occurs;  the  local  out- 
growth, the  bud,  separates  from  the  mother  as  a  smaller,  usually 
incompletely  formed,  animal. 

9.  According  to  the  position  and  number  of  the  buds  we  dis- 
tinguish lateral,  terminal,  and  multiple  budding. 

10.  Sexual  reproduction  is  reproduction  by  means  of  special 
sexual  cells,  which  do  not  take  part  in  the  ordinary  functions  of 
the  body. 

11.  In  sexual  reproduction  two  kinds  of  cells  unite,  the  female 
egg  and  the  male  spermatozoon  (fertilization). 


GENERAL  EMBRYOLOGY.  163 

12.  In   rare    cases   the    egg    develops   without    fertilization: 
parthenogenesis;   this  is  a  sexual  reproduction  with  degenerated 
fertilization. 

13.  Pcedogenesis  is  parthenogenetic  reproduction  by  a  young 
(i.e.,  incompletely  developed)  animal. 

14.  Different  modes  of  reproduction  (asexual,  sexual,  partheno- 
genesis, paedogenesis)  may  occur  in  the  same  species;  then  these 
often  occur  in  a  regular  order,  and  in  such  a  way  that  individuals, 
with  different  modes  of  reproduction  alternate' with  one  another:. 
alternation  of  generations  in  the  wider  sense. 

15.  Alternation  of  generations  in  the  strict  sense  (progressive^ 
generation,  metagenesis]  is  the  alternation  of  two  generations,  of 
which  one  reproduces  by  division  or  budding,  the  other  sexually.. 
The  former  is  called  the  nurse,  the  latter  the  sexual  animal. 

16.  The  alternation  of  parthenogenesis  or  paedogenesis  with 
pronounced  sexual  reproduction  is  called  regressive  alternation  of 
generations,  or  lieterogony. 

17.  Development  which  is  inaugurated  by  sexual  reproduction 
shows  in  nearly  all  multicellular  animals  a  general  agreement  in 
the  incipient  stages:   fertilization,   cleavage,   formation  of  germ- 
layers. 

18.  The  essential  point  of  fertilization  lies  in  the  complete 
fusion  of  egg  and  spermatozoon,  particularly  in  the  fusion  of  the 
nuclei,  egg  and  sperm  nuclei,  to  form  the  cleavage  nucleus. 

19.  The  cleavage  of  the  egg  is  a  cell  division,  a  division  of  the 
fertilized  egg  into  the  cleavage  spheres  (blast omeres).     The  cleav- 
age may  be  total  (holoblastic  egg)  or  partial  (meroblastic  egg) ; 
total  cleavage  is  either  equal  or  unequal,  the  partial  either  discoidal 
or  superficial. 

20.  By  progressive  division  of  the  cleavage  spheres,  and  by  the 
formation  of  a  cleavage  cavity,  there  arises  a  one-layered  embryo, 
the  Uastula  (vesicula  blast odermica). 

21.  By  the  invagination  of  the  blastula  the  gastrula  or  two- 
layered  embryo  arises. 

22.  The  gastrula  contains  a  cavity,   the  primitive   digestive 
tract  or  archenteron,  opening  to  the  exterior  through  the  blasto- 
pore ;  it  consists  of  two  epithelial  layers,  the  entoderm  (hypoblast) 
or  the  inner  germ-layer,  lining  the  archenteron,  and  the  ectoderm 
(epiblast)  or  outer  germ-layer. 

23.  Between  the  inner  and  the  outer  germ-layer  still  a  third, 
the  middle  germ-layer,  mesoderm,  may  be  formed. 

24.  The  middle  germ -layer  arises  either  by  an  infolding    or  • 


164  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

a  cutting  off  of  a  part  of  the  entodermal  epithelium,  epithelia, 
mesoderm,  mesothetium;  or  by  the  migration  of  separate  cells  to 
form  a  gelatinous  tissue  (mesenchyme). 

25.  Many  animals  deposit  their  eggs  before  or  shortly  after 
fertilization  (oviparous)',  others  lay  eggs  which  have  already  been 
fertilized  in  the  maternal  body,  and  at  the  time  of  laying  have 
passed  through  some  of  the  stages  of  development  (ovo-viviparous). 
A  third  series  of  animals  give  birth  to  living  young  (viviparous). 

26.  The  development  of  an  animal  is  either  direct  or  indirect 
(metamorphosis). 

27.  Indirect  development  or  metamorphosis  is  where  the  young 
animal,  as  it  comes  from  the  egg,  differs  from  the  sexually  mature 
animal  in  two  points : 

(a)  by   the   lack   of   certain   organs  which   occur   in   the 

sexually  mature  animals; 

(b)  by  the  appearance  of  organs,  larval  organs,  which  are 

lacking  in  the  sexually  mature  animals. 

III.  RELATION   OF   ANIMALS   TO    ONE   ANOTHER. 

General  Relations. — Just  as  between  the  organs  of  one  and  the 
same  animal  there  exists  a  regular  relation  which  is  termed  corre- 
lation of  parts,  so  also  the  different  individuals  of  the  animal 
population  stand  in  manifold  and  intimate  reciprocal  relations  to 
one  another.  Darwin  has  shown,  in  a  great  number  of  instances, 
liow  the  conditions  of  existence  of  many  animal  species  are  altered, 
if  other  forms  appear  or  disappear,  or  undergo  an  extraordinary 
reduction  or  increase  in  number  of  individuals.  Such  reciprocal 
•effects  are  usually  of  a  more  special  nature  and  can  be  understood 
only  by  individual  study;  a  few  conditions  are  of  wide  occurrence 
and  are  hence  suitable  for  a  general  consideration;  to  such  belong 
oolony  and  society  formation,  parasitism,  and  symbiosis. 
I.  RELATIONS  BETWEEN  INDIVIDUALS  OF.  THE  SAME  SPECIES. 

Colony  Formation. — Colony  and  society  formation  are  condi- 
tions which  exist  between  individuals  of  the  same  species.  An 
animal  colony  is  a  union  of  numerous  individual  animals  by  an 
organic  bodily  connexion;  the  latter  may  arise  in  two  ways:  first, 
&y  animals,  originally  separate,  approaching  one  another  and 
partially  fusing  together;  secondly,  by  individuals,  formed  by 
division  and  budding,  remaining  united  with  one  another  instead 
of  separating.  The  first  is  extremely  rare,  and  in  the  animal 
kingdom  plays  no  role  whatever. 


(ECOLOGY. 


165 


Colony  Formation  by  Fusion. — Many  Protozoa  fuse  with  one 
another  and  form  larger  bodies  in  which  the  individual  animals 
can  still  be  recognized.  Among  the  multicellular  animals,  that  of 
Diplozoon  paradoxum  (fig.  109)  is  the  only  case  known  where  two 
animals  (Diporpa),  sprung  from  different  eggs,  normally  unite 
into  a  double  animal,  which  recalls  certain  double  monsters,  as,  for 
example,  the  Siamese  twins. 


FIG.  109. — Development  of  Diplozoon  parailojrum.  (From  Boas.)  (1)  Larva,  from 
which  comes  (2)  'Diporpa.'  (3)  Two  Diporpee  uniting.  (4)  The  Diporpae  have 
united  into  Diplozoon.  m,  mouth ;  rf,  digestive  tract;  /«,  posterior  adhering  appa- 
ratus; b,  ventral  sucking-disc,  which  serves  for  attachment  to  the  dorsal  cone,  r. 

Colony  Formation  by  Incomplete  Division  and  Budding. — In 

general  it  can  be  said  that  the  important  instances  of  colony 
formation  rest  upon  incomplete  asexual  reproduction.  An  animal 
forms  new  individuals  by  division  or  by  budding,  but  the  process 
is  not  completed  since  the  new  generation  does  not  separate  from 
the  parent.  There  remain  connexions  of  tissue  uniting  the  buds 
with  the  mother  or  the  sisters  with  each  other.  The  colonies  of 
marine  hydroids  and  corals  (figs,  91,  206)  may  consist  of  thousands 
of  individuals  which,  by  repeated  incomplete  budding  or  division, 
have  sprung  from  a  single  sexually  produced  mother  animal. 

Community  of  Functions. — In  the  majority  of  cases  the  con- 
nexion of  the  tissues  results  in  a  considerable  degree  of  community 
of  functions.  Stimuli  which  affect  one  individual  are  transmitted 
by  common  nerves  to  the  others  of  the  colony;  thus  movements  in 
common  are  rendered  possible.  In  a  similar  way  the  food  captured 
and  digested  by  one  animal  serves  for  all.  On  account  of  the 
community  of  its  functions,  a  colony  appears  like  a  unified  whole, 
like  an  individual  of  a  higher  order;  the  same  process  which  led  to 
the  formation  of  multicellular  organisms  is  repeated.  Just  as 
there  the  elementary  organisms,  the  cells,  are  united  into  a  single 
animal,  so  here  the  single  animals  are  united  into  a  colony. 

Polymorphism. — When   a   whole   is   made   up    of    numerous- 
equivalent  parts,  the  conditions  for  division  of  labor  are  present. 
Instead  of  the  functions  of  the  entire  organism  being  distributed 


166 


GENERAL  PRINCIPLES  OF  ZOOLOGY. 


equally  to  the  individual  parts,  many  of  the  latter  become  employed 
solely  for  this,  others  again  solely  for  that  function,  and  acquire  a 
corresponding  structure.  In  case  of  such  animal  colonies  one 
speaks  then  of  multiformity  or  polymorphism.  Polymorphism 
appears  oftenest  in  connexion  with  the  vegetative  functions,  lead- 
ing to  a  distinction  between  sexual  animals  and  nutritive  animals, 
as  in  the  case  of  most  Hydrozoa,  where  often  nutrition  is  accom- 
plished by  animals  without  sexual  organs,  and  reproduction  is 
carried  on  by  animals  without  a  mouth.  But  other  functions, 
movement,  sensation,  offence  and  defence,  may  also  become 
specialized.  Siphonophores  are  the  classical  examples  of  poly- 
morphism (fig.  110).  Here  united  into  a  single  body  are  locomotor 


-FlO.  110.— Praya  diphyes.  (After  Gegenbaur)  A,  the  entire  animal;  J3,  a  single 
group  of  individuals  greatly  magnified  (Eudoxia).  1,  covering  scale;  2,  nutritive 
polyp;  3,  nettle-threads;  4,  sexual  bell. 

animals,  the  swimming-bells,  serving  only  for  locomotion;  cover- 
ing scales,  which  serve  only  to  protect  the  others;  nutritive  polyps, 
which  alone  take  in  and  digest  food;  sexual  animals  and  tactile 
polyps,  which  are  concerned  only  in  sexual  reproduction  and  with 
sensation.  In  regard  to  the  other  functions  each  animal  is  related 


(ECOLOGY.  167 

to  its  brothers  and  sisters;  its  very  existence  therefore  has  become 
dependent  upon  these;  the  single  individual  can  live  only  while  a 
part  of  a  whole.  Thus  also  division  of  labor  leads  to  greater 
centralization;  the  more  polymorphic  an  animal  colony  becomes, 
the  more  unified  it  is,  the  more  it  gives  the  impression  of  being  a 
single  animal  instead  of  an  aggregation  of  single  animals. 

In  Social  Animals  the  reciprocal  dependence  of  the  individuals 
is  much  less,  since  here  there  exists  no  organic  connexion,  only  a 
voluntary  communal  life.  As  asexual  reproduction  is  of  impor- 
tance in  the  case  of  colonies,  so  here  the  sexual  plays  a  prominent 
role.  Under  the  influence  of  the  sexual  impulse,  many  animals, 
even  some  of  the  lowest  organisms,  flock  together,  either  per- 
manently or  periodically;  sea-urchins,  sea-cucumbers,  many  fishes, 
collect  near  the  coast  at  the  time  of  egg-laying.  The  sexual  im- 
pulse draws  together  herds  of  deer,  elephants,  etc.  The  care  of  the 
young  offspring  further  leads  to  a  closer  organization,  to  a  society. 
All  insect  societies  are  built  up  on  this  basis.  Consequently, 
since  the  sexual  life  is  the  starting-point  of  social  life,  it  is  further 
comprehensible  that,  in  the  different  groups  of  individuals  forming 
the  community,  the  sexual  organs  may  be  influenced  in  their 
development.  Besides  males  and  females  (kings  and  queens) 
there  are  still  other  animals  with  degenerated  sexual  organ? 
incapable  of  function,  the  workers;  the  latter  are  either  only 
females  (bees  and  ants)  or  females  and  males  (termites).  While 
the  kings  and  queens  give  rise  to  the  next  generation,  the  workers 
care  for  the  young,  look  after  the  hive,  provide  food  and  protec- 
tion, and  also  serve  for  defence,  if  the  latter  is  not  delegated  to  a 
special  class,  the  soldiers  (termites). 

II.    RELATIONS  BETWEEN  INDIVIDUALS  OF   DIFFERENT  SPECIES. 

Causes  of  Close  Relation. — Where  individuals  of  different 
species  stand  in  close  reciprocal  relations  to  each  other  the  cause 
is  to  be  found  in  the  advantages  which  the  one  species  derives  from 
the  other,  or  which  these  both  furnish  reciprocally;  the  former 
condition  is  called  parasitism,  the  latter  symbiosis. 

Parasitism. — Parasites  are  animals  which  find  their  dwelling- 
place  upon  or  in  another  animal,  the  host,  and  obtain  nourishment 
from  it.  They  have  consequently  come  into  a  dependent  condi- 
tion and  have  undergone  a  more  or  less  extensive  change  in  their 
organization. 

True  Parasitism. — The  fact  that  an  animal  has  settled  down  upon 
another  is  not  sufficient  to  characterize  it  as  a  parasite.  There  are  many 


1G8  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

sedentary  animals  which,  when  opportunity  offers,  attach  themselves  to  a 
stone,  a  plant,  or  another  animal ;  in  such  cases  the  term  parasitism  is  a 
misnomer,  because  it  cannot  be  called  a  dependent  condition.  If  a 
hydroid  fasten  itself  upon  the  back  of  a  crab  instead  of  on  a  stone,  it  is  the 
result  of  chance,  in  which  the  nature  of  the  hydroid  is  in  no  way  con- 
cerned. The  case  would  be  different  if  the  polyp  were  able  to  live  only 
upon  the  crab,  and  perished  if  in  any  other  place.  Such  a  dependent 
condition  usually  occurs  only  when  the  mode  of  nutrition  is  also  depend- 
ent upon  the  place  of  abode ;  when  the  host  not  only  serves  for  a  dwell- 
ing-place, but  also  furnishes  the  dweller  with  food  ;  when,  consequently, 
the  dweller  lives  at  the  expense  of  the  host. 

Degeneration  Caused  by  Parasitism. — The  degree  to  which  a 
parasite  has  become  dependent  upon  its  host  varies  in  the  different 
species;  it  is  determined  by  the  extent  to  which  the  parasite  has 
adapted  itself  to  the  organization  of  its  host.  Therefore  it  is 
necessary  in  speaking  of  parasitism  to  consider  the  changes  of  form 
which  the  parasitic  mode  of  life  has  caused  in  the  structure  of 
animals.  These  concern  most  immediately  the  organs  of  locomo- 
tion and  nutrition.  Since  a  parasite  needs  to  fix  itself  as  firmly  as 
possible  to  the  host,  the  locomotor  apparatus  more  or  less  com- 
pletely disappears  and  an  apparatus  for  fixation  to  the  host 
becomes  necessary;  parasites  of  different  groups  are  provided  with 
hooks,  claspers,  sucking-discs,  etc.  The  blood,  tissue-fluids,  or 
liquid  food  of  the  host  furnishes  nourishment  to  the  parasite: 
these  are  substances  in  solution  which  scarcely  need  digestion. 
Usually,  therefore,  the  digestive  canal  is  simplified  or  quite  dis- 
appears; among  the  parasites  there  are  gutless  worms  as  well  as 
gutless  Crustacea.  The  mode  of  life  of  a  parasite  is  also  simpli- 
fied, since  it  is  no  longer  compelled  to  seek  its  food ;  in  all  parasites 
the  nervous  system  and  sense-organs  undergo  a  high  degree  of 
degeneration;  the  former  becomes  limited  usually  to  the  most 
indispensable  portion;  the  latter,  except  those  of  touch,  may 
entirely  disappear. 

Modification  of  the  Sexual  Apparatus  by  Parasitism. — The 
sexual  apparatus,  on  the  contrary,  undergoes  a  strong  develop- 
ment. While  it  becomes  easier  for  the  parasite  to  maintain  itself, 
the  existence  of  the  species  is  more  precarious.  If  a  man  die, 
then  most  of  his  parasites  die  with  him,  especially  those  which 
exist  in  the  interior  of  his  body.  In  order  that  a  parasitic  species 
may  not  become  extinct  in  a  short  time,  it  is  necessary  that  the 
eggs  be  introduced  into  a  new  host.  Since  this  transmission  is 
attended  with  difficulties,  the  parasites  must  produce  an  enormous 
number  of  eggs.  The  eggs,  too,  are  distinguished  by  great  resist- 


(ECOLOGY. 


169 


00 


ing  power  and  well -developed  protective  organs,  such  as  strong 
shells,  etc. ;  thus  it  is  known,  for  example,  that  the  eggs  of  Ascarids 
continue  to  develop  for  some  time  in  alcohol,  being  protected  by 
their  impermeable  shell. 

Ectoparasites  and  Entoparasites. — All  the  above-mentioned 
phenomena  are  more  conspicuous  in  the  case  of  parasites  which 
live  inside  of  other  animals,  entopara- 
sites, than  in  the  case  of  the  dwellers 
upon  the  skin  or  other  superficial 
organs,  the  ectoparasites.  In  case  of 
entoparasites  the  transforming  influ- 
ence of  parasitism  is  so  considerable 
that  representatives  of  the  most  diverse 
animal  groups  take  on  a  remarkable 
similarity  of  appearance  and  structure. 
Pentastomum  tcenioides  (fig.  112),  for 
example,  belongs  in  the  same  class  with 
the  spiders,  the  Arachnida,  but  in 
external  appearance  it  is  entirely  unlike 
them,  resembling  the  tape-worms  (fig. 
111).  Hence  for  a  long  time  all  ento- 
parasites, on  account  of  their  simi- 
larity, were  united  into  a  single 
systematic  group  under  the  name  of 
'  Helminthes/  comprising  members  of 
the  crustaceans,  worms,  and  spiders, 
as  well  as  animals  of  entirely  different 
groups  of  the  animal  kingdom.  Only 
by  embryology  was  the  unnaturalness 
of  this  grouping  recognized.  Ento- 
parasitism  therefore  is  one  of  the  best 
examples  for  illustrating  convergent 
development,  i.e.,  animals  of  different 
systematic  position  acquiring,  under  similar  conditions  of  life,  a 
great  similarity  of  structure  and  appearance. 

Symbiosis. — Less  frequent  than  parasitism  is  symbiosis,  or  the 
association  of  animals  for  reciprocal  advantages.  Social  animals 
frequently  not  only  hold  certain  animals  in  bondage,  but  even  seek 
to  protect  and  serve  them;  as,  for  example,  in  the  company  of 
ants  are  found  certain  blind  beetles,  like  Claviyer  (Myrmecophily), 
or  some  species  of  plant-lice,  or  even  ants  of  other  species  and 
genera.  But  such  cases  of  association  correspond  in  part  to  the 


FIG.  111. 


FIG.  112. 


(After 


.—Pentastomum  tcenioides 
;er  Leuckart.)  ft, 
hooks  rjght  and  left  of  mouth  ; 
or,  unpaired  ovary,  branching 
into  two  oviducts,  which  unite 
into  the  unpaired  vagina  (ra): 
the  latter  receives  the  outlets 
of  two  receptacula  seminis  (rs\ 
and  winds  around  the  digestive 
tract  (d);  CB,  oesophagus. 


170  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

domestication  of  animals,  or  to  slavery,  as  carried  on  by  man. 
The  ants  keep  the  plant-lice  in  order  to  lick  the  sweet  juice  which 
is  secreted  in  their  honey-tubes;  they  steal  the  pupae  of  other  ants 
and  rear  them,  to  use  them  later  as  slaves.  This  state  of  things 
rests,  consequently,  not  upon  equal  rights,  since  the  one  animal, 
in  the  present  example  the  ant,  brings  about  the  association,  while 
the  other  animal  is  passively  led  into  it. 

An  instance  of  most  complete  equal  rights  and  true  symbiosis  is 
furnished  us,  however,  by  a  hermit-crab  and  an  actinian  (fig.  113),  Eupa- 

gurus  pubescens  and  Epizoanthus  ameri- 
canus.  Like  every  species  of  hermit-crab 
this  also  inhabits  a  snail-shell  from  the 
opening  of  which  only  his  legs  and  pin- 
cers are  protruded.  Upon  this  shell  an 
Epizoanthus  becomes  attached  and  by 
budding  soon  covers  it  with  a  colony  of 

FIG.  m-Ao^TTf  E^anthus  P0^8'  "  ^fter  th"s  Coring  the  shell 
americanus  on  the  shell  occu-  it  is  not  only  capable  of  extending  the 
$eerril!)a  hermit'crab-  (From  aperture  by  its  own  growth,  but  has  the 

power  of  entirely  dissolving  and  absorb- 
ing the  substance  of  the  shell  so  that  no  trace  of  it  can  be  found,  though 
the  form  is  perfectly  preserved  by  the  somewhat  rigid  membrane  of  the 
polyp."  The  advantage  which  the  actinian  derives  from  this  symbiosis  is 
clear  :  it  gains  a  share  of  the  food  which  the  crab  obtains.  It  is  less 
clear  what  the  crab  gains  by  the  association  ;  however,  the  polyp  is  perhaps 
a  protection  to  him,  by  means  of  its  batteries  of  nettle  cells,  while  by 
growth  it  increases  the  size  of  the  '  house '  occupied  by  the  hermit  and 
thus  saves  him  periodic  changes  of  abode. 

Occurrence  of  Symbiosis.— That  animals  in  general  rarely  live  sym- 
biotically  with  one  another  rests  mainly  upon  the  fact  that  the  conditions 
of  life  of  all  animals  to  a  certain  point  are  similar  or  identical.  They  all 
take  in  compounds  rich  in  carbon  and  nitrogen,  decompose  them,  and,  in 
the  presence  of  oxygen,  separate  them  into  carbon  dioxide,  water,  and 
oxidation  products  containing  nitrogen.  All  animals  consequently  are 
competitors  in  the  struggle  for  food.  For  the  same  reason,  conversely, 
symbiosis  between  plants  and  animals  is  not  at  all  uncommon.  In 
particular  there  are  certain  lower  algaB,  the  ZooxanthellaB,  which  often 
live  in  animals.  The  radiolarians  contain  with  such  constancy  in  their 
soft  bodies  green-  or  yellow-colored  cells  that  for  a  long  time  these  were 
regarded  as  constituent  parts  of  the  animal.  Quite  similar  yellow  and 
green  cells  inhabit  the  stomach  epithelium  of  many  actinians,  corals,  and 
even  of  many  worms.  The  ZooxanthellaB  are  nourished  by  the  carbon 
dioxide  which  is  formed  by  the  animal  tissues,  and  breathe  out  oxygen, 
which  in  turn  serves  as  food  for  the  animal ;  further,  they  form  starch 
and  other  carbohydrates,  and  there  is  nothing  to  prevent  any  surplus  thus 


(ECOLOGY.  171 

formed  from  becoming  food  material  for  the  animal.  Thus  there  is  on  a 
small  scale  that  cycle  of  matter  which  exists  on  a  grand  scale  in  Nature 
between  the  animal  and  vegetable  kingdoms.  By  aid  of  chlorophyl  and 
of  the  chemical  influence  of  sunlight  the  plants  decompose  water  and 
carbon  dioxide  and  form  from  them  oxygen,  which  they  breathe  out,  and 
compounds  rich  in  carbon,  which  they  store  up  in  their  tissues :  they 
are  reducing  organisms.  On  the  contrary,  animals  give  off  carbon 
•dioxide  and  water,  but  take  their  oxygen  from  the  air,  and  carbon  com- 
pounds in  their  food ;  they  use  oxygen  to  break  down  the  chemical 
combinations,  to  oxidize  :  they  are  oxidizing  organisms.  This  explains 
why  the  favorable  influence  of  plants  upon  animals  ceases  immediately 
when  they  change  the  character  of  their  metabolism.  With  the  disap- 
pearance of  their  chlorophyl  moulds  and  bacteria  lose  the  power  of  reduc- 
ing carbon  dioxide  ;  they  derive  their  food  from  other  organisms  and 
decompose  this  into  carbon  dioxide,  water,  etc.  ;  like  animals,  they  are 
oxidizing  organisms,  and  consequently  dangerous  competitors.  When 
they  establish  themselves  upon  the  animal  body,  they  almost  always  work 
injury  to  it ;  hence  in  animals  they  are  the  cause  of  many  extremely  dan- 
gerous ailments. 


IV.   ANIMAL   AND    PLANT. 

Distinction  between  Animal  and  Plant. — The  consideration  of 
symbiosis  has  led  us  up  to  the  fact  that  a  distinction  exists  between 
plants  and  animals  in  the  mode  of  metabolism,  which  may  be 
expressed  thus :  plants  usually  take  in  carbon  dioxide  and  give  off 
oxygen,  while  animals  breathe  in  oxygen  and  give  out  carbon 
dioxide.  Hence  it  might  be  concluded  that  it  is  easy  to  discover 
differences  which  generally  obtain  between  plants  and  animals, 
for,  as  a  matter  of  fact,  the  laity  are  never  in  doubt  in  deciding  to 
which  realm  of  nature  the  more  highly  organized  animals  and 
plants,  which  are  the  only  ones  known  to  them,  belong. 

Doubtful  Cases. — But  the  more  one  studies  this  question,  the 
more  difficult  becomes  its  solution.  The  old  zoologists  indeed 
formed  the  conception  that  there  are  organisms  which  stand  on  the 
limits  between  the  animal  kingdom  and  the  vegetable,  and  Wotton 
named  these  directly  zoophytes  or  plant-animals.  Now  we  know 
that  Wotton's  plant-animals  are  true  animals  with  but  a  superficial 
similarity  to  plants;  but,  by  means  of  the  microscope,  we  have 
become  acquainted  with  numerous  lower  organisms,  and  it  is  still 
doubtful  in  which  of  the  two  realms  of  nature  these  belong.  As 
such  may  be  mentioned  the  Myxomycetes  and  many  Flagellata. 


172  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

Physiological  Distinctions. — If  one  wish  to  discover  sharp 
distinctions  between  animals  and  plants,  he  may  take  into  con- 
sideration on  the  one  side  physiological,  on 
the  other  morphological,  characters.  Start- 
ing from  the  physiological  point  of  view, 
Linnaeus  ascribed  to  plants  only  the  capacity 
of  reproduction  and  nutrition,  but  to  animals 
the  power  of  motion  and  sensation  in  addi- 
tion. However,  we  know  that  vegetable, 
like  animal,  protoplasm  is  irritable  and  is 
capable  of  movement,  as  is  shown  by  the 
active  movements  of  the  lower  Algge,  the 
great  sensitiveness  of  the  Mimosa,  and  other 
FIG.  ii4.—  Lepas  anati-  plants;  but  further,  we  know  that  even  many 

/era.     (After  Schmar-          .    ,  •,  1-11  •       i          •        T 

da.)  c,  carina;  t,  ter-  oi  the  more  highly  organized  animals,  e.g., 
Crustacea  (fig.  114),  lose  the  power  of  loco- 
motion and  become  fixed,  and  many  fixed  forms,  e.g.,  the  sponges 
(fig.  84),  even  under  the  closest  examination  appear  immovable 
and  unaffected  by  stimulation ;  thus  we  are  led  to  abandon  the 
idea  that  the  so-called  animal  functions  are  to  be  regarded  as 
accurate  distinctions. 

Metabolism  not  a  Safe  Criterion. — Even  the  difference  in  met- 
abolism is  by  no  means  sufficient.  Every  plant  has  a  double 
exchange  of  material.  In  its  movements  and  other  vital  functions 
the  vegetable  protoplasm  produces  carbon  dioxide  and  consumes 
oxygen;  at  the  same  time  there  goes  on,  under  the  influence  of 
sunlight  and  of  chlorophyl,  the  reduction  of  carbon  dioxide  and 
the  giving  off  of  oxygen.  In  chlorophyl-containing  plants  the 
reducing  process  preponderates  so  considerably  during  the  day 
that  there  is  evident,  as  the  final  result,  the  giving  off  of  a  greater 
quantity  of  oxygen,  and  only  at  night,  when  the  reducing  process 
becomes  interrupted  on  account  of  the  lack  of  sunlight,  does  the 
production  of  carbonic-acid  compounds  become  perceptible.  But 
the  reducing  processes  become  immediately  preponderant  if  the 
chlorophyl  be  absent;  chlorophylless  moulds  and  bacteria  have, 
therefore,  the  same  metabolism,  so  far  as  carbon  dioxide  is  con- 
cerned, as  animals. 

Cellulose  not  a  Sure  Test. — So  also  it  is  incorrect  to  say  that 
only  plants  have  the  power  to  make  cellulose,  for  cellulose  is  found 
in  many  lower  animals,  the  rhizopods,  and  in  the  highly  organized 
group  of  tunicates;  according  to  recent  investigations  it  appears 
to  be  found  even  among  the  arthropods. 


(ECOLOGY. 


173 


Morphological  Distinctions. — Turning  to  the  morphological 
characteristics,  multicellular  animals  and  multicellular  plants  are 
readily  distinguished,  since  the  former  in  the  germ-layers  have  a 
principle  of  cell  arrangement  peculiar  to  them.  With  the  appear- 
ance of  the  gastrula  each  organism  is  undoubtedly  an  animal. 
But  in  unicellular  animals  the  arrangement  of  the  cells  is  lacking, 
and  only  the  constitution  of  the  single  cell  guides  us.  Now  are 
there  unmistakable  morphological  differences  between  the  animal 
and  the  vegetable  cell  ? 

Plant-cells  have  a  Cellulose  Membrane. — In  the  structure  of 
plant  and  animal  cells  an  important  distinction  is  found  in  the 
fact  that  the-  former  has  a  cellulose  membrane,  but  the  latter  is 
usually  membraneless.  To  this  distinction  must  be  referred  in  the 
last  analysis  the  widely  different  appearance  of  the  two  realms. 
Since  the  plant-cell  is  early  surrounded  with  a  firm  coat,  it  loses  a 
large  part  of  its  power  of  further  changing  its  form;  hence  vege- 
table tissues  and  organs  are  uniform  in  comparison  with  the  incon- 
ceivable multiformity  which  animal  histology  and  organology 
disclose.  The  numerous  higher  stages  of  organization  which  the 
animal  kingdom  reaches,  even  in  its  lower  classes,  is  in  great  part, 
indeed,  the  result  of  the  fact  that  the  cells  of  animals  do  not 
become  encapsuled,  but  have  preserved  the  capacity  for  more 
varied  and  higher  development. 

Transitions. — But  even  here  transitions  are  found  between  the 
lower  plants  and  animals.  In  the  lower 
Algae  the  cells  have  power  to  emerge  from 
their  cellulose  membrane,  and  to  swim 
about  freely  (fig.  115),  before  they  encapsule 
themselves  anew.  On  the  other  hand,  most 
unicellular  animals  encyst;  they  pause  in 
their  ordinary  functions  of  life,  become 
spherical,  and  surround  themselves  with  a 
firm  membrane,  in  some  cases  even  of  cel- 
lulose. Since  in  both  cases  an  alternation 
between  the  encapsuled  and  the  free-living 
condition  occurs,  only  the  longer  duration 
of  the  one  or  of  the  other  can  lead  to  a 
distinction.  But  here  occurs  the  possibility 
that  undifferentiated  intermediate  forms  ex- 
ist ;  their  actual  existence  prevents,  even  yet, 
a  sharp  distinction  between  the  animal  and 
vegetable  kingdoms. 


7? 


FIG.  115.— (Edngonium  in 
spore-formation.  (After 
Sachs.)  A.)  a  piece  of 
the  filament  of  the  alga 
with  escaping  cell-con- 
tents; _B,  zoospore 
formed  from  the  con- 
tents; C,  zoospore  fixed 
and  germinating. 


174  GENERAL  PRINCIPLES  OF  ZOOLOGY. 


V.  GEOGRAPHICAL   DISTRIBUTION   OF   ANIMALS. 

The  Different  Faunal  Regions. — Even  a  superficial  knowledge 
of  the  mode  of  distribution  of  animals  shows  that  the  animal  fauna 
in  different  regions  of  the  earth  has  an  essentially  different  char- 
acter. In  part  this  difference  of  fauna  is  the  immediate  result  of 
climatic  differences.  The  polar  bear,  arctic  fox,  eider-ducks,  and 
many  aquatic  birds  are  restricted  to  the  polar  zones,  because  they 
cannot  endure  more  than  a  certain  degree  of  warmth;  on  the  other 
hand,  the  larger  species  of  cats,  the  apes,  the  humming-birds,  etc., 
occur  only  in  tropical  or  sub-tropical  regions,  because  they  are  not 
sufficiently  protected  against  cooler  weather. 

Climate  not  the  Only  Factor. — If  climate  were  the  sole  factor 
determining  distribution,  the  faunal  character  of  two  lands  which 
have  similar  climatic  conditions  would  be  essentially  the  same; 
conversely,  the  separate  regions  within  a  continuous  territory 
extending  through  several  climatic  zones  must  have  quite  different 
faunas,  according  as  they  are  nearer  the  equator  or  the  poles.  But 
such  is  not  the  fact;  two  tropical  countries  may  differ  more  widely 
in  the  characteristics  of  their  fauna  than  the  hot  and  cold  regions 
of  one  and  the  same  country. 

Factors  in  Distribution. — Modern  zoology  endeavors  to  explain 
these  peculiar  conditions  by  regarding  the  present  distribution  of 
animals  as  the  product  of  two  factors:  the  gradual  changes  of  the 
animal  world,  and  further  the  gradual  changes  of  the  earth's  sur- 
face on  which  the  animals  are  distributed.  The  history  of  the 
earth  as  disclosed  by  geology  shows  two  facts:  (1)  that  the  con- 
nexions between  parts  of  the  earth  have  varied  greatly;  that,  for 
example,  at  a  time  when  the  Mediterranean  had  not  yet  reached 
its  present  extent,  Morocco,  Algiers,  Tunis,  and  Egypt  were  more 
closely  united  with  the  European  coast  of  the  Mediterranean  than 
with  the  southern  part  of  the  African  continent  separated  from 
them  by  the  Sahara;  (2)  that  considerable  variations  of  climate 
have  taken  place :  there  prevailed  in  Europe  in  the  tertiary  period 
a  subtropical  climate  which  rendered  possible  the  existence  of 
animals  which  now  occur  in  Algeria  (lions).  But  later  a  glacial 
period  began,  which  introduced  over  a  wide  area  of  the  European 
continent  the  conditions  of  arctic  life,  and  consequently  a  fauna 
of  northern  animals  (reindeer).  Hand  in  hand  with  the  geological 
changes  went  changes  in  the  animal  world,  the  then  existing 
species  dying  out  under  the  change  of  conditions,  or  forming  new 


DISTRIB  UTION.  175 


species  through  gradual  variations.  Thus  the  distribution  of 
animals  constitutes  an  extremely  complicated  problem,  the  solu- 
tion of  which  necessitates  comprehensive  preliminary  work.  It 
must  be  known  with  certainty  how  the  connexions  between  the 
continents  and  the  climates  have  changed,  particularly  in  the 
later  geological  periods;  further,  we  must  study,  not  only  how 
animals  are  distributed  over  the  earth's  surface  at  the  present  time, 
but  also  how  they  were  distributed  in  earlier  times.  Finally,  by 
means  of  comparative  anatomy  and  embryology  we  must  have  clear 
and  detailed  ideas  of  the  relationships  and  interrelationships  of 
animals. 

It  will  be  an  extremely  long  task  to  solve  all  the  problems  of 
the  subject  here  sketched  in  outline.  What  has  been  investigated 
thus  far  can  only  be  regarded  as  a  preliminary  proof  that  zoology 
with  its  prevailing  views  of  the  changes  of  animals  and  of  the  earth 
is  on  the  right  track.  It  would  be  a  test  of  the  correctness  of  this 
view  if  it  were  proved  that  the  faunal  resemblances  of  two  countries 
depends,  in  the  first  place,  upon  how  long  they  have  been  in  close 
connexion  with  each  other,  consequently  allowing  an  interchange 
of  the  animals  inhabiting  them.  Two  regions,  separated  early  in 
the  earth's  history  and  never  again  connected,  must  have  greater 
differences  in  faunal  characters  than  two  lands  still  connected  or 
only  recently  separated.  It  is  instructive  when  we  travel  in  the 
northern  hemisphere  and  find  in  widely  separated  regions  strik- 
ingly similar  faunae,  while  under  the  equator  or  in  the  southern 
hemisphere  under  the  same  conditions  striking  differences  are  seen. 
This  is  explained  on  the  hypothesis  that  in  all  past  periods  as  now 
the  land  masses  of  the  northern  hemisphere  have  been  closely  con- 
nected, while  the  parts  of  the  continents  extending  to  the  south — 
aside  from  hypothetical  temporary  connexions  between  South 
America,  Africa,  and  Australia — have  been  separated  through 
most  of  the  earth's  history. 

In  carrying  out  more  closely  the  points  of  view  mentioned, 
students  of  distribution  have  attempted  to  mark  off  the  great 
faunal  areas  of  the  earth,  the  faunal  provinces  or  regions,  and 
within  these  again  less  important  divisions,  subregions.  These 
provinces  have  been  based  chiefly  upon  the  distribution  of 
mammals,  less  upon  that  of  birds  and  other  animals;  for  the  dis- 
tribution of  mammals  is  chiefly  determined  by  those  changes  of 
the  earth's  surface  which  are  best  known  geologically  and  possess 
most  interest.  Elevation  or  depression  of  the  earth's  surface  often 
opposes  impassable  barriers  to  most  mammals :  rising,  if  it  lead  to 


176  GENERAL   PRINCIPLES   OF  ZOOLOGY. 

the  formation  of  glaciered  mountain-chains;  sinking,  when  arms 
of  the  sea  are  formed,  which,  even  if  only  narrow,  interpose 
between  two  hitherto  connected  land  areas  straits  which  are 
impassable  for  most  mammals.  Birds  and  insects  which  fly  well 
are  less  affected  by  all  such  changes  of  the  earth's  surface;  the 
majority  of  them  can  fly  over  arms  of  the  sea  and  mountain-chains, 
for  there  are  birds  which  can  even  cross  the  Atlantic  Ocean. 

The  Six  Primary  Regions. — Of  the  systems  of  animal  geography 
proposed  up  to  the  present  time,  the  divisions  advocated  by 
Sclater  and  Wallace  finds  most  favor.  These  English  scholars 
distinguish  the  six  following  primary  regions:  (1)  the palcearctic, 
comprising  all  Europe,  northern  Africa  as  far  as  the  Sahara,  and 
northern  Asia  as  far  as  the  Himalayas;  (2)  the  Ethiopian,  all  of 
Africa  south  of  the  Sahara;  (3)  the  oriental,  including  upper  and 
farther  India,  southern  China,  and  the  western  Malay  Islands; 
(4)  and  (5)  the  nearctic  and  the  neotropical  regions,  which  make 
up  the  American  continent  and  are  divided  by  a  line  drawn  at 
about  the  northern  border  of  Mexico;  (6)  the  Australian,  in 
which,  besides  Australia  itself,  are  included  the  larger  and  smaller 
islands  of  the  Pacific  Ocean  and  the  eastern  Malay  Islands,  east  of 
Celebes  and  Lombok. 

(1)  The  Australian  region  is  most  sharply  distinguished  from 
all  the  others  and  by  many  is  set  apart  as  a  distinct  division  called 
*  Notogaea. '  Its  isolated  geographical  position  together  with  the 
fact  that  it  has  long  been  separated  from  other  countries 
(apparently  since  the  beginning  of  the  tertiary)  explains  the  fact 
that  only  the  oldest  mammals,  the  monotremes  and  marsupials, 
have  entered  the  region,  while  the  placental  mammals  have  not 
been  able  to  follow.  While  the  marsupials,  which  in  the  secondary 
period  also  inhabited  the  northern  hemisphere,  were  replaced  there 
in  tertiary  times  by  the  placenta! s,  they  were  able  to  develop 
farther  in  the  Australian  region.  Australia  and  the  adjacent 
islands  are  thus  the  land  of  marsupials,  which  have  persisted  else- 
where only  in  South  America  (Cmnolestes,  Didelphidaa),  the 
opossum  ranging  north  into  the  United  States.  On. the  other 
hand,  at  the  time  of  discovery  Australia  lacked  all  placental 
mammals  except  those  (whales,  dugong,  seals,  bats)  which  were 
not  restricted  by  water  and  the  Muridae,  easily  transported  on 
floating  wood.  Two  larger  mammals,  the  wild  dog  or  dingo 
(Canis  dingo)  and  the  pig  of  New  Guinea  (Sus  papuanus),  may 
have  accompanied  man,  this  being  the  most  probable  for  the  dingo 
in.  spite  of  the  fact  that  his  remains  occur  in  the  pleistocene  along 


DISTRIBUTION.  ITT 

with  those  of  the  giant  marsupials.  Further  peculiarities  of  the 
Australian  region  are  the  birds-of-paradise  in  New  Guinea,  the 
egg-laying  mammals  Omithorhynchus,  Echidna,  and  Proechidna, 
and  the  cassowaries  and  the  Australian  ostrich  (Dromceus  novce- 
liollandice). 

It  is  easily  understood  that  the  isolated  island  groups  of  the 
South  Sea  (Polynesia)  have  developed  many  faunistic  peculiarities, 
as  well  as  that  an  exchange  of  forms  may  have  taken  place  between 
the  islands  of  the  oriental  province  and  the  islands  faunally  related 
to  Australia,  and  that  '  Wallace's  Line '  is  not  so  sharp  a  boundary 
as  it  was  once  thought  to  be  (extension  of  marsupials  into  Celebes, 
of  placentals  into  the  Moluccas).  On  the  other  hand  the  distinct- 
ness of  New  Zealand  needs  mention.  It  is  distinguished  from 
Australia  by  a  large  number  of  peculiar  birds  (Apteryx  and  the 
extinct  Dinornithidae),  reptiles  (the  ancient  Sphenodon),  and 
molluscs.  If  the  bats  and  mice — unimportant  in  matters  of  dis- 
tribution— be  excepted,  New  Zealand  lacks  all  native  mammals, 
even  marsupials. 

(2)  The  neotropical  province  (South  and  Central  America)  is, 
next  to  Australia,  the  most  sharply  characterized,  and,  like  that 
region,  has  been  set  aside  as  a  special  division  '  Neogaea/  especially 
when  considered  with  reference  to  its  geological  history,  which 
shows  that  during  the  cretaceous  and  early  tertiary  time  it  was 
separated  from  North  America  by  the  sea  and  had  developed  a 
peculiar  fauna  (e.g.,  gigantic  edentates,  no  carnivores).  These 
peculiarities  disappeared  towards  the  end  of  the  tertiary  by  the 
entrance  of  carnivores  and  ungulates  from  the  north  and  an 
extension  of  the  edentates  to  the  northern  hemisphere.  To  the 
Neogaea  belong  the  platyrhine  apes,  the  catarrhine  to  the  Old 
World.  Characteristic  edentates  are  the  armadillos,  sloths,  and 
ant-eaters;  the  marsupials  are  represented  by  the  opossums  and 
Ccenolestes;  among  birds  the  humming-birds,  toucans,  the  peculiar 
Cotingidae,  Tanagridae,  Tinamous,  Palamedidae,  Rhea,  etc.  The 
almost  entire  absence  of  insectivores  and  the  considerable  develop- 
ment of  rodents  (cavies,  agoutis,  chinchillas)  are  noteworthy. 

The  four  remaining  provinces  are  still  closely  connected 
geographically  and  form  a  third  great  division,  '  Arctogaea/  charac- 
terized by  the  entire  absence  of  platyrhine  apes,  monotremes,  and, 
except  the  North  American  opossum,  of  marsupials.  In  the 
secondary  and  tertiary  times  the  northern  parts  of  these  lands  were 
connected  and  an  interchange  of  faunas  occurred,  this  being  the 
easier  on  account  of  the  extension  of  the  warm  climate  to  the  far 


178  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

north.     Hence  many  unite  the  palaearctic  and  nearctic  provinces 
into  a  '  holarctic '  province. 

(3)  The  nearctic  region  has  peculiar  to  it  three  mammalian 
families,    the    prong-horned    antelope,    the    opossums,    and    the 
Haplodontae ;  of  the  group  of  Amphibia,  the  Sirenidae  and  Amphi- 
umidae.     The  Nearctic  .is  to  be  distinguished  from   the  nearest 
related  palaearctic  region  through  the  crowding  in  of  neotropical 
forms  like  the  raccoon,  opossum,  humming-birds,  etc. 

(4)  The  palaearctic  region  covers  the  greatest  area  and  conse- 
quently abuts  upon  many  other  provinces.     Hence  there  exist  on 
the   one   side   important   differences   between    the   various   local 
faunas,  which  are  conditioned  by  climate  and  great  distances,  but 
on  the  other  it  explains  the  fact  that  the  palaearctic  region  has  no 
peculiar  families.     The  families  which  here  have  reached  a  great 
development  are  the  deer,  cattle,  sheep,  and  camels;  especially 
conspicuous  genera  are  the  chamois,  squirrel,  badger,  and  marmot. 

(5)  The  Ethiopian  region  has  many  animals  found  only  there; 
among  these  the  hippopotamus  and  giraffe,  the  aardvark,  and,  if 
we  include  Madagascar,  the  lemurs  are  most  characteristic.     To 
these  are  added  a  rich  development  of  antelopes  and  zebras  and 
the   gorilla  and  chimpanzee.     Equally  noteworthy  is  the  entire 
absence  of  striking  families  and  genera,  such  as  the  bears,  moles, 
deer,  goats,  tapirs,  sheep,  the  true  cattle  and  swine,  provided  they 
have  not  been  domesticated  and  introduced. 

Within  the  region  the  island  of  Madagascar  occupies  a  remark- 
able position.  This  island  is  the  land  of  lemurs  and  Insectivora; 
no  land  is  so  rich  in  lemurs,  as  the  majority  of  the  genera  live 
exclusively  in  Madagascar.  On  the  other  hand  the  large  beasts 
of  prey,  the  cats,  hyenas,  dogs,  and  the  bears  (which,  however,  do 
occur  in  Africa),  all  the  true  apes,  antelopes,  elephants,  and  the 
various  species  of  rhinoceros  are  absent.  Consequently,  since 
Madagascar  is  distinguished  quite  conspicuously  from  Africa,  many 
zoologists  separate  the  island  from  the  Ethiopian  region;  many 
even  give  it  the  rank  of  an  independent  province. 

(6)  The  oriental  region  contains,  next  to  Madagascar,  the  most 
lemurs;  among  which  the  Tarsidae  and  Galeopithecidae  (the  latter 
often  considered  an  insectivore)  are  exclusively  oriental.     Remark- 
able inhabitants  of  the  province  are  the  gibbons  and  orang-utans, 
the  musk-deer,  numerous  families  and  genera  of  birds. 

Arctic  and  Antarctic  Provinces. — Of  late  the  view  has  gained 
ground  that,  besides  these  six,  two  other,  circumpolar,  provinces 
must  be  distinguished,  the  arctic  and  the  antarctic.  Both  have  a 


DISTRIBUTION.  179 

fauna  consisting  of  few  species  but  numerous  individuals,  of  which 
the  auks,  polar  bear,  reindeer,  and  arctic  foxes  are  characteristic  of 
the  northern  or  arctic  region,  the  penguins  and  the  entire  absence 
of  land  mammals  of  the  antarctic. 

The  Distribution  of  Aquatic  Animals. — Since  most  seas  are 
connected,  the  faunal  regions  cannot  be  distinguished  so  sharply 
as  in  the  case  of  the  land  faunas;  conspicuous  differences  are 
present  only  when  two  oceans  are  separated  by  continents  extend- 
ing far  to  the  north  and  south;  such,  for  example,  exist  between 
the  Eed  Sea  and  the  geographically  neighboring  Mediterranean, 
between  the  east  and  west  coasts  of  North  America,  even  where 
they  are  separated  only  by  the  narrow  isthmus  of  Panama.  Then, 
too,  considerable  differences  may  exist  where  currents  of  greatly 
different  temperatures  meet. 

Changes  in  the  Fauna  Conditioned  by  Depth. —  Much  more 
remarkable  in  the  marine  fauna  are  certain  differences  brought 
about  by  the  changes  of  the  conditions  of  life  in  the  different 
depths  of  the  sea.  A  deep-sea  fauna,  a  coast  fauna,  and  a  pelagic 
fauna  can  be  distinguished.  The  coast  fauna  embraces  the 
animals,  some  free,  some  fixed,  whicli  inhabit  the  plant-covered 
rocky  or  sandy  shore  to  a  depth  of  a  few  hundred  feet.  The 
deep-sea  fauna  swims,  creeps,  or  is  fixed  at  the  bottom  of  the 
ocean  at  depths  of  1000  to  almost  9000  meters;  it  is  distinguished 
from  the  coast  fauna  in  part  by  its  archaic  character,  for  here  very 
often  genera  and  entire  groups  of  animals  exist,  like  the  Hexac- 
tinellidae,  crinoids,  certain  starfishes  and  sea-urchins,  etc.,  which 
for  a  long  time  were  chiefly  known  through  fossils  from  earlier 
geological  ages. 

The  Plankton. — The  pelagic  animal  world  comprises  all  forms 
which  swim  freely  in  the  water,  the  ' plankton';  here  belong  many 
coelenterates,  medusae,  and  ctenophores,  entire  groups  of  Protozoa, 
like  the  radiolarians,  many  Crustacea  and  crustacean  larvae;  of  the 
molluscs  the  heteropods  and  pteropods.  These  animals  live  either 
at  the  surface  of  the  sea  itself  or  floating  at  greater  or  lesser, 
depths,  to  8000  meters  or  even  more.  Usually  they  are  gelatinous 
and  of  glasslike  transparency;  this  must  be  regarded  as  sympa-, 
thetic  coloring  and  adaptation  to  the  transparency  of  the  water. 

Distribution  of  Fresh-water  Animals.— In  fresh  water  two 
groups  of  animals  must  be  distinguished,  of  which  the  one  com- 
prises mainly  the  more  highly  organized  forms,  the  molluscs, 
fishes,  and  Crustacea,  the  other  the  lower  animal  world.  The 
distribution  of  the  former  is  mainly  determined  by  the  same  factors 


180  GENERAL  PRINCIPLES  OF  ZOOLOGY. 

which  influence  terrestrial  forms;  the  distribution  of  the  latter, 
however,  is  cosmopolitan.  The  same  infusorians  and  rhizopods, 
copepods,  fresh-water  sponges  and  polyps  which  occur  in  America 
seem  to  be  distributed  over  the  entire  earth.  This  is  connected 
with  the  fact  that  all  these  animals  have  resting  stages  in  which 
they  endure  desiccation.  The  resting  stage,  be  it  as  a  hard-shelled 
«gg  or  as  an  encysted  animal,  may  be  borne  about  by  the  wind,  or 
may  be  carried  with  the  mud  by  aquatic  birds,  and  upon  again 
reaching  the  water  resume  its  active  state. 

VI.   DISTRIBUTION    OF   ANIMALS    IN   TIME. 

It  is  the  province  of  a  special  science,  paleontology  or  paleo- 
zoology,  to  treat  of  the  character  and  distribution  of  animals  in 
the  earlier  periods  of  the  earth's  history,  but  since  it  is  necessary 
to  draw  upon  paleontological  facts  to  understand  the  living  forms 
an  outline  of  the  geological  periods  with  the  characteristic  animals 
may  be  given  here. 

g 

I.  Azoic  OR  ARCHEAX  ERA. 

No  organisms  are  certainly  known  from  this  age.  The  animal 
nature  of  Eozoon  canadense  of  the  Laurentian  beds,  once  referred 
to  the  Foraininifera,  is  more  than  doubtful. 

II.  PALEOZOIC  ERA. 

1.  Cambrian.  4.   Carboniferous. 

2.  Silurian.  5.   Permian. 

3.  Devonian. 

The  oldest  paleozoic  period,  the  Cambrian,  contains  only 
invertebrate  fossils:  trilobites,  gigantostracn,  cystoids,  nautiloids, 
gasteropods,  and  a  few  lamellibranchs.  Trilobites,  cystoids, 
gigantostraca,  and  the  blastoids  and  tetracoralla,  which  appear  in 
the  Silurian,  reach  their  culmination  and  become  extinct  in  the 
paleozoic.  Fishes  appear  in  the  Silurian,  and  acquire  a  great 
•development  in  the  Devonian.  The  earliest  Amphibia  come  from 
the  carboniferous,  the  reptiles  appear  in  the  Permian. 

III.  MESOZOIC  ERA. 

1.   Triassic.  2.  Jurassic.  3.   Cretaceous. 

The  mesozoic  era  was  the  age  of  reptiles,  which  were  repre- 
sented by  numerous  forms,  some  of  gigantic  size;  most  of  them 


DISTRIBUTION.  181 

becoming  extinct  in  the  cretaceous.  The  first  mammals  appear  in 
the  triassic,  the  birds  in  the  Jurassic.  Among  the  invertebrates 
the  ammonites,  which  appeared  in  the  Devonian,  reached  their 
greatest  development  and  became  extinct  in  this  era. 

IV.  CENOZOIC  ERA. 

(a)  Tertiary. 

1.  Eocene.  3.   Miocene. 

2.  Oligocene.  4.   Pliocene. 

(b)  Quaternary. 

5.   Pleistocene.  6.   Kecent. 

In  the  tertiary  all  of  the  now  living  orders  of  mammals  andf 
birds  appeared,  among  them  man,  whose  remains  have  been  traced 
with  certainty  to  the  pleistocene. 


SPECIAL  ZOOLOGY. 

comparative  anatomy  and  the  theory  of  evolution  have 
made  their  impression  upon  systematic  zoology  one  recognizes  in 
classification  not  only  a  means  of  arranging  the  species,  but  also 
£he  possibility  of  expressing  the  relations  which  the  larger  and 
smaller  groups  bear  to  each  other.  The  solution  of  these  prob- 
lems demands  .an  accurate  knowledge  of  comparative  anatomy  and 
embryology  and  a  complete  knowledge  of  animal  forms  based  upon 
them.  We  are  yet  far  from  such  a  knowledge,  farther  with  regard 
to  some  groups  than  others,  and  as  a  consequence  systematic  prob- 
lems are  not  all  equally  advanced  towards  solution. 

In  general  it  may  be  said  that  certain  natural  groups  are 
recognized:  (I)  Chordata;  (2)  Mollusca  (after  the  elimination 
of  the  Brachiopoda) ;  (3)  Arthropoda;  (4)  Echinoderma;  (5) 
Ccelenterata  (after  the  separation  of  sponges) ;  (6)  Protozoa.  On  the 
other  hand,  it  is  yet  uncertain  exactly  how  to  regard  the  worms, 
brachiopods,  polyzoa,  and  a  few  other  forms.  The  general  ten- 
dency is  to  distribute  the  worms  into  at  least  three  branches  (flat 
worms,  round  worms,  and  annelids)  and  to  unite  the  Polyzoa  and 
Brachiopoda  in  a  branch  of  Molluscoida.  In  this  way  groups  poor 
in  species  and  of  little  importance  in  a  general  account  of  the  ani- 
mal kingdom  are  placed  on  the  same  basis  as  the  large  and  exceed- 
ingly important  groups  of  vertebrates,  arthropods,  and  molluscs, 
and  thus  obtain,  especially  in  the  eyes  of  the  beginner,  an  impor- 
tance which  does  not  belong  to  them.  It  therefore  seems  better  in 
an  elementary  work  to  pursue  a  more  conservative  course. 

182 


PROTOZOA.  183 


PHYLUM   I— PROTOZOA. 

All  of  the  Protozoa  are  small ;  some  may  be  seen  by'a  sharp  eye 
as  minute  points,  but  the  majority  are  so  minute  that  they  are 
invisible  except  with  a  microscope.  On  the  other  hand,  there  are 
a  few  which  have  a  diameter  to  be  measured  by  millimeters,  this 
being  especially  the  case  where  hundreds  of  individuals  are  united 
in  colonies. 

This  small  size  is  a  necessary  result  of  the  fact  that  the  Protozoa 
are  single-celled  animals.  Like  all  cells  they  consist  of  that  pe- 
culiar substance,  protoplasm,  and  they  have  the  further  cell  attri- 
bute, the  possession  of  one  or  more  nuclei.  Being  unicellular,  it 
follows  that  they  lack  true  tissues  and  true  organs.  They  lack 
alimentary  canal,  nervous  system,  sexual  organs,  etc.  The  funda- 
mental functions  of  nourishment,  sensation,  movement,  and  repro- 
duction are  performed  more  or  less  directly  by  the  protoplasm. 

In  nutrition,  in  so  far  as  it  is  not  produced  by  substances  in 
solution,  foreign  particles  pass  into  the  protoplasm  and  are  digested 
by  it.  They  usually  lie  during  digestion  in  special  collections  of 
fluid,  the  food  vacuoles  (figs.  120,  144,  etc,  no),  more  rarely  in  the 
protoplasm  itself.  All  indigestible  portions  are  cast  out  after  a 
time.  This  taking  in  and  casting  out  of  foreign  matter  can  take 
place  in  the  lower  Protozoa  at  any  point  of  the  surface,  while  in 
the  more  highly  organized  species  there  are  definite  openings  which 
according  to  analogy  with  many-celled  animals  are  spoken  of  as 
mouth  and  anus,  or  more  precisely,  cytostome  and  cytopyge.  The 
mouth  may  connect  with  a  tube,  the  oesophagus  or  cytopharynx, 
which  ends  free  in  the  protoplasm. 

Structures  may  occur  within  the  protozoan  cell  which  recall 
the  organs  of  higher  animals,  and  which  are  called  cell  organs. 
While  motion  is  usually  produced  by  the  protoplasm  and  its  pro- 
cesses— -pseudopodia,  flagella,  and  cilia — there  are  Protozoa,  like 
Stentor  and  the  Vorticellidae,  which  have  true  muscular  fibrillae. 
The  sensitiveness  to  light  is  often  increased  by  the  formation  of  an 
eye  spot,  a  small  pigment  body  in  which  even  a  lens  may  occur, 
More  constant  of  cell  organs  are  the  contractile  vacuoles  (fig.  116? 
etc.,  cv),  structures  rarely  absent  from  fresh-water  species,  but 
commonly  lacking  from  marine  forms.  These  are  distinguished 
from  the  food  vacuoles- by  three  characters:  they  have  a  definite 


18-i  PROTOZOA. 

place  in  the  cell;  their  number  is  approximately  constant  in  most 
species;  they  exhibit  extremely  constant  phenomena.  The  walls 
contract  and  empty  the  fluid  contents  to  the  exterior,  often 
through  a  special  duct.  When  one  empties  it  completely  disap- 
pears and  is  formed  again  anew  in  a  short  time,  and  is  filled  with 
fluid  from  the  surrounding  protoplasm.  It  thus  resembles  the 
contractile  vacuoles  in  the  water  vascular  system  (excretory  organs) 
of  the  worms  to  be  described  later.  Apparently  the  contractile 
vacuoles  are  for  the  elimination  of  injurious  substances  in  solution 
produced  by  the  vital  processes,  among  them  possibly  carbon 
dioxide,  like  a  respiratory  organ. 

Apparently  all  the  vital  functions  are  under  the  control  of  the 
nucleus.  Experiments  show  that  Protozoa,  artificially  deprived  of 
their  nuclei,  perform  their  functions  incompletely  and  soon  perish, 
while  fragments  containing  a  nucleus  remain  alive.  Young  Pro- 
tozoa usually  have  a  single  nucleus,  and  many  have  but  one 
throughout  life  ;  but  others  early  become  multinucleate,  Such 
multinucleate  forms  are  frequently  regarded  as  cell  complexes  or 
syncitia,  but  unnecessarily,  for  aside  from  the  fact  that  in  animal 
and  plant  histology  polynucleate  masses  of  protoplasm  are  regarded 
as  cells,  this  term  makes  a  distinction  between  the  uni-  and  the 
multi-nucleate  forms,  which  does  not  correspond  to  the  actual 
relations,  since  the  phenomena  of  both  are  completely  alike. 

Reproduction  is  accomplished  exclusively  by  fission  or  budding, 
and  under  suitable  conditions,  such  as  abundance  of  nourishment, 
occurs  so  rapidly  that  many  Protozoa  inside  a  few  weeks  can 
number  their  descendants  by  millions.  Many  divide  in  the  free 
state  while  they  are  creeping  or  swimming  about;  others  become 
encysted  before  division.  They  become  spherical  and  secrete  a 
protecting  membrane  around  themselves  (figs.  121,  122).  En- 
cysted individuals  usually  divide  into  more  than  two  pieces,  in 
four,  eight,  or  even  many  hundreds  of  reproductive  bodies.  It  fre- 
quently happens  that  multinucleate  species  divide  into  as  many 
parts  as  there  are  nuclei. 

In  the  Protozoa  may  occur  a  fusion  of  individuals — conjuga- 
tion— which  in  many  respects  has  much  similarity  to  the  process 
of  fertilization  in  Metazoa  and  in  plants.  In  some  (conjugation 
of  many  Rhizopods)  this  does  not  correspond  to  true  fertilization, 
since  only  the  protoplasm  unites  (plastogamy],  while  the  fusion  of 
nuclei  (caryogamy)  necessary  to  fertilization  does  not  occur.  In 
others  a  fusion  of  nuclei  takes  place.  In  the  cases  which  have 
been  accurately  studied  there  has  been  seen,  before  the  fusion  of 


PROTOZOA.  185 

the  nuclei,  a  process  comparable  to  the  formation  of  the  polar  glob- 
ules in  the  egg,  to  this  extent,  that  in  each  of  the  conjugating 
individuals  the  nucleus  divides  twice  and  of  the  products  of  divi- 
sion only  one,  the  nucleus  intended  for  caryogamy,  persists  while 
the  others  (polar  globules)  degenerate. 

These  cases  of  true  fertilization  permit  of  great  diversity.  The 
conjugating  individuals  can  be  equal  in  size  (most  Infusoria,  many 
Rhizopoda),  or  there  is  a  disparity  in  size  (sexual  dimorphism),  in 
which  smaller  and  consequently  more  mobile  '  males '  (microga- 
metes,  zoospores)  fertilize  the  larger  fixed  or  slowly  moving 
'  females '  (macrogametes,  oospores)  as  in  Volvox  globator,  Vorti- 
cellidae,  and  many  Sporozoa.  In  conjugation  of  individuals  of 
equal  size  there  is  frequently  a  mutual  fertilization — A  fertilizes 
B,  and  is  in  turn  fertilized  by  B — after  which  the  animals  sepa- 
rate (most  Infusoria,  Gregarines,  Noctiluca). 

Twenty  years  ago  it  could  be  laid  down  as  a  universal  fact  that  the 
Protozoa  in  contrast  to  the  Metazoa  lacked  sexuality.  In  the  mean  time 
observations  on  Protozoa  belonging  to  different  classes,  even  the  Rhizop- 
oda, have  so  increased  that  the  conclusion  is  that  fertilization  occurs  in 
all  Protozoa,  although  the  rarity  of  the  process  in  many  species  renders 
the  complete  demonstration  difficult.  Still  there  remain  certain  interest- 
ing differences  from  the  Metazoa.  The  Protozoa  lack  special  sexual  cells 
— eggs  and  spermatozoa.  On  the  contrary,  the  whole  body  functions  as  a 
sexual  cell.  Further,  the  relations  of  fertilization  to  reproduction  are  not 
the  same  as  in  the  Metazoa.  It  does  indeed  occur  (swarm-spore  formation 
in  Noctiluca,  formation  of  pseudonavicellee  in  gregariues)  that  fertilization 
precedes  rapid  division,  but  much  more  commonly  fertilization  is  the 
result  of  rapid  division  and  a  cause  of  slower  reproduction  (Infusoria)  or 
even  of  complete  rest  (Actinosphcerium,  Actinophrys,  Volvox).  One  can 
therefore  only  speak  of  fertilization,  not  of  sexual  reproduction,  in  the 
Protozoa.  These  facts  are  of  great  importance  in  the  consideration  of  the 
nature  of  impregnation,  for  they  show  that  it  has  not  only  the  purpose  of 
stimulating  the  developmental  processes,  but  that  it  accomplishes  other 
functions,  and  that  these  functions,  obscure  as  they  at  present  are,  are 
the  more  important  since  they  are  the  more  primitive  and  the  more  widely 
distributed. 

With  Noctiluca,  many  Sporozoa,  and  perhaps  in  Rhizopods  a  period 
follows  impregnation  in  which  the  division  (*  sexual  reproduction ')  has  a 
special  character  (swarm-spore  formation  in  Noctiluca,  formation  of  sporo- 
blasts  and  sporozoites  in  the  Sporozoa)  and  differs  from  the  customary 
'  vegetative  '  reproduction.  This  alternation  of  methods  of  reproduction 
recalls  the  alternation  of  generations  of  the  Metazoa  and  is  called  by  the 
same  name. 

The  Protozoa  with  thin  small  and  soft  protoplasmic  bodies  are 
but  little  if  at  all  protected  against  drying  up,  and  therefore  they 


186  PROTOZOA. 

are  aquatic.  A  few  forms,  like  Amoeba  terricola,  are  terrestrial,  and 
these  only  occur  in  moist  places.  Salt  and  fresh  water,  of  the  latter 
.stagnant  pools  rich  in  vegetation,  are  the  favorite  places  for  Pro- 
tozoa. The  fresh-water  forms  are  cosmopolitan,  so  that  the  forms 
in  the  most  diverse  lands  are  very  similar.  This  depends  upon  cer- 
tain peculiarities.  The  fresh-water  Protozoa  can  become  encysted 
independent  of  reproduction,  and  in  the  encysted  stage  can  endure 
times  of  unfavorable  conditions  such  as  lack  of  food,  freezing,  or 
•complete  evaporation  of  the  water.  When  thus  protected  they 
may  be  blown  about  by  the  wind  or  carried  far  on  the  feet  of 
birds.  Hence  it  is  that  one  group  bears  the  name  Infusoria,  for  if 
•dry  earth  or  dry  plants  (e.g.,  hay)  be  soaked  in  water  and  this 
infusion  allowed  to  stand  for  some  time,  a  more  or  less  rich 
Protozoan  fauna  will  develop  in  it.  The  encysted  animals  in  the 
earth  or  on  the  plants  are  awakened  by  the  moisture  to  new  life 
.and  leave  the  cyst.  Spontaneous  germination,  as  was  once  believed, 
does  not  occur  here,  for  if  one  sterilize  the  materials  and  prevent 
xthe  entrance  of  germs  the  water  will  remain  uninhabited. 

Historical. — On  account  of  their  practical  invisibility  the  Protozoa  were 
unknown  until  1675  ;  they  were  discovered  in  infusions  by  the  Dutch 
Leeuwenhoek,  the  discoverer  of  the  microscope.  Wrisberg  in  the  eight- 
eenth century  called  them  Animalcula  infusoria — infusion  animals,  and 
Siebold  in  the  century  just  closed  gave  them  the  name  Protozoa.  The 
proposition  of  Haeckel  to  place  a  portion  of  the  Protozoa  in  a  kingdom 
Protista  between  animals  and  plants  has  found  but  little  acceptance.  In 
the  accounts  of  the  structure  the  views  of  Dujardin  and  Ehrenberg  were 
long  at  variance.  Ehrenberg  maintained  with  all  confidence  that  the 
Protozoa  like  all  animals  possessed  the  most  important  organs,  alimen- 
tary canal,  nervous  system,  muscles,  excretory  and  sexual  organs.  Du- 
jardin denied  all  this  and  ascribed  to  the  Protozoa  only  a  single  homo- 
geneous substance,  '  sarcode '  (p.  60)  which  was  sufficient  to  produce  all 
vital  phenomena.  Dujardin's  view  later  found  important  support  in 
^iebold's  discovery  that  the  Protozoa  were  unicellular.  Still  for  a  long 
time  Ehrenberg's  ideas  persisted  in  various  modified  forms  and  were  not 
totally  overthrown  until  after  the  middle  of  the  nineteenth  century.  The 
fact  that  there  are  unicellular  animals  without  organs  and  yet  capable  of 
existence  was  an  extremely  valuable  addition  to  knowledge,  for  it  not  only 
broadens  our  conception  of  animal  life,  but  it  furnishes  ;for  the  theory  of 
evolution  from  simple  organisms  the  strongest  link  in  the  chain,  the  first 
of  the  series. 

''TJie -different  appearances  of  Protozoa  depend  upon  the  grade  of 
organological  and  histological  differentiation.  Since  these  are  most  promi- 
nent in  the  nourishing  and  locomotor  structures,  these  become  important 
in  subdividing  the  group.  The  organs  for  these  purposes— pseudopodia, 


/.    RHIZOPODA. 


187 


flagella,  cilia — furnish  the  basis  for  the  differentation  of  these  classes,  to 
which  are  added  forms — the  class  of  Sporozoa — modified  by  parasitism. 

Class  I.  Rhizopoda. 

In  the  lowest  position  in  the  Protozoa  must  be  placed  those 
organisms  which  lack  permanent  structures  for  locomotion  and 
nourishment,  but  in  which  the  protoplasm  of  the  body  performs 
these  functions.  The  term  Rhizopoda  refers  to  the  fact  that  the 
protoplasm  sends  out  root-like  processes — false  feet  or  pseudopodia 
— for  locomotion  and  for  taking  nourishment.  These  differ  from 
true  appendages  in  that  they  are  not  constant  cell  organs,  but  are 
formed  according  to  demand  and  again  disappear.  A  pseudopo- 
dium  arises  when  the  protoplasm 
streams  to  one  point  of  the  body  and 
extends  as  a  process  beyond  the  sur- 
face. Since  the  process  becomes 
attached  and  draws  the  body  after 
it,  or  since  the  protoplasm  of  the 
body  may  flow  into  it,  a  slow  change  ( 
of  place  occurs.  Thus  the  process 
disappears  and  is  absorbed  in  the 
organism,  and  new  pseudopodia  are 
formed  at  other  places  which  after  a 
time  are  retracted  in  turn.  This 
type  of  locomotion  is  called  amoeboid 
after  the  Amceba,  in  which  it  was 
first  accurately  studied.  When  the 
Ehizopoda  in  their  wanderings  meet 
particles  of  nourishment,  they  en- 
close them  with  their  protoplasm  FIG.  116.  —  Amoeba  proteus.  (After 

,     Leidy.)    cv,  contractile  vacuole;  en, 

and   take   them    into    the    interior  OI     entosarc;   efc,  ectosarc;   n,  nucleus; 
,,     '        T       ,„        .,.,„      ,^x  N,  food-body. 

the  body  (fig,  116,  J\  ). 

The  form  of  the  pseudopodia  is  approximately  constant  for 
each  species,  but  as  a  whole  very  variable,  so  that  it  may  be  used 
not  only  for  separating  species  but  families  and  larger  groups.  Qn 
the  one  hand,  there  are  finger-like  pseudopodia  (fig.  116),  on  the 
other,  those  of  such  delicacy  that  even  under  strong  magnification 
they  appear  like  fine  threads  (fig.  117).  Between  these  two 
extremes  are  many  intermediate  forms.  Thread-like  pseudopodia 
usually  branch,  and  when  the  branches  meet  they  may  fuse  and 
form  anastomoses,  from  which  it  follows  that  it  is  not  true,  as  was 
once  supposed,  that  the  psendopodia  are  .covered,  by  a  membrane. 


188 


PROTOZOA. 


The  fine  granules  of  the  protoplasm  can  enter  the  pseudopodia 
and  produce  here,  as  they  move  back  and  forth,  the  phenomenon 
of  '  streaming/  Since  foreign  particles,  like  grains  of  carmine 
taken  up  by  the  protoplasm,  can  participate  in  this  streaming,  it 


\ 


FIG.  117.— Rotalia  freyeri.    (From  Lang,  after  M.  Schultze.) 

follows  that  the  movements  depend  not  upon  the  granules  but  on 
the  protoplasm  itself.  We  have  already  used  the  fact  (p.  62) 
that  granules  in  the  finest  thread  can  move  in  opposite  directions 
at  the  same  time,  to  demonstrate  the  extraordinary  complexity  of 
protoplasmic  structure. 

When  Rhizopoda,  in  the  free  or  en- 
cysted condition,  increase  by  division,  the 
division  products  frequently  exchange  the 
amoeboid  motion  for  that  of  the  Flagellata, 
and  become  flagellate  spores  or  zoospores. 
The  body  becomes  oval  and  develops,  on  the 
anterior  end  which  contains  the  nucleus, 
one  or  more  flagella,  which  move  more  ener- 
getically than  pseudopodia,  and  are  perma- 
nent as  long  as  the  zoospore  stage  persists 
(fig.  121).  Since  many  Protozoa  possess 
flagella  along  with  pseudopodia,  the 
FIG.  iM.-3forttoa«io*a  as-  boundary  between  Rhizopods  and  Flagel- 

pera.   (After  F.  £.  Schulze.)    lates  is  not  distinct  (fig.   118). 

The  Rhizopoda  form  an  ascending  series  in  which  the  systematic  char- 
acters become  more  and  more  pronounced;  such  are  the  assumption  of  a 


/.   RHIZOPODA:  MONERA,  LOBOSA. 


189 


definite  form,  as  in  the  Radiolaria  and  Heliozoa,  the  formation  of  a  skele- 
ton of  regular  character,  as  in  the  Thalamophora,  or  the  development  of 
a  peculiar  reproduction,  as  in  the  Mycetozoa.  At  the  bottom  stand  the 
Monera  and  the  Lobosa  whose  characters  are  mostly  negative,  for  neither 
form,  skeleton,  nor  reproduction  affords  systematic  distinctions. 

Order  I.  Monera. 

The  most  important  character  of  the  Monera  is  the  lack  of  a  nucleus. 
As  with  other  negative  characters  this  is  somewhat  uncertain.  In  many 
•cases,  especially  when  the  protoplasm  is  filled  with  chromatin  granules, 
the  nucleus  is  recognized  with  difficulty,  and  hence  animals  have  been 
described  as  anucleate  in  which  the  nucleus  was  overlooked.  The  num- 
ber of  '  Monera '  was  formerly  very  great,  but  has  diminished  with  the 
development  of  microscopic  technique.  So  it  is  possible,  even  probable, 
that,  in  the  few  forms  now  remaining  in  the  group,  the  nucleus  has  merely 
escaped  observation.  On  the  other  hand  there  are  several  theoretical 
reasons  which  support  the  idea  of  anucleate  organisms.  It  is  easier  to 
suppose  that  with  the  appearance  of  life  there  were  organisms  consisting 
of  but  a  single  substance  thau  that  these  organisms  had  nucleus  and 
protoplasm  already  differentiated.  Several  species  of  Protamcsba  are 
placed  in  the  Monera. 

Order  II.  Lobosa  (Amoebina). 
Lobosa  are  primitive  Ehizopoda  with 
one  or  several  nuclei.  The  species  of 
Amoeba,  forms  which  owe  their  name  to 
their  constant  change  of  shape,  are  typical 
(figs.  116,  119).  This  change  of  form  is 
due  to  the  formation  and  disappearance 
of  a  few  finger-like  (lobose)  pseudopodia. 
Body  and  pseudopodia  consist  of  two 
layers,  a  soft  granular  inner  entosarc 
(en)  and  a  firmer,  clear,  outer  ectosarc 
(ek).  In  the  entosarc  is  usually  a  single 
(sometimes  several)  nucleus  (ri),  which  is 
vesicular,  and  contains  either  one  large 
or  several  smaller  nucleoli.  A  contractile 
vacuole  is  usually  present.  Reproduction 
occurs  by  division  (fig.  119),  and  in  some 
instances  encystment  has  been  observed,  FVJ- 
the  protoplasm  dividing  into  manv  him-  Bchnlae.)  cv,x  contractile 

J  vacuole ;    ek,  ectosarc  ;    en, 

dred  small  amoebae.  entosarc;  w,  nucleus. 

Most  Lobosa  occur  in  fresh  water ;  the  larger  forms,  like  Pelomyxa 
palustris  (2  mm.  in  diameter)  live  in  the  ooze  of  pools,  the  smaller,  like 
Amceba  proteus  and  A.  princeps,  on  plants  or  free  in  the  water.  The  very 
small  A.  terricola  lives  in  moist  earth.  There  are  also  parasites  among  the 
Amceba3,  like  A.  coli  (0.02  to  0.035  mm.  large),  rare  in  colder  climates,  fre- 


190 


PROTOZOA. 


quently  observed  in  the  tropics  in  liver  abscesses  and  in  ulcers  of  the 
colon  of  men  ill  with  dysentery,  and  perhaps  the  cause  of  the  disease. 
Some  of  the  Monothalamia  (p.  198)  are  sometimes  referred  to  the  Lobosa. 

Order  III.  Heliozoa,  Sun  Animalcules. 

The  Heliozoa  owe  their  name  to  the  shape  of  the  body,  with 
the  pseudopodia  arranged  like  rays.  In  the  pseudopodia  are  a 
firm  axial  thread  forming  a  skeleton  and  a  thin  coating  of 
protoplasm.  Branching  and  anastomoses  of  the  pseudopodia 
are  rare,  and  usually  occur  only  when  the  radial  arrangement  is 
modified  by  pressure.  The  axial  threads  frequently  converge  at 
the  centre  of  the  body.  Here  lies  a  granule,  the  centrosome 
separated  from  the  nucleus,  which  plays  an  important  part  in 
division.  The  body  consists  of  cortical  and  medullary  portions 


No, 


cv     Na 

FIG.  120.— Actinosphcerium  eichhorni.     Jtf,  medullary  substance  with  nucleus   (n) ; 
R,  cortical  substance  with  contractile  vacuoles  (cv);  IV,  food-body. 

(fig.  120),  distinguished  by  differences  in  the  protoplasm,  but 
not  separated  by  membrane.  In  the  cortex  are  the  contractile 
vacuoles  (cv);  the  medulla  contains  the  usually  single  nucleus. 
Among  the  polynucleate  forms  is  the  largest  and  one  of  the  most 
beautiful  of  fresh- water  species,  Actinosphcerium  eichhorni.  Many 


I.  RHIZOPODA:  HELIOZOA. 


191 


Heliozoa  possess  a  silicious  skeleton,  which  may  be  a  lattice-work 
sphere    (fig.   121),  needles   radially  arranged   or   placed   tangen- 


\ 


FIG.  121.— Clathrulina  elegam.    A,  with  extended  pseudoppdia;  JB,  divided  into  two 
cysts ;  C,  zoospore ;  n,  nucleus ;  cv,  contractile  vacuole. 

tially.     The  forms  without  skeletons  are  few,  but  these  have  the 
power  (fig.  122)  of  forming  silicious  envelopes  during  encystment. 

Reproduction  takes  place  by  divi- 
sion, and  one  or  both  moieties  may 
become  swarm  spores,  i.e.,  assume  an 
oval  form  bearing  at  one  pole  one  or  two 
flagella  (fig.  121,  C).  These  swarm 
spores  become  widely  distributed  by 
the  flagella  before  they  resume  the 
spherical  shape,  lose  their  flagella  and 
form  pseudopodia.  It  frequently  occurs 
that  several  Heliozoa  of  the  same 
species  become  connected  by  proto- 
plasmic bridges,  and  so  form  unions  of 
from  two  to  ten  individuals.  True  fer- 
tilization preceded  by  a  kind  of  polar-  Flo.  122.-cyst  with  germinal 
globule  formation  has  only  been  ob- 
served  in  Actinoplirys  sol  and  Act ino splicer ium  eiclihorni. 


192 


PROTOZOA. 


Forms  with  skeleton  and  those  without  are  distinguished.  To  the  first 
belong  Clathrulinaelegans,  with  a  spherical  lattice-work  skeleton  supported 
on  a  stalk  (fig.  121).  Acanthocystis  turfacea,  skeleton  of  radial,  branching 
needles.  To  the  forms  without  skeleton  belong  first  Actinosphcerium 
eichhorni,  as  large  as  a  pin-head,  milk-white,  protoplasm  foamy  from  the 
numerous  fluid  vacuoles,  the  different  sizes  of  which  markedly  distinguish 
cortical  from  medullary  proportions.  The  contractile  vacuoles  are  in  the 
cortex,  the  nuclei  in  the  medulla.  In  encystment  the  foamy  appearance 
and  most  of  the  nuclei  are  lost,  and  a  cyst  is  formed  with  numerous  uni- 
nucleate  daughter  cysts.  Each  daughter  cyst  divides  into  secondary  cysts 
which,  after  the  formation  of  polar  globules,  fuse^  (fertilization)  and 
produce  germ  spheres.  From  these  then  escape,  after  a  long  rest,  the 
young  Actinosphceria.  The  reproduction  of  Actinophrys  sol,  a  smaller 
form,  is  essentially  similar. 

Order  IV.  Radiolaria. 

The  Radiolaria,  the  most  beautiful  and  most  highly  organ- 
ized of  the  Rhizopoda,  strongly  recall,  in  their  appearance,  the 


? 

/**: 

X 


FIG.  123.— Thalassicolla  pelagica.  In  the  centre  the  nucleus  with  coiled  nucleolus, 
around  it  the  central  capsule  with  oil  globules;  still  outside  the  extracapsular 
soft  body  with  vacuoles  (extracapsular  alveoli),  yellow  cells  (black)  and  pseu- 
dopodia. 

Heliozoa.     They  are  spherical,  only  rarely  by  flattening  converted 
into  disks,  or  by  unequal  growth  into  conical  or  lobular  shapes. 


/.   RHIZOPODA:  RADIOLARIA. 


193 


A  second  resemblance  lies  in  the  delicate  pseudopodia,  often 
with  an  axial  filament.  The  distinguishing  characteristic  is  the 
central  capsule.  This  is  the  central  portion  of  the  body  surrounded 
by  a  membrane,  outside  of  which  is  the  extracapsulum.  The 
central  capsule  is  the  most  important  part  of  the  animal.  If  it  be 
dissected  out  from  the  extracapsulum  it  not  only  lives  but  regen- 
erates the  lost  parts,  while  the  extracapsular  portions  die.  Since 
the  protoplasm  of  both  parts  is  identical,  the  difference  in  regen- 
erative powers  can  only  depend  on  the  nuclei,  which  are  confined 
to  the  central  capsule. 

The  central  capsule  may  be  uni-  or  polynucleate.  In  the  first  case  the 
nucleus  (fig.  123),  a  vesicle  of  appreciable  size,  lies  in  the  centre  of  the 
capsule,  in  the  others  the  capsule  is  thronged  by  hundreds  of  small  homo- 
geneous nuclei.  All  Radiolaria  are  uninucleate  in  the  young  stage,  and  only 
at  the  time  of  swarm-spore  formation  polynucleate.  The  fact  that  certain 
species  have  almost  always  one  nucleus,  while  others  usually  have  many,  is 
explained  in  the  first  case  by  the  long  continuance  of  the  uninucleate  con- 
dition, only  giving  place  to  the  polynucleate  condition  just  before  the 
formation  of  swarm-spores,  while  in  the  second  the  polynucleate  condition 
is  reached  early.  In  the  central  capsule  are  also  included  various 
deposits  which  serve  as  food  during  reproduction,  such  as  concretions,  oil 
globules,  etc. 

The  membrane  surrounding  the  central  capsule  is  either  per- 
forated on  all  sides  by  numerous  pore- 
canals  or  by  small  openings  in  certain 
places.  Through  these  pores  and  open- 
ings the  intracapsular  protoplasm  passes 
out  and  spreads  itself  in  the  extracap- 
sulum. This  consists  of  a  gelatinous 
mantle  through  which  the  protoplasm 
•extends  as  a  fine  network  before  it  forms 
pseudopodia  on  the  surface.  In  the 
larger  Radiolaria  it  contains  vacuoles 
(extracapsular  alveoli)  developed  in  the 
protoplasmic  net  (fig.  123). 

With  few  exceptions  the  Radiolaria 
possess  skeletons  of  wonderful  beauty; 
latticed  spheres,  single  or  one  within 
another,  and  bound  together  with  radial 
rods  (fig.  85),  frequently  ornamented  on 
the  outer  surface  with  spines,  or  latticed 
discs,  helmet-like  or  cage-like  structures  (fig.  124)  or  spongy 
structures.  In  other  cases  occur  rings,  tubes,  spines,  which  meet 


FIG.  124.— Eucyrtidium   crani- 
oides.    (After  Haeckel). 


194 


PROTOZOA. 


FIG.  125.— Acanthometra  elastica.    c/r,  central  capsule;  n,  nucleus;  p,  pseudopodia; 
St,  spines ;  Wk,  extracapsulum. 


FIG.  )£6.— Collozoum  inerme.    a,  jelly:  5,  oil  globules  in  the  central  capsule;  c,d, yellow- 
cells;  e,  vacuoles. 


1.   RIIIZOPODA:  RADIOLARIA.  195 

in  the  central  capsule  (fig.  125),  etc.  In  rare  cases  the  skeleton  is 
formed  solely  of  organic  substance  (acanthin)',  usually  it  is  sili- 
cious  and  of  much  firmness.  Hence  skeletons  of  Radiolaria  occur 
in  rocks  of  various  ages,  as  in  Caltanisetta,  Sicily,  the  Nicobars 
(both  tertiary),  and  the  Barbadoes. 

In  reproduction  first  comes  fission,  which  begins  a  division  of 
the  central  capsule  (in  uni-nucleate  forms  with  a  division  of  the 
nucleus)  and  usually  extends  through  the  extracapsulum.  If  this 
latter  does  not  divide  a  colony  results,  in  which  a  gradually 
increasing  jelly  contains  numerous  central  capsules,  bound  together 
by  protoplasmic  threads,  which  form  the  pseudopodia  on  the  sur- 
face (fig.  126).  A  second  type  is  reproduction 
by  swarm  spores,  which  begins  when  the 
nucleus  has  divided  into  hundreds  or  thousands 
of  daughter  nuclei.  The  central  capsule  then 
divides  into  as  many  portions  as  there  are 
nuclei,  which  become  oval  and  develop  two 
flagella  (fig.  127),  which  soon  begin  to  vibrate 
so  that  the  central  capsule  is  filled  with  a 
tumultuous  crowd.  With  the  breaking  of  the  Flo  127_ Zoosporeg  of 
capsular  membrane  these  swarm  spores  escape,  SfcrospHlre-l6/h"e'zoo' 
and  at  this  point  our  knowledge  of  this  type  of  gP«rye.  ^^Jroapor!? 
development  ceases.  Since  in  many  species 
there  are  large  and  small  spores — macrospores  and  microspores — 
it  is  probable  that  for  the  further  development  a  copulation  of 
different  swarm  spores  is  necessary. 

Common,  if  not  constant,  in  the  bodies  of  the  Radiolaria  are  the  yellow 
cells  which  were  formerly  regarded  as  a  part  of  the  animal;  they  are  uni- 
cellular algae  (Zooxantliellce),  which  are  also  present  in  other  animals. 
(Thalamophora,  actinians,  sponges,  etc.)  They  afford  an  instance  of  sym- 
biosis, or  the  living  together  of  different  organisms  for  mutual  good.  This 
new  view  rests  upon  the  facts  that  the  Zooxanthetta  have  a  membrane, 
secrete  starch-like  substances,  divide  independently  of  the  radiolarian  and 
continue  to  live  after  its  death. 

The  Radiolaria  are  exclusively  marine.  In  fair  weather  they  float  at 
the  surface,  but  sink  in  times  of  storm.  Certain  species  and  even  large 
groups  like  the  Phaeodaria  occur  only  at  great  depths  (1500-4000  fathoms) 
where  the  temperature  is  about  0°  C. 

Sub  Order  I.  PERIPYLEA  or  SPUMELLARIA.  The  capsule  mem- 
brane everywhere  perforated  by  pore  canals;  skeleton  absent  or  formed  of 
loose  needles,  of  siiicious  latticed  spheres,  often  reduced  to  a  spongy  net- 
work or  flattened  to  discs.  The  latticed  spheres  can  be  provided  with 
spines  and  connecting  rods.  SPH^ROZOID^E,  colonial  (fig.  126);  THALASSI- 


196 


PROTOZOA. 


(fig.  123);  HALIOMMID^E  with  latticed  spheres  (fig.  85);  DISCID.E,  disc 
like. 

Sub  Order  II.  ACANTHARIA.  Capsular  membrane  perforated  every- 
where by  pore  canals;  twenty  spines  of  acanthin  which  radiate  from  the 
centre  in  an  extremely  regular  manner,  form  the  skeleton,  as  in  Acantho- 
metra  (fig.  125),  or  the  spines  are  bound  together  by  a  latticed  sphere 
formed  of  twenty  separate  plates,  as  in  Acanthophracta. 

Sub  Order  III.  MONOPYLEA  or  NASSELLARIA.  The  pores  of  the 
central  capsule  occupy  a  pore  field  at  one  end.  Best  known  are  the  CYRTID.E 
(Eucyrtidium,  fig.  124)  with  graceful  helmet  or  cage-like  skeletons. 

Sub  Order  IV.  PH^EODARIA.  The  central  capsule  has  a  principal 
opening,  often  drawn  out  into  a  tube  and  surrounded  by  dark  pigment 
(phteodium)  around  which  may  be  smaller  openings.  The  skeleton  is  sili- 
cious  and  formed  of  hollow  pieces.  Mostly  from  the  deep  seas.  Aida- 
cantha,  Aulosphcera,  Ccelodendron,  measuring  from  0.5  to  1.0  mm.,  are 
pelagic. 

Order  V.  Foraminifera  (Thalamophora,  Reticularia). 
The  Foraminifera,  though  not  equalling  the  Radiolaria  in 
beauty  and  variety  of  forms,  excel  them  in  numbers  of  indi- 
viduals, and  hence  have  a  greater  importance  in  the  history  of  the 
earth.  No  group  of  animals  at  present  or  in  the  past  have  had 
so  great  a  part  in  the  formation  of  beds  of  rock. 

The  most  prominent  characteristic  of  the  order  is  afforded  by 
the  shell.  This  is  an  envelope,  closed 
at  one  pole,  and  usually  opening  to  the 
exterior  at  the  other,  the  pseudopodia 
passing  through  the  aperture  (fig.  128). 
Accordingly  as  the  axis  connecting 
these  poles  is  shortened  or  lengthened, 
the  shell  becomes  disc-like,  spherical, 
flask  formed  or  even  coiled  in  a  spiral. 
An  additionalfeature,  the  division  of  the 
interior  of  the  shell  by  transverse  par- 
titions into  numerous  chambers,  is  com- 
mon. Such  many-chambered  shells 
(Polythalamia)  are  at  first  small,  and 
consist  of  one  or  few  chambers,  but 
FIG.  128.  —  Ouadruia  symmetrica.  as  the  animal  grows  new  chambers  are 

(After  F.  E.   Schulze.)    cv,  con- 
tractile vacuoie;    n,  nucleus;   continually  added  at  the  mouth  01  the 
JV,  food-body.  v  -n          X         •  •      AI  11    .     *   ^i 

stiell.       Openings  in  the  walls  of  the 

shell  (foramina)  connect  the  adjacent  chambers.  The  spiral 
shells  with  many  chambers  have  a  striking  resemblance  to  the 
much  larger  shells  of  the  Nautilus  (fig.  386),  which  led  to  the 
view  once  held  that  the  Foraminifera  were  small  cephalopoda. 


7.   RI11ZOPODA:   FORAMINIFERA. 


197 


In  the  fresh-water  forms  the  shell  is  built  of  a  chitinuous 
organic  substance  which  may  be  strengthened  by  silica  or  the 
incorporation  of  foreign  particles.  The  more  typical  members, 
exclusively  marine,  have  almost  invariably  calcareous  shells  which, 
when  dissolved  by  acid,  leave  but  the  slightest  trace  of  organic 
matter.  The  presence  of  minute  pores  in  the  shell  is  of  syste- 
matic importance,  the  group  of  Perforata  (fig.  117)  being  char- 
acterized by  them. 

The  animal  portions  form  a  more  or  less  complete  cast  of  the 
inside  of  the  shell  (fig.  129),  and  in  the  polythalamate  forms  con- 
sist of  as  many  pieces  as  there  are  chambers  in  the  shell,  connected 


FIG.  129.— Protoplasm  of  Globigerina 
after  solution  of  the  shell,    n,  nucleus. 


FIG.  130.— Young  Miliola  with  several 
nuclei.    (From  Lang.) 


together  by  plasma  bridges  passing  through  the  foramina  of  the 
partitions.  In  the  protoplasm  there  is  a  large  nucleus  (figs.  128, 
129,  ri),  which  in  some  cases  is  early  replaced  by  a  daughter  gen- 
eration of  small  nuclei.  Contractile  vacuoles  usually  occur  only 
in  the  fresh-water  forms.  The  pseudopodia  project  through  the 
chief  opening  of  the  shell  and  in  the  Perforata  probably  through 
the  pores  in  the  shell  wall.  They  are  rarely  finger-like  (fig.  128); 
usually  they  are  thread-like,  branched,  richly  granular  and 
anastomosing,  and  hence  favorable  objects  for  the  study  of  the 
streaming  of  protoplasm. 

Eeproduction  is  generally  accomplished  by  fission,  but  presents 
many  variations.  Only  rarely  do  both  animal  and  shell  divide; 
frequently  the  protoplasm  protrudes  from  the  mouth  of  the  shell, 
a  new  shell  is  formed  on  the  outgrowth  and  the  protoplasm  then 
divides,  one  of  the  resulting  individuals  retaining  the  old  shell.  In 
the  marine  Polythalamia  the  following  process  is  general:  The 


198  PROTOZOA. 

poiynucleate  protoplasm  divides  into  numerous  uninucleate  por- 
tions ('  embryos')  which  frequently,  while  still  within  the 
mother,  develop  a  shell  of  one  or  several  chambers.  In  several 
species  (Polystomella  crispa,  Hyalopus  dujardinii,  Miliolidae,  etc.) 
there  is  apparently  a  dimorphic  alternation  of  generations.  A 
megalospheric  generation  (characterized  by  the  long  persistence  of 
a  large  'principal  nucleus/  and  often  of  a  large  central  chamber) 
produces  zoospores,  These  develop  into  the  microsphgeric  genera- 
tion (early  poiynucleate,  often  with  small  central  chamber),  which 
form  '  embryos  '  (supra),  which  in  turn  become  megalospheres. 

Sub  Order  I.  MONOTHALAMIA.  Mostly  fresh-water  forms.  These 
species  never  have  calcareous  shells,  but  shells  of  chitin  or  silica,  often 
strengthened  by  foreign  bodies.  Contractile  vacuole  usually  present. 
Pseudopodia  either  lobose  or  filiform,  branched  or  simple.  A.  Forms 
with  finger-form  pseudopodia:  Arcella,  brown  disc-like  shell,  two  or 
several  nuclei;  Quadrula,  shell  of  many  square  plates  (fig.  128);  Diffiugia, 
(fig.  131)  with  shells  of  sand.  These  forms  may  be  regarded  as  merely 
shelled  Amoeba  and  are  frequently  referred  to  the 
Lobosa.  All  other  Foraminifera  are  characterized 
by  the  filiform  branching  or  anastomosing  pseudo- 
podia. B.  Forms  with  branching  and  anastomosing 
filiform  pseudopodia.  Euglypha,  shell  of  oval  plates; 
Gromia  (fig.  17),  marine,  shell  a  horny  sac. 

Sub  Order  II.    POLYTHALAMIA.      Exclusively 
marine,  living  on  aquatic  plants,  on  the  bottom  or 
pelagic.     The  shells,  when  not  dissolved,  fall  to  the 
bottom  in  such  numbers  that  a  gram  of  the  sand 
50,000  of  them.     Thick  beds  of  rock  like 


FIG       —Difflu  ia 

(Orig.)  the  chalk,  the  nummulitic  limestone,  and  the  green- 

sand  are  largely  foraminiferal  in  origin.  The  living  species  have  an  average 
diameter  of  about  1  mm.  Rarely  species  have  a  diameter  of  several  cen- 
timeters (Psammonyx  vulcanicus,  5-6  cm.).  The  fossil  nummulites  reach 
•a  diameter  of  6  cm.  A.  Shell  wall  massive,  the  terminal  pseudopodal  open- 
ing being  the  only  communication  with  the  exterior.  Miliola  (fig.  130). 
B.  PERFORATA.  Shell  perforated  by  many  pores;  the  terminal  opening  may 
be  lacking.  Polystomella,  Rotalia  (fig.  117),  bottom  dwelling;  GloMgerina 
bulloides  (fig.  129),  pelagic.  Among  the  fossils  the  Nummulites  need  men- 
tion as  well  as  the  Eozoon  canadense  of  the  extremely  old  Laurentiau  beds 
•of  Canada,  the  animal  nature  of  which  is  denied  by  most  students. 

Order  VI.  Mycetozoa. 

The  Mycetozoa  or  slime  animals  are  regarded  by  some  as  ani- 
mals, by  others  as  plants  under  the  older  name  Myxomycetes 
(slime  moulds).  The  first  position  is  supported  by  the  structure 
of  the  motile  stage,  the  plasmodium,  the  second  by  the  reproduc- 
tion resembling  that  of  many  fungi.  The  plasmodia  appear  in 


/.    RHIZOPODA:  MJCETOZOA. 


199 


damp  weather  as  networks  of  bright-red,  orange  or  yellow  slime  on 
decaying  wood.  They  are  giant  Amosbae,  several  centimeters  in 
extent,  of  reticulate  protoplasm  containing  many  nuclei  and  much 
foreign  matter  taken  as  food.  They  creep  slowly  by  means  of 
pseudopdia  (fig.  132).  On  drying  the  p]asmodium  encysts  in  a 


FIG.  132. 


FIG.  133. 


FIG.  132.— Chondrinderma  difforme.  (After  Strasburger.)  a,  dry  spore;  b,  swollen  in 
water;  c,  spore  with  escaping  contents;  d,  zoospore;  e,  amoeboid  modification  of 
zoospores  which  are  uniting  to  form  a  plasmodium;  /,  part  of  a  plasmodium; 
in  d  and  e,  nuclei  and  contractile  vacuoles. 

FIG.  133.— Spore-sacs  of  Arcyria  incarnata.  (After  de  Bary.)  At  the  left  the  sporan- 
gium ruptured  by  the  expanding  capillitium,  which  has  discharged  the  spores. 

peculiar  manner,  and  if  at  the  proper  stage  of  maturity,  it  forms  the 
reproductive  bodies,  the  sporangia  (fig.  133).  These  are  firm-walled 
vesicles,  frequently  stalked,  the  stalk  sometimes  extending  into  the 
axis  of  the  sporangium  as  a  columella.  The  space  between  the 
wall  of  the  sporangium  and  the  columella  is  filled  with  fine  powdery 
spores  and  an  exploding  apparatus,  either  a  network  of  fine  fila- 
ments (capillitium)  or  many  spirally  coiled  threads  (elaters). 
When  wet,  as  by  rain,  the  elaters  or  capillitium  expand,  rupture 
the  sporangium  and  scatter  the  spores.  The  spores  germinate  in 
water  or  on  moist  surfaces,  and  from  each  comes  out  a  small 
amoeba-like  embryo,  frequently  furnished  with  a  nagellum  (fig.  132). 
Several  of  these  embryos  fuse  to  form  a  plasmodium :  ^Efhalium 
septicum,  flowers  of  tan,  plasmodium  yellow,  on  spent  tanbark; 
Comatricha,  Arcyria  (fig.  133). 


200 


PROTOZOA. 


Class  II.  Flagellata  (Mastigophora). 

In  many  Khizopoda,  as  described  in  the  foregoing  pages,  the 
pseudopodia  disappear  from  time  to  time  and  are  replaced  by  one 
or  two  flagella  ;  others  have,  besides  pseudopodia,  permanent 
flagella  for  locomotion  and  taking  of  food.  Such  flagellate  spores 
and  flagellate  Ehizopods  form  the  transition  to  the  Mastigophora, 
which  are  permanently  flagellate,  the  flagella  serving  as  organs  of 
locomotion  and  feeding.  There  are  three  orders  which  must  be 
described  separately. 

Order  I.  Autoflagellata. 

All  autoflagellates  at  first  sight  are  closely  similar,  a  usually 
oval  body  with  a  vesicular  nucleus  at  one  end,  a  contractile  vacuole 
at  the  other.  At  the  anterior  end  there  is  often  added  a  small  red 
or  brown  pigment  spot  (fig.  134),  apparently  for  the  recognition 
of  light,  and  hence  a  primitive  eye. .  At  this  same  pole  are  also 
one  or  two  flagella ;  when  a  greater  number  occur  they  are  scat- 


Fio.  134. 


FIG.  135. 


FIG.  136. 


FIG.  134.— Euglena  viridis.  (After  Stein.)  c,  contractile  vacuole^  n,  nucleus ;  o,  pig- 
ment spot. 

FIG.  135.—Dinobryon  sertularla.  (After  Stein.)  a,  a  parasitic  flagellate  often  found  in 
the  lorica;  ft,  contractile  vacuole;  n%  nucleus. 

FIG.  136.— Conocladium  umbellatum.    (After  Stein.) 

tered  over  the  body.  The  body  surface  is  frequently  naked,  and 
may  be  capable  of  aniceboid  motions;  at  other  times  it  is  covered 
with  a  more  or  less  evident  cuticle.  Very  common  are  closed 
envelopes  and  open  goblet-shaped  cases  (loricse,  fig.  135),  and 
also  simple  or  branched  stalks  (fig.  136),  on  which  the  animals 


//.   FLAGELLATA:  AUIOFLAGELLATA. 


201 


sit  in  small  groups.  There  are  great  differences  in  the  feeding 
and  in  the  organs  connected  therewith.  Many  feed  like  animals, 
being  provided  with  pseudopodia  like  the  Rhizopoda  or  with  a 
mouth  like  the  Infusoria.  In  the  Choanoflagellata  there  is  an 
interesting  structure,  the  collar.  This  is  a  funnel-like  process  of 
the  body  protoplasm  on  which  foreign  particles  are  thrown  by 
the  flagellum  in  the  centre  (fig.  136)  and  thence  are  conveyed 
to  the  interior.  (According  to  recent  researches  the  collar 
consists  of  a  plasma  membrane  rolled  up  spirally  with  two  turns. ) 
Besides  these  animal  forms  are  plant-like  species  which  contain 
chlorophy]  (Volvocinae,  Euglenidae)  or  brown  chromatophores- 
(Chromomonadineae),  aiding  in  assimilation  and  enabling  the 
organism  to  produce  paramylum  or  even  starch.  It  is  note- 
worthy that  forms  which  are  plant-like  in  this  respect  are 
closely  allied  anatomically  to  forms  which  resemble  the  animals. 
Indeed,  there  are  species  which  possess  a  cytostome  without  taking 
solid  nourishment,  assimilating  by  means  of  chlorophyl  or  living 
on  fluid  food  (fig.  137).  All  this  renders  more  difficult  the  syste- 


FIG.  137. 


FIG.  138. 


FIG.  137.— Chilimonas  paramcecium.     (After  Butschli.)    oe,  cytostome;  n,  nucleus;  v, 

contractile  vacuole. 
FIG.  138.— Flagellata.     -4,  Mastigamceba;  B,  Codosiga;  (7,  Bicoseca;  D,  Hexamita;   Er 

Noctiluca,  side  view. 

matic  valuation  of  the  differences  appearing  in  the  food,  and  also 
shows  that  the  Flagellata  have  relations  in  different  directions: 
with  the  Rhizopoda,  the  Infusoria,  and  the  lower  plants. 


202  PROTOZOA. 

Eeproduction  is  nearly  always  by  fission.  In  many  species 
conjugation  is  known,  best  in  those  plant-like  forms,  the  Volvo- 
cina, where  two  individuals  fuse  completely  to  a  resting  spore.  In 
the  colonial  Volvocina  the  conjugating  individuals  are  unequal  in 
size,  some  animals  of  the  colony  growing  to  large  immobile 
oospheres,  while  others  by  continued  division  form  groups  of 
minute  active  zoospores  or  spermatozoids.  When  fertilized  by 
the  zoospore  the  oospheres  fall  to  the  ground,  become  encysted, 
become  brown  in  color,  and  enter  a  resting  stage  before  they  form 
a  new  colony  by  division. 

Sub  Order  I.  PHYTOFLAGELLATA.  Plant-like  chlorophyl-bearing 
flagellates,  mostly  with  eye-specks.  Volvocina :  Volvox  globator,*  green 
sphere  0.2-0.7  mm.  in  diameter,  consisting  of  thousands  of  individuals 
which  propel  the  colony  by  their  flagella.  Euglenidse  :  Euglena  viridis  * 
(fig.  134),  solitary,  coloring  small  pools  bright  green  (a  red  variety  colors 
them  purple)  by  their  immense  numbers.  Chrysomonadina,  plant-like  in 
nourishment  but  rarely  taking  solid  food  :  Dinobryon  *  (fig.  136). 

Snb  Order  II.  CHOANOFLAGELLATA.  With  collars ;  mostly  small 
colonial  forms.  Codosiga  *  (fig.  138,  B)  ;  Conocladium,  numerous  indi- 
viduals united  on  a  stalk  (fig.  136). 

Sub  Order  III.  EUFLAGELLATA.  Animal  flagellates,  taking  solid 
particles  of  food  either  by  pseudopodia  or  by  a  more  or  less  developed 
•cytostome.  MONADINA.  Here  belong,  besides  numerous  free  forms,  several 
parasites  of  man :  LamUia  (Cercomonas)  intestinalis,  fig.  139  (Mega- 


FIG.  139.  FIG.  HO. 

PIG.  IW.—Lamblia  intestinalis.    (After  Grassi.)    Front  and  side  views,  n,  nucleus. 
FIG.  140.— Trichomonas  vaginalis.    (After  Blochmann.)    n,  nucleus  (fourth  flagellum 
lacking  in  figure). 

stoma  entericum);  also  in  rats  and  mice  :  Trichomonas  hominis  (T.  intes- 
tinalis), both  in  small  intestine.     T.  vaginalis  (fig.  140). 


II.   FLAGELLATA:   DINOFLAOELLATA. 


203 


Order  II.     Dinoflagellata  (Cilioflagellata). 


These  forms,  occurring  in  both  fresh 
recently  placed  near  the  plants  because, 
with  their  brown  chromatophores,  their 
food  relations  are  like  those  of  plants, 
.although  the  taking  of  solid  food  by  a 
mouth  opening  has  been  observed.  The 
.armor  formed  of  cellulose  plates  is  also 
plant-like.  This  armor  is  divided  by  a 
transverse  groove  into  two  parts  which 
recall  somewhat  a  box  and  its  lid.  There 
is  also  a  longitudinal  furrow  which  crosses 
the  other.  At  the  point  of  crossing 
.are  two  flagella,  one  of  which  lies  in 
the  transverse  groove  and  was  for  a  long 
time  regarded  as  a  circle  of  cilia,  whence 
the  old  name  cilioflagellates  given  the 
order.  Peridinium  tcibulatum  and  Cera- 
tium  cornutum  (fig.  141);  Ceratium  tri- 
pos,* marine. 


water  and  the  sea,  are 


,r 


rsfi 


FIG.  141.— Ceratium  cornutum. 
(After  Stein.)  opo,  anterior 
horn  with  opening;  aaTi, 
rsh,  posterior  and  right 
horn ;  0,  flagellum ;  ##,  flagel- 
lar  groove;  If,  longitudinal 
groove;  r,  rhomboidal  plate; 
i\  vacuole. 


Order  III.    Cystoflagellata. 

The  cystoflagellates,  characterized  by  a  gelatinous  body  sur- 
rounded by  a  membrane,  include  two  very  interesting  species, 
both  marine,  which  differ  markedly  in  external  appearance. 

Noctiluca   miliaris*  (figs.   142,  138,  U),    among   all   marine 


FIG.  142.— Noctiluca  miliaris  (in  part  after  Cienkowski).  A,  entire  animal;  /,  flagel- 
lum; ?i,  nucleus;  o,  cytostome,  beside  it  the  k  tooth'  and  klip';  t,  tentacle;  J3,  C, 
upper  end  with  two  stages  in  the  formation  of  zoospores;  D,  zoospores. 

animals,  best  shows  the  phenomenon  of  phosphorescence.     These 
spherical  forms,  about  1  mm.    large,  sometimes   occur   in   such 


204 


PROTOZOA. 


numbers  at  night  as  to  make  the  whole  surface  light  at  the  slight- 
est agitation.  The  phosphorescence  is  apparently  caused  by  oxi- 
dation processes  in  the  protroplasm,  but  it  persists  for  some  time 
after  deprivation  of  oxygen.  The  membrane  covering  the  body  is 
interrupted  by  a  pit  at  one  point,  the  cytostome,  near  which  is  the 
nucleus  surrounded  by  an  aggregation  of  protoplasm  which  sends 
branching  threads  into  the  jelly  of  the  body.  At  the  entrance  to 
the  cytostome  is  a  flagellum,  of  no  use  in  locomotion,  and  a  band- 
like  tentacle  consisting  of  an  outgrowth  of  the  body  membrane 
with  a  transversely  banded  muscular  axis;  it  moves  slowly  with  a 
swinging  motion. 

Noctiluca  reproduces  by  simple  fission  and  by  formation  of 
swarm  spores  (fig.  142,  B,  (?,  D).  In  the  latter  two  individuals 
lose  tentacles,  flagella,  and  cytostomes,  and  conjugate  ;  after 

mutual  nuclear  fertilization  the 
animals  separate,  while  the  proto- 
plasm in  each  collects  in  a  disc 
which,  by  successive  divisions,  is 
converted  into  numerous  uni- 
nucleate  oval  germs.  These  at  first 
project  from  the  sphere,  but  later 
separate  and  form  small  flagellate 
spores  whose  later  history  is  not 
certainly  known. 

Leptodisciis  medusoidcs  of  Eu- 
rope (fig.  143)  has  the  appearance 
of  a  medusa  1  to  1.5  mm.  in 
diameter.  Its  gelatinous  disc  is 
covered  by  a  membrane,  and  at  the 
FIG.  uz.-Leptodiscus  medusoides,  sur-  highest  point  of  the  concave  surface 

face  and  optical  section.  /,  nagellum;  js  a  mass  Of  protoplasm  with  a  sin- 
m,  cytostome;  n,  nucleus;  o,  preoral 

tract;  p,  protoplasmic  band.  gie  nucleus.     On  one  side  of  this  a 

band  goes  to  the  mouth,  on  the  other  a  canal  bearing  a  fine 
nagellum  at  its  end.  The  animals  swim  well,  like  medusae,  by 
closing  the  umbrella,  the  motions  of  which  are  caused  by  delicate 
muscles  on  the  concave  side. 

Class  III.     Ciliata. 

The  Ciliata  rival  the  Ehizopoda  in  numbers  and  variety  of 
form.  They  are  so  complicated  in  structure  that  they  were  long 
held  as  multicellular,  a  view  which  was  entirely  abandoned  only 
in  the  last  quarter-century.  All  have  a  form  definite  for  the 


III.    CIL1ATA. 


205 


species;  this  in  the  ' ametabolous '  forms  is  unalterable,  in  the 
4  metabola '  it  can  be  pressed  out  of  shape  in  passing  through  a 
narrow  space.  This  constancy  of  form  is  due  to  the 'development 
of  more  or  less  cuticle  on  the  outside  of  the  body,  which  in  the 
'  ametabola '  acquires  an  armor-like  firmness;  in  the  others  is  more 
flexible.  The  cuticle  is  covered  with  cilia — small  vibrating  pro- 
cesses which  move  not  singly  but  together  in  large  numbers,  and 
serve  not  only  as  organs  of  locomotion,  but  by 
creating  vortices  in  the  water  bring  food  to  the 
organism.  They  furnish  the  most  important 
characteristic  of  the  class  (fig.  144). 

The  presence  of  a  cuticle  necessitates  a 
cytostome  (except  in  the  parasitic  species), 
since  food  particles  cannot  be  taken  in  at  any 
point.  At  the  cytostome  the  cuticle  with  its 
cilia  forms  a  funnel-like  extension  (cyto- 
pharynx)  into  the  protoplasm.  At  the  bottom 
the  cuticle  is  interrupted  so  that  water  and  pro- 
toplasm are  in  contact.  By  the  action  of  the 
cilia  food  particles  are  taken  into  the  cyto- 
pharynx  and  pressed  into  the  protoplasm, 
formkiff  a  small  enlargement  which  finally  sinks  FIG 

J  caudatum     (half    dia 

into  the  substance  as  a  lood  vacuole  (na), 
which  by  the  streaming  of  the  protoplasm  is 
carried  about  in  the  body.  The  digestible  por- 
tions are  absorbed,  and  those  not  capable  of 
digestion  are  cast  out  of  the  body  at  a  fixed 
point  (cytopyge)  usually  not  recognizable  at  other  times  (fig.  151). 
Contractile  vacuoles  (cv)  are  lacking  only  in  parasites  and 
marine  species.  They  are  constant  in  number  and  position,  and 
frequently  have  afferent  ducts  which  empty  into  the  vacuole,  the 
vacuole  in  turn  forcing  the  fluid  to  the  exterior.  Trichocysts,  nettle 
bodies,  and  muscular  fibrillse  occur  in  some  species.  Trichocysts 
are  minute  rods  placed  vertically  to  the  surface  in  the  cortical 
layer,  which  under  the  influence  of  reagents  (chromic  acid  is  best) 
elongate  into  threads  penetrating  the  cuticula.  These  have  been 
compared  by  some  to  the  nettle  cells  of  coelenterates,  and  have 
been  ascribed  defensive  functions;  others  regard  them  as  tactile 
structures.  They  have  no  connexion  with  the  cilia.  Kettle 
bodies  are  extremely  rare.  Muscle  fibres  are  more  common ;  they 
lie  between  ectosarc  and  cuticle,  and  cause  quick  convulsive  motions 
of  the  animal. 


grammatic).  cv,  con- 
tractile vacuole  in 
systole,  cu',  in  dias- 
tole; to,  nucleus;  na, 
food  vacuole,  ?ia',  iu 
formation;  nfc,  micro- 
nucleus;  f,  tricho- 
cysts,  at  t'  protruded. 


206 


PROTOZOA. 


The  nuclear  relations  are  extremely  interesting  in  that  there  are 
two  nuclei  physiologically  unlike.  The  larger  of  these  (nucleus 
of  older  writers,  macronucleus}  is  a  large  oval,  rod-like,  or  spiral 
body,  readily  and  deeply  staining  with  microscopic  stains,  and  sur- 
rounded with  a  membrane.  It  appears  to  control  all  the  common 

vital  functions  of  the  animal  (motion, 
feeding,  etc.).  Beside  it  or  in  a  depres- 
sion in  it  is  the  much  smaller  micro- 
nucleus  (nucleolus  or  paranucleus  of 
older  authors)  which  stains  less  deeply 
and  only  plays  a  part  in  reproduction. 
In  all  sexual  processes  it  comes  to  the 
front  and  can  be  called  the  sexual 
nucleus. 

Multiplication  of  Ciliata  occurs  by 
binary  fission  (fig.  145);  more  rarely,  and 
then  only  in  the  encysted  condition,  by 
division  into  numerous  (up  to  64)  parts. 
Budding  is  known  in  the  Peritricha  and 
Suctoria.  First  the  micronucleus  divides 

no.   M-ParamKcium    aurelia  mitotically>  and   the*   th«   macronucleus 

in  division,  ft,  macronucleus;  separates  bv  elongation  and  construction. 

n/r,  micronuclei;  o,  cytostome  J 

of  the  separating  individuals.  The  old  cytostome  persists  in  the  anterior 

At  2  an  early  stage  of  division 

of  cytostome.  offspring,  but  often  an  outgrowth  from 

it  (fig.  145,  2,  of)  passes  into  the  posterior  half  and  develops  into 
a  new  mouth. 

The  periods  of  fission  are  interrupted  from  time  to  time  by  the 
sexual  process  of  conjugation,  which  will  be  described  as  it  occurs 
in  Paramcecium  (fig.  146).  Two  individuals  touch  at  first  in  front, 
and  then  by  their  whole  ventral  surfaces,  so  that  their  cytostomes 
come  together.  In  the  neighborhood  of  the  latter  a  plasma  bridge 
connects  the  two.  Later  the  individuals  separate.  While  these 
easily  observable  external  processes  are  occurring  there  is  a  com- 
plete modification  of  the  nuclear  apparatus  in  the  interior.  The 
macronucleus  increases  in  size,  and  breaks  into  small  portions 
which  disappear  within  the  first  week  after  copulation  (probably 
absorption),  and  give  place  to  a  new  nucleus  derived  from  the 
micronucleus.  At  the  beginning  of  copulation  the  micronucleus 
becomes  spindle-shaped,  divides  and  repeats  the  process,  the  result 
being  the  formation  of  four  spindles,  three  of  which  break  down, 
thus  recalling  the  polar  globules  in  the  maturation  of  the  egg 
(p.  146).  The  fourth  or  primary  spindle  places  itself  in  the  neigh- 


III.    CILIATA. 


207 


borhood  of  the  cytostome  at  right  angles  to  the  surface  and  divides 
into  two   nuclei,  the  superficial   being  called  the  wandering  or 


FIG.  146.  —  Conjugation  in  Paramcecium.    fc,  macronucleus;  ?i 

stomes. 


micronucleus;  o,  cyto- 


I.  Changes  of  micronucleus;  left  sickle  stage,  right  spindle  stage. 

II.  Second  division  of  micronucleus  into  primary  spindles  (1,  5)  and  secondary 
spindles  (2,  3,  4;  6,  7,  8). 

III.  Degeneration  of  secondary  spindles  (2,  3,  4;  6,  7,  8);  division  of  primary  spindle 
into  male  (1m,  5m)  and  female  spindles  (I?/;,  5w). 

IV.  Exchange  of  male  spindles  nearly  complete  (fertilization),  one  end  still  in  the 
parent  animal,  the  other  united  with  the  female  spindle,  1m  with  5w  and  5m  with  Ito; 
macronucleus  broken  up. 

V.  The  cleavage  spindle  t  formed  by  male  and  female  spindles  dividing  into  the 
secondary  cleavage  spindles  t',  t". 

VI.  VII.  End  of  conjugation.    The  secondary  cleavage  spindle  dividing  into  the 
anlage  of  the  new  micronucleus   (nfc')»  and   that   of   the   new   macronucleus,  pt 
(placenta).    The  fragments  of  the  old  macronucleus  begin  to  degenerate. 

Since  P.  caudatum  shows  the  earlier  and  P.  aurelia  the  later  stages  better,  these 
forms  have  been  used,  P.  caudatum  for  I-III,  P.  aurelia  for  the  rest.  The  differences 
consist  in  the  existence  of  one  micronucleus  in  P.  caudatum,  two  in  P.  aurelia^  and 
that  in  the  latter  the  nuclear  degeneration  begins  in  I. 

male  nucleus,  the  deeper,  the  stationary  or  female  nucleus.     The 
male  nuclei  of  the  two  copulating  animals  are  exchanged,  travers- 


208  PROTOZOA. 

ing  the  protoplasmic  bridge  in  their  course.  Both  male  and 
female  nuclei  become  spindle-shaped,  and  the  immigrant  male 
.spindle  fuses  with  the  female  spindle,  forming  a  single  spindle  of 
division.  At  last  the  division  spindle  produces  (usually  by  indi- 
rect means)  two  nuclei,  one  of  which  becomes  the  new  macronu- 
cleus,  the  other  the  new  micronucleus. 

In  a  comparison  of  the  fertilization  of  the  Metazoa,  the  female 
nucleus  corresponds  to  the  egg  nucleus,  the  male  nucleus  to  that 
of  the  spermatozoa.  As  the  fusion  of  egg  and  sperm  nuclei  forms 
a  segmentation  nucleus,  so  here  the  division  nucleus  is  formed  in 
a  similar  manner.  As  the  egg  cell  through  fertilization  acquires 
the  capacity  not  only  to  produce  sex  cells  but  somatic  cells — cells 
which  carry  on  the  common  functions  of  the  body — the  fertilized 
micronucleus  forms  not  only  the  new  micronucleus,  but  also  the 
macronucleus  which  controls  the  body  processes,  and  hence  is  the 
somatic  nucleus.  In  other  words,  fertilization  in  the  Ciliates 
leads  to  a  complete  new  formation  of  the  nucleus  and  thus  to  a 
new  organization  of  the  organism. 

In  most  Ciliata  the  conjugating  individuals  are  equivalent, 
the  fertilization  is  mutual,  and  the  individuals  separate  later.  In 
the  Peritricha  (mostly  sessile  forms,  fig.  147),  on  the  contrary,  the 


Fio.  \it.-Epistylis  umbellnria.  (After  Greeff.)  Part  of  a  colony  in  'bud-like '  conju- 
gation r,  microspores  arising  by  division;  fr,  microspore  conjugating  with  a 
macrospore. 

resemblance  to  fertilization  in  the  Metazoa  is  strengthened  in  that 
there  is  a  sexual  differentiation  and  a  permanent  fusion  of  the 
conjugating  individuals.  Some  animals — the  macrospores — retain 
their  size  and  sessile  habits;  others  by  rapid  division  produce 


III.    CILIATA:  HOLOTRICHA,  HETEROTRICHA. 


209 


groups  of  markedly  smaller  microspores.  The  latter  separate  and 
fuse  completely  with  the  macrospores,  only  a  small  cuticular  sac 
persisting  to  indicate  the  fusion.  The  nuclear  phenomena  are 
much  the  same  as  with  Paramcecium,  allowance  being  made  for 
the  permanance  of  the  fusion. 

Order  I.  Holotricha. 

The  Holotricha  are  doubtless  the  most  primitive  Ciliates,  since 
the  cilia  on  all  parts  of  the  body  are  similar;  being  at  most  slightly 
stronger  at  one  end  of  the  body  or  on  the  inside  of  the  cytostome. 
Best  known  are  the  species  of  Paramcecium*  (fig.  144)  occurring 
in  stagnant  water.  Opalina  ranarum  *  lives  in  the  intestine  of  the 
frog.  It  lacks  mouth,  has  numerous  similar  nuclei,  no  micronu- 
oleus  and  no  conjugation.  The  small  encysted  Opalines  pass  out 
with  the  faeces,  and  are  eaten  by  the  tadpoles,  which  thus  become 
infected. 

Order  II.  Heterotricha. 

Like  the  Holotricha  the  Heterotricha 
are  everywhere  ciliated,  but  they  have  a 
tract  of  stronger  cilia,  the  adoral  ciliated 
spiral.  This  is  a  band  of  cilia  beginning 
at  some  distance  from  the  cytostome  and 
leading  in  a  spiral  course  into  the  mouth. 
It  consists  of  rows  of  cilia  united  into 
'  membranellse '  placed  at  right  angles  to 
the  course  of  the  spiral.  In  the  best- 
known  heterotrichans,  the  Stentors  *  (fig. 
148),  the  peristomial  area,  surrounded  by 


FIG.  148.  FIG.  149. 

FIG.  148. — Stentor  polymorphus.    (After  Stein.)    a,  peristomial  area;  b,  roof  of  hypo- 

storae;  0,  contractile  yacuole;  n,  nucleus;  o,  cytostome;  r,  adoral  ciliated  spiral; 

t,  hypostome  (excavation  for  mouth). 
FIG.  UU.—Balantidiuni  coli.    (After  Leuckart.) 


210 


PROTOZOA. 


the  spiral,  forms  the  broader  end  of  the  body,  which  gradually 
tapers  toward  the  other  end,  by  which  the  animal  may  attach 
itself  by  small  plasma  threads.  Muscle  fibres  which  run  length- 
wise immediately  under  the  cuticle  produce  energetic  move- 
ments. Stentor  polymorphus  *  when  attached  builds  a  gelatinous 
case.  S.  cceruleus.*  Balantidium  coli  (fig.  149)  appears  in  the 
large  intestine  of  men  ill  with  diarrhrea;  it  also  occurs  in  swine 
without  causing  sickness.  Other  parasites  of  man  are  B.  minu- 
tum  and  Nyctotherus  faba. 

Order  III.  Peritricha. 

In  the  Peritricha  there  is  always  a  broad  peristome  area  with 
the  cytostome;  the  opposite  end  has  a  corresponding  pedal  disc 
or  is  narrowed  like  a  goblet  and  ends  in  a  stalk  (fig.  150).  Only 


FIG.  150.— Carchesium  polypinum.  (After  Biitschli.)  Left,  a  single  animal;  right,  three 
stages  of  division,  cv,  contractile  vacuole;  n,  macronucleus:  n',  micronucleus; 
JVv,  food  vacuoles:  os,  cytopharynx;  per,  peristome;  vs,  reservoir  of  contractile 
vacuole;  «»*,  undulating  membrane  ;  vst,  vestibule;  wk,  ring  on  which  a  posterior 
circle  of  cilia  may  develop. 

the  adoral  ciliated  spiral  is  constant.  It  arises  from  the  swollen 
margin  of  the  peristomial  area,  and  continues  on  the  '  operculum/ 
a  ciliated  disc  which  projects  free  from  the  peristomial  area,  but 
in  contraction  is  drawn  close  against  it,  tliQ  peristome  lips  folding 
over  all.  Besides,  there  may  be  a  temporary  or  permanent  circle 
of  cilia  near  the  hinder  end.  The  nucleus  is  usually  sausage- 
shaped,  much  bent,  and  with  the  small  micronucleus  in  its  hinder 
angle  (fig.  150,  nf). 

The  best  known  representatives  are  the  VORTICELLID^E  (figs.  147,  150), 
attached  by  a  long  stalk  which  is  usually  hollow  and  contains  a  slightly 


///.    CILIATA:  HJPOTRICHA. 


211 


spiral  muscle.  This  extends  into  the  body  and  divides  up  into  fine  fibrilla3 
which  extend  under  the  cuticle  to  the  peristome.  When  the  muscle  in  the 
stalk  contracts  it  becomes  coiled  into  a  corkscrew  spiral,  drawing  back  the 
animal,  and  folding  in  the  anterior  end.  Vorticella*  is  solitary;  Carche- 
sium*  forms  colonies  with  dichotomously  branched  stalks;  Zoothamnion* 
colonies  imbedded  in  a  common  jelly  ;  Epistylis*  (fig.  147),  branched  col- 
onies with  rigid  stalks,  the  muscle  being  confined  to  the  base  of  the  body. 

Order  IV.  Hypotricha. 

In  this  order  the  body  is  more  or  less  flattened  and  a  ventral 
and  a  weakly  arched  dorsal  surface  are  differentiated.  The  back 
lacks  cilia,  but  often  bears  spines  and  tactile  bristles.  On  the  ven- 
tral side  are  several  longitudinal  rows  of  cilia,  and  besides  straight 


ID1.. 


FIG.  151.  FIG  152. 

FIG.  151.— Stylonychfa  mytilns.  (After  Stein.)  «,  anal  hooks;  6,  ventral  hooks;  c,  con- 
tractile vacuole:  rf,  frontal  ridge;  g,  canal  leading  to  contractile  vacuole;  I,  upper 
lip;  71,  nucleus  with  micronucleus:  p,  adoral  ciliated  spiral;  r,  marginal  cilia;  s, 
caudal  cilia;  .sf,  frontal  spines;  z,  anus  (cytopyge). 

FIG.  152.— Division  of  Sti/ionychia  mytilns.  (After  Stein )  c,  c',  contractile  vacuoles  of 
the  two  individuals;  n,  ?V,  nucleus  and  micronucleus;  p,  p',  adoral  ciliated  spiral; 
r,  »•',  marginal  cilia;  w;,  «;',  ciliated  ridges. 

spines  and  hooked  cilia  composed  of  united  cilia.  These  latter  are 
of  use  in  creeping.  The  strongly  developed  adoral  cilia  are  of  use 
in  locomotion  and  in  producing  vortices  which  bring  food.  The 
macronucleus  is  often  divided  into  two  oval  bodies  connected  by  a 
thread;  the  micronuclei  vary  in  number  from  2  to  4  in  the  same 


212 


PROTOZOA. 


species.     These  are  the  best  forms  for  studying  the  micro  nuclei. 
The  species  of  Stylonychia*  (figs.  151,  152)  are  best  known. 

Order  V.  Suctoria  (Acinetaria). 

The  Suctoria  differ  from  other  Infusoria  in  the  absence  of  cilia 
from  the  adult  and  consequently  have  no  means  of  locomotion. 
They  are  fixed  to  some  support  either  by  the  base  or  by  a  slender 
stalk.  The  body  is  usually  spherical  and  is  covered  with  a  cuticle, 
which  in  the  genus  Acineta  is  produced  into  a  cup-like  lorica. 
There  is  no  mouth,  but  in  its  place  tentacles  or  sucking  feet,  very 
fine  tubes  with  contractile  walls  which  begin  in  the  protoplasm  and 
protrude  through  the  cuticle  (fig.  153,  F).  The  Acinetaria  kill 
other  animals,  especially  infusoria,  with  their  tentacles,  and  then 


FIG.  153.— Forms  of  Suctoria.  (After  various  writers.)  A,  Dendrosoma;  B,  Rhyncheta; 
C,  Opliryodendron;  D,  Tokophrya;  £",  ciliated  young  of  Sphceroplirya;  F,  diagram  of 
structure  showing  capitate  and  styliform  tentacles  arising  from  the  ectosarc 
and  corresponding  canals  in  the  entosarc. 

suck  the  substance  through  these  tubes.  The  contractile  vacuole, 
rarely  lacking,  lies  near  the  compact  macronucleus;  micronuclei 
are  generally  present. 

In  contrast  to  the  immobile  adults  the  young  which  are  ciliated 
(fig.  153,  E)  after  the  pattern  of  ciliates,  are  good  swimmers. 
They  arise  either  as  buds  from 'the  surface  of  the  mother  or  as 
(  embryos 9  in  her  interior.  This  latter  condition  is  only  a  modifi- 
cation of  the  other,  for  parts  of  the  outer  surface  become  pushed 
into  the  interior,  and  there  form  a  brood  cavity  in  which  the 
•embryos  arise.  After  swimming  for  a  while  the  young  come  to 
rest,  lose  the  cilia,  and  develop  the  tentacles. 

Some  species  of  Podoplirya  are  widely  distributed  in  fresh  water,  also 
Sphcerophrya,  parasitic  in  Infusoria.  The  species  of  Acineta  as  well  as 
Podophrya  gemmipara  (fig.  20)  are  marine,  living  on  hydroids  and  Poly- 
zoa. 


IV.   SPOROZOA:    OREGARINA. 


213 


Class  IV.     Sporozoa. 

Under  the  name  Sporozoa  are  united  several  groups  of  Protozoa 
which,  while  they  differ  much  in  structure,  have  much  in  common 
in  life  and  development.  They  are  parasites  in  Metazoa,  many  of 
them  in  the  cells  themselves,  causing  their  degeneration  (Cytospo- 


FIG.  154. — Sporozoa.  -4,  cyst  of  Clepsidrina  with  sporoducts;  B,  Clepsidrina,  two  indi- 
viduals (after  Schneider);  C,  Eimeriafalcifonnis,  from  mouse;  £>,  same,  falciform 
embryos:  JK,  Hoplorhynchus  dujnrdinii^  from  Litkobius;  F,  Gregarina  atgantea.  from 
lobster;  Cr,  Sarcocystis  miescheri,  from  pig;  H,  Myxidium  (after  Th6olan);  I,Rhopa- 
locephalus,  alleged  cause  of  cancer  (after  Korotneff). 

ridae).  They  take  no  solid  food,  but  are  nourished  by  fluid  mate- 
rial absorbed  through  the  whole  surface.  In  reproduction  they 
form  a  large  number  of  l  sporoblasts/  which  when  enveloped  with 
a  membrane  are  called  '  spores/  the  contents  of  which  usually 
break  up  into  several  small  bodies  or  '  sporozoites/  The  sporozo- 
ites  for  their  development  must  leave  the  host.  The  resemblances 
to  the  Rhizopods  (Mycetozoa)  are  unmistakable,  especially  those 
Sporozoa  which  have  pseudopodia  for  much  of  their  life. 

Order  I.  Gregarina. 

The  typical  and  longest  known  sporozoa  are  the  Gregarines, 
parasites  of  oval  or  thread-like  form  (recalling  round  worms), 
usually  somewhat  flattened,  which  so  far  have  only  been  found  in 
invertebrates,  where  they  live  in  the  intestine  or  gonads,  more 
rarely  in  the  body  cavity.  The  protoplasm  (fig.  155,  /)  is  sepa- 
rated more  sharply  than  in  other  Protozoa  into  a  clear  ectosarc 
(elc)  and  a  granular  entosarc  (en).  The  ectosarc  is  covered  by 
a  cuticle  (not  always  easily  seen,  but  frequently  with  a  double  con- 
tour) (cu),  which  must  be  permeable  by  fluid  food,  for  no  cyto- 
stome  exists.  In  many  (perhaps  all)  there  is  a  double  striping 
of  the  body,  a  longitudinal  recognizable  by  furrows  on  the  outer 
surface  and  hence  cuticular,  and  a  transverse  marking  in  the 


PROTOZOA. 


ectosarc,  produced  by  circular  or  spiral  muscle  fibrillae.  These 
muscles  explain  the  peristaltic  motion  and  the  occasional  sharp 
bending  of  the  body,  but  not  the  peculiar  gliding  motion  like  that 
of  diatoms  by  which  locomotion  is  usually  effected.  This  is 


pm.— 


I. 


en. 


FIG.  155.— Development  of  Gregarina  blattarum.  I,  conjugation ;  II,  A-C,  a  cyst  in 
transformation  into  pseudonavicellae;  III,  A,  a  pseudonavicella  greatly  enlarged; 
B,  same  with  sickle-formed  sporozoites;  CM,  cuticle;  dm,  deutomerite;  ek,  ecto- 
sarc; en,  entosarc;  ?i,  nucleus;  pm,  protomerite;  pn,  pseudonavicellae;  rJc,  re- 
sidual body;  sfc,  sickle-form  sporozoites. 

explained  by  the  view  that  the  gregarines  secrete  stiff  gelatinous 
threads  from  the  posterior  end,  and  the  elongation  of  these  forces 
the  body  forward. 

In  many  gregarines  (Poly cyst idae)  the  body  is  divided  by  a  cir- 
cular incision  into  a  smaller  anterior  part,  the  protomerite,  and  a 
larger  deutomerite.  Internally  this  division  is  marked  by  a  bridge 
of  ectosarc  across  the  entosarc.  The  vesicular  nucleus  (there  is 
but  one  in  any  gregarine)  lies  in  the  deutomerite.  An  epimerite — 
a  structure  connected  with  the  peculiar  type  of  parasitism — occurs 
in  many  species.  All  gregarines  are  parasitic  in  youth  inside  of 
cells.  They  later  leave  these,  but  many  remain  for  a  long  time 
with  a  process  of  the  protomerite  imbedded  in  the  cells.  This 
process — the  epimerite — is  provided  with  threads  or  hooks  for 


IV.   SPOROZOA:    COCCIDIAE.  215 

anchorage,  and  is  lost  when  the  animal  gives  up  its  connexion 
with  the  host  cell.  Among  the  intestinal  gregarines  frequently 
occur  l  associations '  where  two  or  more  animals  are  fastened  to- 
gether head  to  tail  in  a  row.  Perhaps  these  associations  are  prep- 
arations for  conjugation  which  occurs  in  development. 

Eeproduction  occurs  exclusively  in  an  encysted  condition  (fig. 
155,  II,  ^4).  Usually  two  animals  (sometimes  one,  rarely  more  than 
two)  occur  in  a  cyst.  A  fusion  of  the  two  encysted  animals  does 
not  take  place,  but  it  is  probable  that  a  nuclear  exchange  (recalling 
that  of  ciliates)  takes  place.  After  each  individual  has  become 
polynucleate  by  division  of  its  nucleus,  it  divides  at  first  super- 
ficially, later  internally  into  small  particles,  the  sporoblasts  (II,  B), 
which  change  into  spores,  here  called  pseudonavicellae.  The 
pseudonavicellae  are  inononucleate  bodies  with  firm  membrane  and 
usually  spindle  form  in  shape  (III,  AA).  In  these  processes  a 
part  of  the  gregarine  takes  no  part.  This  residual  body  appears 
under  proper  conditions  to  swell  up  and  rupture  the  cyst,  thus 
freeing  the  pseudonavicellae.  In  many  gregarines  there  are  sporo- 
ducts  for  the  escape  of  the  pseudonavicellae  (fig.  154,  A).  The 
contents  of  the  pseudonavicellae  divides  into  (usually  eight)  sporo- 
zoites  or  falciform  spores  which  must  pass  out  from  the  spores  and 
into  the  cells  of  the  host  in  order  to  form  gregarines.  This  escape 
of  the  sporozoites  depends  upon  entrance  into  the  proper  host. 
Often  the  transformation  of  the  contents  of  the  cysts  into  pseudo- 
navicellae takes  place  when  the  cysts  have  left  the  original  host. 

Best  known  are  the  Monocystis  tenax  of  thespermatheca  of  earthworms, 
and  Gregarina  (Clepsidrina)  Uattarum  of  the  cockroach.  The  American 
species  have  scarcely  been  touched.  One  species  is  abundant  in  the  intes- 
tine of  Geophilus. 

Order  II.  Coccidiae. 

The  gregarines  of  all  Sporozoa  are  nearest  the  Coccidiae,  which 
are  also  cell  parasites  with  a  single  nucleus,  but  without  either  cell 
membrane  or  division  into  protomerite  and  deutomerite.  In  most 
species,  as  in  Coccidium  cuniculi,  there  are  two  types  of  reproduc- 
tion, an  endogenous,  leading  to  '  autoinfection/  and  an  exogenous, 
concerned  in  the  transfer  of  the  germs  to  other  hosts.  In  the 
first  (lacking  in  many  species)  the  Coccidium  divides  into  many 
falciform  germs  which  separate  from  each  other  and,  without 
alternation  of  hosts,  enter  other  cells.  The  second  type  is  begun 
by  fertilization.  Certain  individuals,  by  rapid  division  form 
microgametes,  small  bodies  swimming  with  serpentine  motions  or 
by  one  or  two  flagella.  Other  individuals  do  not  divide,  but  form 


216 


PROTOZOA. 


macrogametes  which  are  fertilized  by  the  microgametes,  and  then 
encyst,  pass  to  the  outside,  and  serve  for  the  infection  of  other 
animals.  The  contents  of  the  cyst  begin  to  divide,  sooner  or 
later,  into  sporoblasts  (in  Coccidium,  four)  containing  spores,  the 


FIG.  156.— Coccidium  cuniculi,  from  the  liver  of  the  rabbit  (from  Wasielewski.  a,  &r 
young  Coccidia  in  the  epithelial  cells  of  bile  duct,  the  nucleus  of  the  cell  in  the 
upper  process;  c,  encysted;  d,  e,  contraction  of  protoplasm;  t,  h,  g,  spore  forma- 
tion; /,  ripe  spore  with  two  germs  and  a  residual  body. 

process  being  completed  only  after  entrance  into  a  new  host. 
Each  spore  forms  one  or  more  sporozoites,  a  portion  of  the  sub- 
stance being  left  behind.  Coccidium  cuniculi  (oviforme)  in  the 
liver  of  mammals,  especially  rabbits  (rare  in  man),  producing 
cheesy  granules.  C.  perforans  in  the  intestine  of  rabbits,  rare  in 
man. 

Order  III.  Haemosporida. 

In  structure  and  development  these  are  much  like  the  Coccidiae ; 
they  live  in  blood  corpuscles.  The  forms  occurring  in  man  pro- 
duce malaria.  Here,  also,  there  are  endogenous  (autoinfecting) 
and  exogenous  generations  transferring  the  parasites  to  other 
hosts.  The  parasites  in  the  corpuscles  (fig.  157,  a  to  d)  grow  and 

abed  e  f  g 


FIG.  157.— Plasmodium  laverni,  var.  quartana  (from  Wasielewski,  after  Labb6),  from 
the  blood  of  a  malarial  man.  a,  newly  infected  blood  corpuscle;  b,  somewhat 
larger  germs;  c,  full-grown  parasite  with  strong  pigmentation;  d,  rounded  form 
with  large  nucleus;  e,  beginning  of  germ  formation;  /,  rosette  of  germs  around  a 
residual  body;  g,  germs  set  free  by  degeneration  of  corpuscle. 

divide,  producing  ( daisy-like  forms'  characterized  by  little  accu- 
.mulations  of  pigment  derived  from  the  haemoglobin  of  the  blood. 


IV.   SPOROZOA:  MYXOSPORIDA. 


217 


These  division  products  are  set  free  by  a  breaking  down  of  the 
corpuscle  (period  of  chill)  and  infect  other  corpuscles.  Thus 
autoinfection  can  continue  until  at  length  sexual  forms  appear — 
'  spheres '  or  macrogametes,  flagellate  microgametes — incapable  of 
infecting  the  corpuscles.  The  conjugation  of  these  seems  to  take 
place  when  they  are  taken  into  the  stomach  of  a  blood-sucking 
mosquito.  After  fertilization,  the  oosphere  wanders  into  the 
intestinal  wall  of  the  mosquito,  grows  larger,  encysts,  and  produces 
many  sporoblasts,  which  in  time  form  many  sporozoites.  These 
migrate  into  the  salivary  glands  of  the  mosquito  and  thence  are 
transferred  to  man  with  the  sting  of  these  insects.  Since  a  tem- 
perature above  20°  C.  (68°  F.)  is  best  for  the  development  of  the 
stages  in  the  mosquito,  and  since  mosquitos  need  water  for  their 
development,  the  prevalence  of  the  disease  in  moist,  warm  regions 
is  easily  understood.  For  the  transfer  of  human  malaria 'not  all 
mosquitos  will  serve,  but  apparently  only  those  of  the  genus 
Anopheles.  The  species  of  Culex  convey  bird  malaria.  The  differ- 
ent kinds  of  malaria  seem  to  be  produced  by  different  parasites. 

Order  IV.  Myxosporida. 

The  Myxosporida  (fig.  158)  are  mostly  large  (sometimes  visible 
to  the  naked  eye)  and  occur  especially 
in  fish  and  arthropods.  When  they  occur 
in  hollow  organs  they  are  naked  and  have 
pseudopodia,  but  in  parenchymatous  or- 
gans like  the  heart,  liver,  brain,  kidney, 
etc. ,  they  are  usually  enclosed  in  a  mem- 
brane, and  here  they  produce  the  great- 
est injury.  At  first  binucleate,  they  soon 
become  polynucleate,  and  it  would  appear 
that  they  can  reproduce  by  fission.  Even 
before  the  growth  is  ended  they  begin  Flo  158_M 
the  process  of  sporulation.  In  the  in- 
terior single  spherical  protoplasmic  bodies 
separate,  these  having  at  first  a  single 
nucleus,  later  more  (as  many  as  ten). 
From  each  of  these  bodies  arise  from  two 
to  many  spores,  the  so-called  psorosperms.  These  (fig.  158,  3)  are 
enclosed  in  a  bivalve  shell  which  includes,  besides  a  binucleate 
germ,  one,  two,  or  four  polar  capsules,  these  resembling  somewhat 
the  nettle  organs  of  the  coelenterates.  They  are  oval  and  contain 
threads  which,  under  certain  conditions,  are  protruded  (fig.  158,  2) 


MJIX- 


«««"*  itAerkuhni  ;  fc,  degen- 

eratmg  nucleus;   n,  vacuole 
formerly  regarded  as 


cleus;    p, 

body,   in 

threads. 


nu- 
cnidocil-like    pole 

s   with    exserted 


218 


PROTOZOA. 


and  serve  to  fix  the  capsule,  while  the  amoeboid  germ  creeps  out 
and  penetrates  the  tissues  of  the  host.  Experiment  shows  that 
fishes  are  infected  through  the  alimentary  canal. 

The  Myxosporida  frequently  cause  serious  epidemics  in  fish.  This  was 
noticeably  the  case  with  the  fish  in  the  aquaria  at  the  Chicago  exposition. 
Myxobolus,  Myxidium.  Invertebrates  may  also  be  infected,  the  celebrated 
pe*brine  of  the  silkworm  being  caused  by  Nosema  (Glugea)  bombycis. 

Order  V.  Sarcosporida. 

The  Sarcosporida  (fig.  159) — also  called 
Rainey's  or  Miescher's  corpuscles — occur  in 
the  voluntary  muscles  of  vertebrates, especially 
mammals.  They  are  oval  cysts  lying  in  sar- 
colemma  sacs  between  the  fibrillse.  They  have 
a  cyst,  the  wall  of  which  is  radially  striped, 
and  inside  this,  in  the  ripe  condition,  are 
spores,  imbedded  in  a  stroma,  each  spore  con- 
taining numerous  renif orm  or  falciform  sporo- 
zoites.  Sarcocystis  miescheriana  in  muscles  of 
pig;  S.  muris  in  the  mouse;  8.  lindemanni 
rare  in  human  muscle. 

Summary  of  Important  Facts. 

1.  The  Protozoa  are  unicellular  organisms 
without  true  organs  or  true  tissues. 

2.  All  vital  processes  are  accomplished  by 
the  protoplasm  (sarcode),  digestion   directly 
by  its  substance,  locomotion  and  the  taking 

of  food  by  means  of  protoplasmic  processes  (pseudopodia)  or  by 
;appendages  (cilia  and  flagella). 

3.  Excretion  takes  place  by  special  accumulations  of  fluid,  the 
contractile  vacuoles. 

4.  Reproduction  is  by  budding  or  by  fission.     Conjugation  has 
been  witnessed  in  many,  and  possibly  occurs  in  all.     True  con- 
jugation is  a  process  of  fertilization  (caryogamy),  in  contrast  to 
fusion  of  plasma  (plasmogamy). 

5.  Protozoa  are  aquatic,  a  few  living  in  moist  earth;  they  can 
only  exist  in  dry  air  in  the  encysted  condition,  surrounded  by  a 
capsule  which  prevents  desiccation. 

6.  Since  encysted  Protozoa  are  easily  carried  by  the  wind,  the 
occurrence  of  these  animals  in  water  which  originally  contained 
none  is  easily  explained. 

7.  The  mode  of  locomotion  serves  as  the  basis  for  division  of 


FlO.  159.  —  Sarcocystis 
miescheriana^  from 
diaphragm  of  pig. 
(After  Biitschli.)  6s, 
cyst;  sp,  spheres  of 
spores. 


IV.   SPOROZOA:   SUMMARY  OF  IMPORTANT  FACTS.     219 

the  Protozoa  into  the  classes  Rhizopoda,  Flagellata,  Ciliata,  and 
Sporozoa. 

8.  The  RHIZOPODA   have  changeable  protoplasmic  processes, 
the  pseudopodia. 

9.  The  Rhizopoda  are  subdivided  into  Monera,  Lobosa,  Helio- 
zoa,  Radiolaria,  Foraminifera,  and  Mycetozoa. 

10.  The   Lobosa   and   Monera  have  no   definite  shape.     The 
Lobosa  have  a  nucleus,  the  Monera  are  anucleate. 

11.  Heliozoa  and  Radiolaria  are  spherical  and  have  fine  radiat- 
ing pseudopodia  and  frequently  silicious  skeletons.     They  are  dis- 
tinguished by  the  occurrence  of  a  central  capsule  in  the  Radiolaria 
which  is  lacking  in  the  Heliozoa. 

12.  The  Thalamophora  (Foraminifera)  have  a  shell,  closed  at 
one  end,  at  the  other  with  opening  for  the  extension  of  pseudopodia. 
The  shell  is  chitinous  or  calcareous,  one  or  several  chambered, 
straight  or  spiral,  sometimes  with  close  walls,  sometimes  perforated 
with  pores;  the  pseudopodia  are  occasionally  lobular,  but  usually 
filiform,  branching  and  anastomosing. 

13.  The  Foraminifera  are  of  great  geological  importance  on 
account  of  their  numbers  and  their  shells,  which  have  built  and 
are  still  building  extensive  beds  of  rock  (chalk,  nummulitic  lime- 
stone).   The  silicious  skeletons  of  the  Radiolaria  are  less  important. 

14.  Mycetozoa  (Myxomycetes  of  botanists)  are  mostly  enormous 
Amoebae  with  branched  reticulate  protoplasm  (plasmodium).    They 
form  complex  reproductive  structures  (sporangia,  etc.),  recalling 
those  of  the  fungi. 

15.  FLAGELLATA  have  one  or  a  few  long  vibratile  processes — 
flagella — which  serve  for  locomotion  and  for  the  taking  of  food. 

16.  The  Autoflagellata  have  only  flagella;  they  feed  like  plants 
(Volvocina)  by  means  of  chlorophyl,  or  have  a  mouth  for  the  tak- 
ing of  food,  or  a  collar  (Choanoflagellata). 

17.  The  Dinoflagellata  have  two  kinds  of  flagella  and  usually 
an  armor  of  cellulose. 

18.  The  Cystoflagellata  have  a  gelatinous  body  enclosed  in  a 
firm  membrane  (Noctiluca). 

19.  The   CILIATA    (INFUSORIA    in  the  narrower  sense)   have 
numerous  slender  vibrating  processes,  the  cilia,  a  cuticle,  and  hence 
fixed  openings  for  the  ingestion  of  food  (cytostome)  and  for  extru- 
sion of  indigestible  matter  (cytopyge). 

21.  Of  great  interest  is  the  occurrence  of  two  kinds  of  nuclei, 
a  functional  nucleus  (macronucleus)  and  a  sexual  nucleus  (micro- 
nucleus,  paranucleus). 


PROTOZOA. 


22.  In  conjugation  portions  of  the  micronucleus  are  exchanged 
and  accomplish  impregnation.  The  macronucleus  degenerates  and 
is  replaced  by  part  of  the  fecundated  micronucleus. 

22.  The  classification  of  the  Ciliata  is  based  on  the  structure 
and  arrangement  of  the  cilia. 

23.  The  Holotriclia  have   similar  cilia  over  the  whole  bod}^ 
The  Heterotricha  have  besides  the  total  ciliation  stronger  cilia  in 
the  neighborhood  of  the  mouth  (adoral  ciliary  spiral).     The  Peri- 
triclia  have  only  adoral  ciliation.     The  Hypotriclia  have,  on  the 
ventral  surface,  the  ciliary  spiral  and  rows  of  cilia  and  coalesced 
cilia.     The   Suctoria  have    cilia  only  in  the    young,   later  they 
become  attached  and  feed  through  suctorial  tentacles. 

24.  SPOKOZOA  are  parasitic  Protozoa,  usually  without  organs 
of  locomotion  or  mouth.     They  take  no  solid  food,  but  live  by 
osmosis  on  tissue  fluids.     In  reproduction  the  encysted  animals 
produce  spores  (apparently  always  beginning  with  fecundation  and 
accompanied  by  a  change  of  host).     The  spores  divide  again  into 
sporozoites.     Besides,  multiplication  without  change  of  host  (auto- 
infection)  can  occur. 

25.  The  Gregarinida  are  temporary  or  permanent  parasites  in 
cells.    (Spores  =  pseudonavicellae,  sporozoite  =  falciform  embryo). 
Coccidite,    Hcemosporida     (cause   of   malaria,    parasitic   in   blood 
corpuscles). 

26.  The  Sarcosporida    (Rainey's  -or  Miescher's  corpuscles  of 
mammalian  muscles)  and   Myxosporida   (psorosperm  capsules  of 
fishes,  psorosperm  —  spore)  live  in  tissues  or  hollow  organs. 

APPENDIX. 

According  to  the  evolution  theory  one  should  expect  forms  between  the 
Protozoa  and  Metazoa.  The  CATALLACTA — spheres  of  ciliated  cells  which  in 
reproduction  break  up  into  single  cells — have  been  described  as  such. 


FlQ.  160.— Section  of  half  of  Trichoplax  adhcerens.    (After  Schulze.) 

Peculiar  many-celled  animals  whose  position  in  the  system  is  difficult  to 
decide  are,  further,  Trichoplax  adhcerens,  Salinella  salve,  the  ORTHO- 
NECTIDA  and  the  DICYEMIDA.  Trichoplax  (fig.  160)  is  discoid  and  consists 
of  twoepithelial-like  cell  layers  separated  by  gelatinous  tissue.  The  Ortho- 


PORIFERA.  221 

nectida  and  Dicyemida  have  a  many-celled  ectoderm,  enclosing  a  solid  mass 
of  cells  in  the  Orthonectida,  a  single  giant  cell  in  the  Dicyemida.  Sali- 
wella  consists  of  a  single  layer  of  cells  enclosing  a  central  digestive  space. 
Since  the  Dicyemida  live  as  parasites  in  the  nephridia  of  cephalopoda,  the 
Orthonectida  in  worms  and  echinoderrns,  it  is  possible  that  their  low  or- 
ganization is  the  result  of  degeneration. 


METAZOA. 

Excluding  the  Protozoa,  all  the  "branches  of  the  animal  kingdom 
may  be  included  under  the  head  Metazoa,  i.e.  higher  animals. 
The  point  of  union  is  that  they  consist  of  numerous  distinct  cells, 
and  that  these  cells  are  arranged  in  several  layers.  At  least  two 
layers  are  present,  a  layer — the  ectoderm — which  bounds  the  body 
externally,  and  a  second,  lining  the  digestive  tract — the  entoderm. 
Between  these  two  a  third  layer  can  occur,  which  frequently  is 
separated  by  a  body  cavity  into  an  outer  or  somatic  layer  forming 
part  of  the  body  wall,  and  an  inner  or  splanchnic  layer  forming 
part  of  the  intestinal  wall.  This  middle  layer  is  called  mesoderm 
no  matter  whether  there  be  a  body  cavity  or  not. 

The  multicellular  condition  allows  a  higher  development  of  the 
organization,  which  appears  in  varying  grades  in  the  specialization 
of  tissues  and  organs.  In  no  metazoan  is  there  lacking  a  true  sexual 
reproduction,  that  is  one  by  sexual  cells,  but  the  fact  must  not  be 
overlooked  that  many  species  reproduce  (possibly  exclusively)  by 
unfertilized  eggs  in  a  parthenogenetic  manner.  Besides  the  sexual 
reproduction  many  species,  especially  the  lower  worms  and  coelen- 
terates,  reproduce  by  budding  and  fission. 

For  all  the  Metazoa  the  segmentation  of  the  egg  is  characteristic 
to  a  high  degree.  The  fecundated  egg  divides  into  numerous 
cells  which,  as  segmentation  cells  (blastomeres),  remain  united  and 
form  the  germ.  .N~o  Protozoan  has  a  true  segmentation.  Division 
there  produces  new  individuals  which  either  separate  completely  or 
exceptionally  remain  in  slight  connexion  as  a  colony. 

PHYLUM  II.  PORIFERA  (SPONGIDA). 

The  Porifera,  or  sponges,  the  most  familiar  representative  of 
which  is  the  bath  sponge  (Euspongia  officinalis),  are,  with  few 
exceptions,  marine.  In  fresh  water  occur  but  a  few  species  of 
Spongilla  (recently  subdivided  into  several  subgenera).  The  ani- 
mals have  no  powers  of  locomotion,  but  are  attached  to  stones  or 
plants,  along  the  shores  or  at  depths  up  to  6000  metres  (4000 


222 


PORIFERA. 


fathoms) .  They  form  spherical  masses,  thin  crusts,  small  cylinders, 
or  upright  branching  forms.  Frequently  the  shape  varies  so  that 
one  cannot  speak  of  a  typical  form.  It  was  also  difficult  to  decide 
about  the  animal  nature  of  the  sponges.  Striking  movements  of 
the  body  are  rare ;  only  by  aid  of  the  microscope  can  one  see 
motion — the  opening  and  closing  of  the  pores  and  the  streaming 
of  the  gastrovascular  system. 

The  simplest  sponges,  the  Ascons  (fig.  161),  are  thin-walled 
sacs,  fixed  at  one  end,  and  with  an  opening,  the  osculum  (func- 
tional anus),  at  the  other.  The  cavity  of  the  sac,  the  s  stomach/  is 
a  wide  digestive  cavity  into  which  water  bearing  food  obtains 
entrance  through  numerous  small  openings  or  pores  in  the  body 
wall.  The  basis  of  the  body  is  a  homogeneous  or  fibrous  connective 


en. 


ek.. 


FIG.  161. 


Fio.  162. 


FIG.  161.— Olynthus.    (After  Haeckel.)    e,  spicules;  z,  eggs;  o,  osculum;  p,  pores;  tt,, 

'stomach.' 
FIG.  162. — Section  of  wall  of  Sycmidra  raphanus.    (After  Schulze.)     e,  ectodermal 

epithelium;  e?i,  collared  flagellate  cells;  m,  mesoderm  with  connective-tissue 

cells;  o,  eggs;  s£,  calcareous  spicules. 

tissue  permeated  with  branching  cells  (fig.  162)  covered  externally 
by  a  thin  layer  of  pavement  epithelium  which  is  easily  destroyed. 
This  epithelium  (earlier  called  ectoderm)  and  the  connective 
tissue  (mesoderm)  are  now  regarded  as  a  common  layer,  '  meso- 
ectoderm/  since  it  has  been  shown  that  the  pavement  epithelium 
is  often  genetically  only  connective-tissue  cells  which  have  spread 
over  the  surface.  On  the  other  hand  there  is  a  distinctly  differen- 
tiated entoderm  in  the  shape  of  a  one-layered  flagellate  epithelium 
lining  the  stomach,  the  cells  of  which  (en)  recall  the  Choanofl agellata 


PORIFERA. 


223- 


(p.  202),  since  they  have  collars  surrounding  the  flagella.  It  has 
therefore  been  attempted  to  regard  each  flagellate  cell  as  an  indi- 
vidual, and  the  whole  sponge  as  a  colony  of  Flagellata,  a  view  which 
neglects  the  other  tissues,  not  only  the  connective  tissue  and  the 
epithelium  already  mentioned,  but  sex  cells,  amoeboid  wandering 
cells,  and  contractile  fibre  cells  which  close  the  pores.  The  taking 
of  food  is  accomplished  by  the  collared  cells. 

Sponges  of  this  simple  ascon  type  are  few.  As  a  rule  the 
sponges  are  more  massive  and  have  a  more  complicated  canal 
system  (figs.  164-166).  The  first  step  towards  complication  is 
seen  in  the  Sycon  sponges  (fig.  163),  in  which  the  gastral  cavity/ 


FIG.  163. 


FIG.  164. 


FIG.  163.— Stereogram  of  Sycon  sponge  (orig.).    a,  ampullae  with  pores  in  their  walls;. 

c,  cloacal  chamber,  with  the  openings  of  excurrent  canals;  i,  incurrent  canals; 

o,  osculum. 
FIG.  164. — Section  of  Leucortis  pulvinar.     (After  Haeckel.)    a,  aboral  pole ;  c,  efferent 

canals  from  the  ampullae  to  the  cloaca;  e,  ampullae;  t,  mesoderm;  o,  osculum;  v, 

cloaca. 

consists  of  numerous  radial  outpushings  (the  flagellate  chambers 
or  ampulla)  which  alone  contain  the  collared  cells,  while  the  cen- 
tral cavity,  now  called  cloaca,  is  here  lined  with  pavement  epi- 
thelium. By  increase  of  mesoderm  and  corresponding  thickening 
of  the  body  wall  the  ampullae  become  separated  from  external  and 
cloacal  surfaces  by  the  ingrowth  of  tissue  (Leucon  type).  The 
ampullae  nevertheless  retain  their  connexion  with  both  surfaces: 
by  means  of  a  system  of  canals.  .  This  canal  system  is  double;  one- 
part  is  incurrent  and  leads  from  the  dermal  pores  to  the  ampullae;; 


224 


PORIFERA. 


the  other  or  excurrent  from  the  ampullae  to  the  cloaca,  the  two 
being  connected  by  the  ampullae  alone.  Both  may  consist  of  lacunar 
spaces  (fig.  164),  or  have  a  more  regular  arrangement  (fig.  165), 


FIG.  165.— Section  of  cortex  of  Chondrilla  nucula,  the  skeleton  omitted.  (After  Schulze.) 
c1,  afferent  canals;  c2,  efferent  canals;  <;,  ampullae;  m,  cloaca;  o,  osculum. 

the  canals  from  the  pores  uniting  in  trunks  and  these  in  turn 
branching  to  go  to  the  ampullae.  The  excurrent  canals  also  show 
-a  similar  tree-like  arrangement.  Not  infrequently  extensive 
subdermal  or  subcloacal  spaces  occur.  The  relations  may  be  more 
•complicated  by  the  development  of  several  cloacae,  or  these  may  be 


FIG.  166.  FIG.  167. 

FIG.  166. — Surface  view  of  dermal  pores  of  Aplysina  aerophobia,    (After  Schulze.) 
FIG.  167.— Ascyssa  acufera.^  (After  Haeckel.) 

repressed;  again  by  the  branching  of  the  sponge  (fig.  167),  while 
still  further  the  branches  may  anastomose  (fig.  168),  giving  rise  to 
a  network. 

Sponges  may  reproduce  asexually,  small  portions  separating  as 
buds  and  producing  new  animals.  Usually  sexual  reproduction 
prevails.  Eggs  and  spermatozoa  arise  from  mesoderm  cells  (fig. 
162),  are  fertilized  and  undergo  segmentation  at  the  point  of  origin, 
and  leave  the  parent  as  flagellate  larvae  (fig.  169,  A).  At  fixation 


CALCISPONGI^.  225 

a  kind  of  gastrulation  takes  place,  the  blastopore  (fig.  169,  B] 
closes,  and  the  osculum,  an  entirely  new  formation,  arises  at  the 
opposite  pole. 


FIG.  168.  FIG.  169. 

FIG.  168.— Leucetta  sagittate*.    (After  Haeckel.) 

FIG.  169.— Development  of  Sycandra  raphanus.    (After  Schulze.)    A,  blastula;  B,  gas 
trula  at  the  moment  of  fixation;  ek,  ectomesoderm;  en,  entoderm. 

The  sponges  are  frequently  regarded  as  Ccelenterata,  but  scarcely  a 
single  homology  can  be  drawn  between  the  two.  The  ccelenterate  mouth 
is  different  from  either  pores  or  oscula.  Indeed  it  is  disputed  whether  the 
collared  cells  are  entoderm.  Nearly  all  sponges  possess  a  skeleton  secreted 
by  special  mesoderm  cells,  and  this  skeleton  affords  the  means,  according 
as  it  is  composed  of  calcic  carbonate  or  of  silica,  of  dividing  the  sponges 
into  two  classes.  Besides,  there  are  two  groups,  the  Ceraospongiae  and  the 
Myxospongiae,  in  which  the  skeleton  is  respectively  of  horny  substance  or 
spongin  or  is  lacking  entirely.  These,  however,  seem  to  be  descendants  of 
the  silicious  forms. 

Order  I.  Calcispongise. 

The  calc  sponges  are  exclusively  marine  and  mostly  live  in  shal- 
low water.  They  are  grayish  or  white  in  color,  of  small  size, 
rarely  exceeding  an  inch  or  so  in  length.  The  skeletal  spicules 
which  arise  in  the  mesoderm  usually  project  through  the  epithelium 
and  form,  especially  in  the  neighborhood  of  the  osculum,  silky 
crowns.  One-,  three-,  and  four-rayed  spicules  are  recognized,  these 
ground  forms  presenting  by  unequal  development  a  great  variety 
of  shapes  (fig.  170). 

Sub  Order  I.  ASCONES.  Sponges  with  thin  porose  walls  and  a  cen- 
tral 'stomach'  (figs.  161,  167).  Leucosolenia* 

Sub  Order  II.  SYCONES.  A  cloaca  present  surrounded  by  ampullae 
radially  arranged  (fig.  163).  Grantia,*  Syeon,*  Sycandra* 


226  PORIFERA. 

Sub  Order  III.  LEUCONES.  A  complicated  system  of  branching  in- 
current  and  excurrent  canals  in  the  thick  walls  connects  the  ampulla 
with  the  outer  surface  and  the  cloacal  cavity  (figs.  164,  168).  Leucetta, 
Leucortis. 


FIG.  170.— Sponge  spicules.    (From  Lang.) 

Order  II.  Silicispongise. 

The  siliceous  sponges  are  richest  in  species  and  occur  at  all 
depths  of  the  sea,  being  frequently  noticeable  from  their  size  (up 
to  a  yard)  and  their  bright  colors.  They  are  subdivided  into 
Triaxonia  and  Tetraxonia.  In  the  Triaxonia  the  spicules  com- 
posing the  skeleton — appearing  as  if  of  spun  glass  (hence 
Hyalospongia,  or  glass  sponges) — have  three  crossed  axes  (six 
threads  radiating  from  a  common  point) — hence  Hexactinellidae. 
The  mesoderm.  is  scanty  and  in  consequence  the  afferent  and 
efferent  canals  are  loose-meshed  lacunar  spaces  and  the  ampullaB 
large-  and  barrel-formed.  In  the  Tetraxonia,  on  the  other  hand, 
the  mesoderm  is  usually  abundant  and  the  canal  system  well 
developed.  The  four-axial  spicules  of  the  Tetractinellidse  must 
be  regarded  as  the  fundamental  skeletal  type.  From  this  are 
derived  the  compact  agglutinated  frameworks  of  the  Lithistidae 
and  the  monaxial  spicules  of  the  Monactinellidae. 

In  both  groups  the  spicules  may  be  united  by  secondary  deposits  of 
silica  to  an  extensive  framework ;  or  the  union  is  effected  by  spongin, 
which,  if  the  spicules  disappear,  forms  the  whole  skeleton  (horny 
sponges),  or,  as  in  slime-sponges,  the  whole  skeleton  may  be  lost. 

Sub  Order  I.  TRIAXONIA.  The  HEXACTINELLID^:  belonging  here  live 
chiefly  in  the  deep  sea,  and  for  a  long  time  only  a  few  species  were  known  : 
Euplectella  aspergillum,  Venus'  flower-basket,  tubular,  consisting  of  fine 
spicules.  Hyalonema.  Apparently  the  horny  sponges  Aplysina  and 
Aplysilla,  as  well  as  the  slime-sponges,  Halisarca,*  have  descended  from 
this  group. 

Sub  Order  II.  TETRAXONIA.  Typical  representatives  are  the  largely 
extinct  LITHISTID.E  (of  which  some  genera — Discodermia — persist  in  deep 


SUMMARY  OF  IMPORTANT  FACTS.  227 

seas)  and  the  TETRACTINELLID.E :  Geodia.*    Near  here  apparently  belongs 
Oscarella,*  without  a  skeleton  (Myxospongia). 

In  the  MONACTINELLID^:  the  spicules  are  united  by  spongin  (Corna- 
cuspongia),  and  can  even  be  entirely  replaced  by  that  substance.  Numer- 
ous marine  forms,  among  them  Chalina,*  and  also  the  fresh-water 
SPONGILLID.E  (Spongilla*  Ephydatia  *),  widely  distributed  as  encrusting 
masses  on  submerged  sticks  and  stones.  The  natural  color  is  light  grayv 
but  they  are  usually  colored  green  by  Algae.  They  are  distinguished! 
from  most  marine  relatives  by  the  formation  of  gemmulae  or  statoblasts. 
At  times  the  protoplasm  divides  into  round  bodies,  as  large  as  the  head  of 
a  pin  and  these  become  surrounded  by  a  firm  membrane  strengthened  in. 
many  forms  by  collar-button-like  spicules,  the  amphidiscs.  These  stato- 
blasts  remain  entangled  in  the  skeleton  and  survive  times  of  freezing  or 
drought.  On  return  of  good  conditions  the  contents  escape  and  form- 
small  Spongillce,  often  utilizing  the  old  skeleton.  This  process  recalls, 
encystment  among  the  Protozoa. 

When  the  spicules  entirely  disappear  and  nothing  but  the  spongiro 
fibres  remain  we  have  the  horny  sponges  or  CERAOSPONGLE.  The  skeleton 
consists  of  a  horny  substance,  spongin,  which  differs  chemically  from  the 
substances  of  true  horn — keratin.  This  spongin  is  always  laid  down  in 
long  fibres  by  peculiar  cells,  the  spongioblasts,  and  it  always  consists  of 
concentric  layers.  The  fibres  interlace,  branch,  and  unite  into  a  skeleton. 

The  best  known  horny  sponges  are  the  bath  sponges,  Euspongict 
officinalis*  occurring  in  the  Mediterranean,  West  Indies,  Florida,  and 
other  seas  in  many  varieties.  Best  of  all  are  the  Levant  sponges  (var. 
mollissima).  Sponges  of  commerce  consist  only  of  the  skeleton,  the  ani~ 
mal  parts  being  killed  and,  after  decay,  washed  away  with  fresh  water. 
Less  valuable  are  Euspongia  zimocca  and  Hippospongia  eqirina*  the? 
horse-sponge,  while  the  Cacospongice  are  useless. 

Summary  of  Important  Facts. 

1.  The  sponge  body  is  largely  a  mass  of  connective  tissue  cov- 
ered externally  with  pavement  epithelium  (meso-ectoderm)   and 
penetrated  by  canals. 

2.  An  entoderm  of  collared  flagellate  cells  occurs  only  in  the 
ampullae  or  flagellate  chambers  which  are  intercalated  between 
incurrent  and  excurrent  canals  (in  ascons  in  the  central  cavity). 

3.  The  animals  receive  nourishment  through  fine  pores  in  the 
body  wall;  indigestible  bodies  are  cast  out  through  one  or  several 
oscula. 

4.  Since  nerves,  muscles,  and  sense  organs  are  lacking  or  very 
weakly  developed,  the  animals  show  the  most  inconspicuous  move- 
ments. 

5.  Sponges  are  divided  into  Calcispongiae  and   Silicispongiae 
according  to  the  character  of  the  skeleton. 


228  CWLENTERATA. 


PHYLUM  III.     CCELENTERATA    (CNIDARIA,    JSTEMATO- 

PHOKA). 

The  animals  belonging  to  the  coelenterates  were  formerly  called 
Zoophyta  (plant-animals).  They  were  united  by  Cuvier  with  the 
Echinoderma  to  form  the  type  Radiata,  a  union  which  Leuckart,  the 
father  of  the  name  Ccelenterata,  set  aside  because  a  special  intes- 
tine and  a  special  body  cavity  occur  in  the  Echinoderma,  while  in 
the  Coelenterata  there  is  but  a  single  system  of  cavities  in  the 
body.  Each  of  the  three  names  indicates  certain  important 
characters  of  the  group. 

(1)  The  name  Zoophyta  was  selected  with  regard  to  the  gen- 
eral appearance.     Most  coelenterates,   like  the  plants,  are  fixed 
and  form  bush-like  or  mossy  colonies  by  incomplete  budding.     This 
resemblance,  is  but  superficial,  for  in  any  accurate  investigation 
there  cannot  be  the  slightest  doubt  of  the  animal  nature  of  any 
Ccelenterate.      The  name  therefore  must  not  be  understood  to 
imply  that  these  are  doubtful  forms  which  stand  on  the  border 
between  plants  and  animals.     Besides,  there  are  not  only  fixed  but 
free-moving  forms  which  swim  in  the  water  with  great  ease. 

(2)  Most   Coelenterata  are  radially  symmetrical.     There  is  a 
main  body  axis  one  end  of  which  passes  through  the  mouth  and 
the  other  through  the  blind  end  of  the  digestive  tract,  and  the 
organs  of  the  body  are  radially  arranged  around  this  so  that  the 
body  may  be  divided  into  symmetrical  halves  by  numerous  planes. 
In  the  higher  Coelenterata  this  may  be  replaced  by  a  biradial 
symmetry  or  even  by  bilaterality  (Ctenophora,  many  Anthozoa). 

(3)  The  term  Coelenterata  is  given  these  animals  because  they 
contain  a  single  continuous  coelenteron  or  gastrovascular  cavity. 
In  the  simplest  species  this  is  a  wide-mouthed  sac  into  which  food 
passes  for  digestion.     The  single  opening  into  it  serves  at  once  as 
mouth  and  anus;  the  sac  itself  is  the  alimentary  tract.    Frequently 
lateral  diverticula  or  branched  canals  are  given  off  from  the  central 
sac  which  distribute  the  nourishment  to  the  peripheral  parts  of  the 
"body,  and  thus  functionally  replace  the  vascular  system  of  higher 
forms. 

Since  this  gastrovascular  system  is  primarily  for  nourishment, 
it  is  an  error  to  call  it  a  body  cavity  and  to  say  that  the  coelenterates 
are  stomachless.  On  the  other  hand,  the  term  '  coelenteron, '  that 
is  a  cavity  which  is  at  once  gastric  and  ccelomic  (p.  158),  is  perfectly 
defensible,  since  in  many  higher  animals  which  possess  a  true  body 


CCELENTERATA. 


229 


cavity  this  is  seen  in  development  to  arise  as  diverticula  from 
the  primitive  stomach  (enteron).  Since  such  diverticula  occur  in 
coelenterates  without  becoming  independent,  one  can  say  that  the 
gastrovascular  system  consists  not  only  of  intestinal  portions  but, 
in  potentia,  of  the  coelom  as  well. 

To  even  a  superficial  observation  the  Coelenterata  are  more 
clearly  animals  than  are  the  sponges.  The  single  animals,  though 
often  united  in  colonies,  and  fixed  to  some  support,  are  capable  of 
quick  and  energetic  motion.  These  movements  are  most  striking 
in  the  tentacles — long  tactile  threads,  in  the  neighborhood  of  the 
mouth,  which  have  the  functions  of  feeling  for  food,  grasping  it, 
and  conveying  it  to  the  mouth.  The  means  of  killing  the  prey  are 
the  cnidae,  nematocysts,  or  nettle  cells  (fig.  171),  which  with  rare 


FIG.  171.— Nettle  cells  of  Coelenterata     (After  Hertwig,  Lendenfeld,  and  Hamann.) 

exceptions  in  Protozoa,  Turbellaria,  and  molluscs  occur  in  no 
other  group.  These  structures,  of  great  systematic  importance, 
are  oval  vesicles  with  fluid  .contents  and  firm  membrane.  Each  is 
drawn  out  at  one  end  into  a  long  tube,  so  delicate  as  to  appear  as 
a  thread  (hence  an  additional  name,  thread  cells).  This  thread  is 
sometimes  armed  throughout  its  length  with  retrorse  hooks,  or  it 
may  have  only  a  few  stronger  hooks  on  its  basal  portion,  which  is 
thicker  than  the  rest.  In  the  resting  stage  the  thread  is  spirally 
coiled  inside  the  cell.  On  stimulation  the  thread  is  quickly 
extended  (<  explosion  of  cell ')  and  produces  a  wound  into  which 
passes  the  irritating  fluid  contents.  Some  coelenterates  (e.g+ 
Physalia)  can  produce  in  this  way  very  painful  nettling  even  in  man.. 

The  nettle  capsule  arises  as  a  plasma  product  inside  a  cell. 
When  fully  developed  the  nettle  cell  extends  to  the  surface  and 
ends  with  a  tactile  process  (cnidocill)  which  upon  contact  stimu- 
lates the  protoplasm  and  causes  the  explosion.  The  cell  itself  is. 
frequently  enclosed  by  a  muscular  sheath  or  a  network  of  muscle 
fibres. 

Among  the  ccelenterates  both  sexual  and  asexual  reproduction. 


230  C(ELENTERATA. 

may  occur,  the  latter  usually  by  budding,  more  rarely  by  division. 
Sexual  and  asexual  types  of  reproduction  can  be  combined  in  the 
same  species,  producing  an  alternation  of  generations. 

In  comparison  with  the  sponges  the  Co3lenterata  may  be  called 
epithelial  organisms.  A  mesoderm  ('  mesogloaa')  may  be  entirely 
lacking  or  may  have  but  a  subordinate  development.  The  ectoderm 
and  entoderm,  on  the  other  hand,  are  the  important  tissues — pro- 
ducing muscles,  nerves,  sense  organs,  sexual  products  and  cnidae. 
Hence  the  group  is  often  called  Diploblastica — two-layered  animals. 

Class  I.  Hydrozoa  (Hydromedusae). 

According  to  varying  standpoints  the  Hydrozoa  can  be  placed 
either  higher  or  lower  than  the  Anthozoa  in  the  system,  since  in  the 
former  group  two  forms  are  frequently  introduced  into  the  life 
Mstory,  one  agreeing  well  in  structure  with  the  Anthozoa,  the 
other  standing  on  a  higher  grade.  The  first  is  the  sessile  and 
usually  colonial  polyp,  the  second  the  free-swimming  medusa,  well 
provided  with  sense  organs.  These  are  usually  related  to  each 
other  by  an  alternation  of  generations.  The  polyp  is  asexual  and 
by  budding  produces  medusae;  the  medusa,  on  the  other  hand,  is 
the  sexual  stage,  and  from  its  eggs  polyps  arise. 

The  polyp  of  the  Hydrozoa  is  the  hydropolyp,  forming  in  the 
branch  of  coelenterates  an  important  archetype  from  which  all 
other  conditions — medusae,  scyphopolyp,  and  even  the  coral  polyp — 
may  be  derived.  Our  best  example  of  this  is  the  fresh-water 
Hydra,  so  common  in  pools  and  streams.  The  body  (fig.  172)  is  a 
sac,  the  hinder  closed  end  of  which,  the  pedal  disc,  is  used  for 
attachment.  The  other  end  bears  the  mouth  which  leads  to  the 
internal  gastrovascular  (digestive)  cavity.  Around  the  mouth  is  a 
circle  of  tentacles  used  in  capturing  food  (mostly  small  Crustacea). 
These  are  outgrowths  of  the  body  wall;  the  circle  dividing  the 
body  into  a  peristome  inside  the  circle  and  a  column  constituting 
the  rest  of  the  outer  wall. 

Hydra  has  but  two  body  layers  (fig.  173),  an  entoderm  of 
flagellate  cells  lining  the  gastrovascular  space,  and  the  ectoderm 
•covering  the  outer  surface.  Between  the  two  is  the  supporting 
layer  (mesogloea),  a  structureless  membrane  without  cells  and  hence 
not  a  body  layer.  Both  layers  consist  of  epithelial  muscular  cells 
(cf.  p.  92),  the  basal  ends  of  which  are  produced  into  smooth 
muscle  fibres,  those  of  the  ectoderm  running  lengthwise,  those  of 
the  entoderm  around  the  body.  The  ectoderm  further  contains 
ganglion,  nettle  and  sex  cells.  The  nettle  cells  on  the  tentacles 


HYDROZOA. 


231 


are  crowded  into  small  ridges  or  batteries.  The  sex  cells  (at  cer- 
tain times)  produce  swellings  on  the  column;  a  circle  of  male 
swellings  close  beneath  the  tentacles,  the  female  cells  farther  down 
the  column  (fig.  172).  Individuals  reproducing  by  budding  are 
more  common  than  the  sexually  mature  (fig.  90).  Small  eleva- 


ek 


s  ek  c 


FIG.  172. 


FIG.  173. 


FIG.  172.— Hydra  viridis,*  testes  above;  ovarian  enlargement  below. 
FIG.  173.— Body  layers  of  Hydra.    (After  Schulze,  from  Hatschek.)    c,  cuticula;  en, 
nettle  cells;  eto,  ectoderm;  en,  entoderm;  s,  supporting  layer. 

tions  appear  on  the  column,  enlarge,  form  tentacles,  and  at  last  a 
mouth,  after  which  they  may  separate  from  the  parent. 

In  the  sea  are  numerous  hydroid  polyps  which,  while  agreeing  in 
the  main  with  Hydra,  are  distinguished  from  it  in  two  important 
respects:  (1)  they  do  not  directly  produce  sexual  organs;  (2)  they 
reproduce  asexually,  and  by  incomplete  budding  form  persistent 
colonies.  In  this  formation  of  colonies  a  series  of  parts  have 
arisen  which  require  special  designations  (fig.  174).  The  separate 
animals  are  the  hydranths,  and  are  connected  together  by  a  system 
of  tubes,  the  ccenosarc,  which,  like  the  hydranths,  consist  of  ecto- 
derm, entoderm,  and  mesogloea,  and  since  the  gastro vascular  space 
continues  in  them,  these  effect  a  distribution  of  food  throughout 
the  colony.  The  coenosarcal  tubes  may  creep  over  some  support 
(stone,  alga,  snail-shell,  etc. )  and  form  a  network,  the  hydrorhiza, 
or  it  may  stand  erect  and  tree-like,  forming  a  liydrocaulus.  Usually 
both  hydrorhiza  and  hydrocaulus  occur  in  the  same  colony. 


232 


CCELENTERA  TA. 


FiQ.llL—Campanulnrfajohnstoni.  (After  Allman.)  o,  hydranth  with  hydrotheca; 
6,  retracted;  rf,  hydrocaulus;  /,  gonotheca,  with  blastostyle  and  medusa  buds; 
g,  free  medusa. 


.  —en 


FIG.  175.— Section  of  Eudendrium  ramnsum.    ek,  ectoderm;  en,  entoderm;  p,  perisarc; 

,s,  supporting  layer. 


HYDROZOA. 


233 


Usually  the  colony  is  strengthened  and  protected  by  the  perisarc, 
a  cuticular  tubular  secretion  of  the  ectoderm.  In  some  (fig.  175) 
the  perisarc  stops  at  the  base  of  the  hydranth;  in  others  (fig.  176) 
it  expands  distally  into  a  wide-mouthed  bell,  the  hydrotheca,  into 
which  the  hydranth  may  retract  at  times  of  danger.  In  rare  cases 


FIG.  176.— Campanularia  geniculata.    eft,  ectoderm;  en,  entoderm;   p,  perisarc,  ex- 
panded around  hydranth  to  a  hydrotheca;  s,  supporting  layer. 

this  perisarc  may  be  greatly  increased  and  calcified,  forming  large 
coral-like  masses  with  openings  from  which  the  hydranths  may 
protrude  (fig.  177). 


FIG.  177.— A  bit  of  Millepora  alcicornis,  enlarged.    (After  Agassiz.) 

The  lack  of  sexual  organs,  which  distinguishes  the  marine 
species  from  the  fresh-water  Hydra,  is  explained  by  the  fact  that 
sexual  individuals  of  special  form  are  produced  from  the  colony 


234 


CGELENTERATA. 


by  budding.     These,  the  medusae,  may  separate  early  from  the 
colony  and  swim  freely.     A  medusa  (figs.  178,  179)  has  the  form 


FIG.  178.— Rhopalonema  velatum.  c,  ring  canal;  e,  exumbrella;  g,  gonads;  h,  otocysts: 
m,  stomach;  n,  nerve  ring;  o,  mouth;  s,  subumbrella;  t',  t'\  tentacles  of  first 
and  second  order ;  v,  velum. 

of  a  dome-like  or  disc-like  bell  and  consists  chiefly  of  an  ex- 
traordinarily watery  jelly.  The  bell  or  umbrella  of  the  medusa  is 
covered  on  both  its  surfaces — the  concave  or  subuinbrella,  the  con- 


HTDROZOA. 


235 


vex  or  exumbrella — with  ectodermal  epithelium.  At  the  margin 
of  the  bell  the  sub-  and  exumbrellar  ectoderm  is  produced  into  a 
two-layered  sheet  with  a  central  opening,  the  velum  or  craspedon 
(fig.  178,  B,  v)  of  systematic  importance,  since  these  medusae  are 
often  spoken  of  as  Craspedota.  Tentacles  (usually  4,  8,  or 
multiples  in  number)  also  arise  from  the  edge  of  the  bell  just 
outside  the  velum. 


FIG.  179.— Tiara  pileata.    (After  Haeckel,  from  Hatschek.) 

Comparable  to  the  tongue  of  the  bell  or  the  handle  of  the 
umbrella  is  the  manufrrium,  hanging  from  the  highest  point  of  the 
subumbrella  and  bearing  the  mouth  at  its  tip.  It  contains  the 
chief  digestive  space  from  which  radial  canals  run  on  the  sub- 
umbrellar  surface  to  a  ring  canal  in  the  margin  of  the  umbrella. 
The  radial  canals  are  usually  four  in  number,  but  in  some  species 
the  number  is  increased  during  growth  even  to  a  hundred  or  more. 
Manubrium  and  canals  are  lined  by  entoderm,  which  also  extends 
into  the  tentacles  and  forms  their  axes. 


236 


C(ELENTERATA. 


All  other  important  organs  arise  from  the  ectoderm.  Gonads 
arise  in  many  species  (fig.  179)  from  the  ectoderm  of  the  manu- 
brium;  in  others  from  the  same  layer  covering  the  subumbrellar 
surface  of  the  radial  canals  (fig.  178),  forming  in  either  case  con- 
spicuous, often  orange  or  red,  thickenings.  Longitudinal  ectoder- 
mal  muscles  move  the  tentacles  in  a  snaky  fashion,  whence  the 
name  medusa.  Circular  striped  muscles  run  on  the  subumbrellar 
side  of  bell  and  velum,  causing  the  characteristic  motion.  By  this 
contraction  the  bell  becomes  more  arched  and  narrowed,  while  the 


J 


B 


D 


FlG.  180.— Otocysts  of  Medusae.— A,  Cunina  ;  B,  Rhopalonema  ;  C,  Carmarinn  (Trachy- 
medusae) ;  D.  Octorchis  (Leptomedusan).  a,  epithelium ;  /i,  auditory  cells  ;  hfy 
origin  of  hairs  ;  hh,  auditory  hairs  ;  hp,  auditory  cushion  ;  o,  otoliths  ;  ?i,  audi- 
tory nerve  ;  nr,  nerve  ring. 

velum  (which  hangs  down  when  at  rest — fig.  178,  A)  contracts 
like  a  diaphragm  across  the  mouth  of  the  bell  (fig.  178,  B).  Since 
water  is  thus  forced  out  through  the  opening  the  medusa  is  forced 
forward  by  the  reaction. 

The  circular  muscles  of  umbrella  and  velum  are  separated  by 
the  nerve  ring,  with  which  are  connected  the  sensory  organs — 
eyes  of  the  simplest  type;  red  pigment  spots  with  or  without  a 
lens  (fig.  81);  and  open  or  closed  auditory  vesicles  (otocysts). 
Tactile  hairs  are  abundant  on  the  tentacles. 


HYDROZOA.  237 

The  auditory  organs  are  of  two  types,  both  beginning  as  free  organs 
and  receiving  their  highest  development  as  closed  vesicles  (otocysts).  One 
type,  the  tentacular  organs,  occur  in  the  Trachymedusae,  the  other,  or 
velar  organ,  in  the  Leptomedusae.  The  tentacular  organs  are  modified 
tentacles,  the  entodermal  axis  of  which  forms  the  otoliths  and  the 
ectodermal  covering  the  sense  cells.  In  the  ^Eginidae  (Fig.  180,  A)  the 
club-like  tentacles,  seated  on  an  auditory  cushion,  project  freely  into  the 
water;  in  the  Trachynemidse  (Fig.  180,  B)  they  are  partially  transformed 
into  vesicles  by  the  upgrowth  of  epithelium,  and  in  the  Geryonidae  (Fig. 
180,  C)  they  are  completely  enclosed  arid  are  sunk  in  the  jelly  of  the  bell. 
The  velar  organs  of  the  Leptomedusa3  are  placed  on  the  subumbrellar  sur- 
face of  the  velum.  They  may  be  either  simple  pits  (Fig.  180,  J?),  or  the 
mouths  of  the  pits  may  close  (Fig.  180,  Z>).  In  these  both  sense  cells  and 
otoliths  are  ectodermal.  Eyes  and  otocysts  occur  in  different  forms,  a 
fact  which  formerly  lead  to  a  division  of  medusae  into  ocellate  and  vesicu- 
late  groups. 

While  polyps  and  medusae  apparently  differ  so  greatly  from  each 
other,  their  morphology  shows  that  the  medusae  are  only  highly 
modified  polyps  adapted  to  a  swimming  life.  The  long  axis  of 
the  polyp  has  been  greatly  shortened  (fig.  181)  and  the  cylindrical 


FIG.  181.— Diagram  of  sections  of  (A)  a  polyp  and  (B)  a  medusa,  ek,  ectoderm;  ck',  of 
exumbrella;  efc2,  of  subumbrella;  ek\  of  manubrium;  e7,  endoderm  (cathamnal) 
layer  arising  from  obliteration  of  digestive  space ;  en,  entoderm ;  r,  ring  canal ; 
s,  subumbrella ;  t,  tentacles;  V,  velum ;  x,  supporting  layer  (gelatinous  in  B). 

body  developed  into  a  disc;  the  mesoglcea  of  column  and  disc  thick- 
ened to  a  conspicuous  layer  of  jelly;  while  manubrial  cavity, 
radial  and  ring  canals  are  to  be  interpreted  as  remnants  of  the 
large  gastrovascular  space  of  the  polyp,  obliterated  in  part  by  the 
pressure  of  the  mesogloea.  To  the  parts  thus  formed  only  the 
yelum  and  sense  organs  are  added. 

This  comparison  of  medusa  with  polyp  is  of  importance  in 
understanding  the  development,  which  usually  is  complicated  by 
an  alternation  of  generations.  From  the  eggs  of  the  medusae  a 
small  ciliated  embryo  (planula)  escapes,  which  becomes  attached, 


238 


C(ELENTERATA. 


develops  mouth  and  tentacles,  and,  by  budding,  produces  a. 
hydroid  colony.  This  hydroid  colony  lacks  sexual  organs.  It 
produces  by  budding  the  sexual  individuals,  the  medusae,  which 
separate  and  swim  freely.  Since  polyp  and  medusas  are  morpho- 
logically comparable,  there  is  a  time  before  the  escape  of  the 
medusae  when  the  colony  is  polymorphic,  consisting  of  asexual 
individuals  (hydranths)  which  reproduce  only  asexually  and  of 
others  which  have  taken  over  the  sexual  reproduction  (medusas). 
Hence  we  conclude  that  the  alternation  of  generations  here  has 
arisen  from  a  division  of  labor  or  polymorphism  of  individuals 
originally  of  equivalent  value,  in  which  some  individuals  (the 
sexual)  have  separated  and  acquired  a  peculiar  structure. 

While  alternation  of  generation  has  arisen  from  polymorphism, 
it  can  again  produce  it.  This  occurs  when  the  medusae,  instead 
of  separating,  remain  permanently  attached  to  the  colony.  They 
then  degenerate  into '  sporosacs/ which  always  lack  mouth,  tentacles, 
and  velum  (fig.  182),  often  also  radial  and  ring  canals,  so  that  at  last 


FIG.  182.— Comparison  of  a  medusa  and  a  sporosac  (orig.).  A,  fully  developed  medusa; 
B,  medusa  with  the  manubrium  closed,  still  attached  to  the  blastostyle ;  C, 

',  last  stage,  eggs  being  pro- 


medusa  reduced  to  a  simple  manubrium  (sporosac) ;  D 
duced  in  the  body  wall  (Hydra). 


there  remains  only  the  manubrium  ('spadix')  and  the  sexual 
organs,  the  latter  enveloped  by  the  rudiments  of  the  umbrella. 
Since  medusae  and  sporosac  replace  each  other  in  closely  allied 
species,  a  common  name,  gonopliore,  has  been  applied  to  both. 

This  developmental  history  may  be  modified  in  two  ways: 
either  the  polypoid  or  the  medusan  generation  may  be  suppressed: 
In  the  first  case  we  have  polyps  which  reproduce  both  sexually  and 
asexually,  in  the  other  medusae  whose  eggs  develop  directly  into 
other  medusae.  (A  few  medusae  may  produce  new  medusae  by 
budding.)  Thus  we  can  have  four  conditions:  (1)  Polyps  which 
produce  sometimes  asexually,  sometimes  sexually,  but  always 


HYDROZOA. 


239 


polpys;  (2)  Medusae  which  always  produce  medusae;  (3)  Polyps 
and  medusae  in  alternating  generations;  (4)  Polyps  and  sessile 
medusae  (sporosacs)  united  in  a  polymorphic  colony. 

The  Hydrozoa  are  almost  exclusively  marine.  The  colonial  forms  occur 
mostly  on  rocky  coasts  down  to  a  depth  of  100  fathoms,  but  have  been 
found  in  water  4000  fathoms  deep.  The  medusae  belong  to  the  pelagic 
fauna.  For  a  long  time  the  only  fresh-water  species  known  belonged  to 
the  cosmopolitan  genus  Hydra,  but  more  recently  both  hydroid  (Proto- 
hydra  ryderi,*  America  ;  Polypodium  hydriforme,  Russia)  and  medusan 
forms  (Limnocodium  soivei'byi,  Brazil ;  Limnocnida  tanganyicce,  Africa ; 
Halomises  lacustris,  Trinidad)  have  been  found.  Cordylophora  lucustris  * 
occurs  in  the  brackish  waters  of  Europe  and  America. 

The  Hydrozoa  may  be  classified  according  to  characters,  derived  either 
from  the  hydroid  or  the  medusan  stage.  The  former  basis  gives  us  four 
groups : 

(1)  Hydraria.     Polyps  with  asexual  and  sexual  reproduction  ;  no  per- 
sistent colonies,  no  perisarc,  no  gonophores  (fig.  172). 

(2)  Tubulariae.     Mostly  colonial,  with  perisarc  but  without  hydrothecae. 
Reproduction  by  gonophores  (medusae  or  sporosacs,  figs.  91,  175). 

(3)  Campanulariae.     Colonial,  with  perisarc  and   hydrotheca.     Repro- 
duction by  gonophores  arising  in  special  perisarcal  envelopes,  the  gonotheca 
(figs.  174,  176). 

(4)  Hydrocorallina.    Colonial,  with  massive,  calcified  perisarc,  resem- 
bling coral.     Reproduction  by  sporosacs  or  short-lived  medusae. 


FIG.  183.— American  Trachy  and  Narcomedusae.  A,  Liriope  scutigera.  (After  Fewkes.) 
.B,  Cunocantha  octonaria.    (After  Brooks.) 


The  characters  derived  from  the  medusae  also  give  five  groups : 

(1)  Anthomedusae   (Ocellatae).     Gonads  on  the  manubrium  ;  no  audi- 
tory organs  ;  eyes  usually  present ;  polyp  generation  present. 

(2)  Leptomedusae.     Gonads  on  radial  canals  ;  usually  velar  auditory 
organs  ;  polyp  generation  present. 

(3)  Trachymedusae.     Gonads  on  the  radial  canals  ;  tentacular  auditory 
organs  ;  develop  directly  to  medusae  (fig.  183,  A.) 

(4)  Narcomedusae.     Gonads  on  the  manubrium  or  gastral  pouches ; 
tentacular  auditory  organs  ;  no  polypoid  stage  (fig.  183,  B.) 


240 


C(ELENTERATA. 


(5)  Siphonophora.  Polymorphic,  free-swimming  colonies  of  Anthome- 
<dusae  ;  no  polyp  generation. 

From  this  it  is  seen  that  there  are  medusae  without  polyp  stages  and 
polyps  without  medusas,  so  that  a  true  system  must  take  into  account  both 
these  features.  When  this  is  done  and  life  histories  are  traced  it  is  seen 
that  the  Anthomedusse  and  the  Tubulariae  are  connected  by  an  alternation 
•of  generations,  and  the  same  holds  good  for  Leptomedusse  and  Campanu- 
lariae.  There  are  three  groups — Trachymedusae,  Narcomedusae,  and 
;Siphonophora — without  a  hydroid  stage,  and  two  in  which  the  polyp 
plays  the  chief  role,  the  medusa  being  rudimentary  in  the  Hydrocorallinae, 
lacking  in  the  Hydraria.  The  hydroid  polyps  are  usually  but  a  few 


FIG.  184.—  American  hydrqzoan  medusae.  (Mostly  after  Fewkes.)  A,  Eutima  gracilis; 
B,  Hydrichthys  mirabilis;  C,  Obelia;  D,  Euchilota  ventricularis;  E,  Lizzia  grata;  F, 
Turritopsis  nutricola;  (?,  Dipurena  strangulata. 

millimetres  or  fractions  of  a  millimetre  in  size,  but  the  huge  Monocaulis 
imperator,  of  the  deep  seas,  a  yard-  in  length,  forms  an  exception.  The 
colonies  are  usually  only  a  few  inches  in  extent.  The  medusae  have  bells 
varying  between  a  millimetre  and  a  few  inches  in  diameter,  reaching  in 
forskalea  a  diameter  of  sixteen  inches. 


Order  I.  Hydraria. 

Until  recently  only  the  cosmopolitan  species  of  Hydra  were  known. 
During  most  of  the  year  they  reproduce  by  budding  (fig.  90),  only  occa- 
sionally developing  gonads  (fig.  172).  The  eggs  remain  in  connexion 
with  the  mother  during  segmentation,  and  later  form  an  embryonal  shell, 
protecting  them  from  drought  or  cold.  In  this  *  encysted  stage  '  they 
can  be  distributed  by  wind  or  water  birds.  These  animals  formed  the 
basis  of  the  celebrated  researches  of  Trembley  on  regeneration.  He 
.showed  that  small  portions  when  they  contained  both  body  layers  could 


/.  HTDROZOA:  EJDROCOEALLIN^J. 


regenerate  the  whole  animal.  His  experiments  upon  turning  the  animals 
inside  out  have  not  been  fully  confirmed  ;  for  in  such  cases  the  layers 
resume  their  normal  positions.  Hydra  grisea  *  (fusca).  large  brown 
species  ;  H.  viridis*  green,  from  the  presence  of  symbiotic  algae.  Pro- 
tohydra  ryderi*  without  tentacles.  Polypodium  hydriforme,  parasitic 
on  sturgeon  eggs  in  Russia,  needs  more  study.  The  marine  Haleremita 
cumulans  may  belong  here. 

Order  II.  Hydrocorallinae. 

Exclusively  marine,  forming  colonies  of  thousands  of  individuals  whose 
calcareous  skeletons  so  closely  resemble  true  corals  that  they  were  asso- 
ciated with  them  until  the  animals  were  studied.  Millepora  alcicornis* 
stag-horn  coral,  in  Florida.  The  rosy  Stylasters  occur  in  tropical  seas. 

Order  III.  Tubulariae  =  Anthomedusae  (Gymnoblastea). 
As  a  rule  these  colonial  forms  with  perisarc  but  without  hydrotheca 
produce  anthozoan  medusae,   but  there  are  forms  like  Clava  *   (pink,  on 


FIG.  185.— American  Tubularian  hydroids.  A,  Myrioihelia  phryqiana  (after  Danielssen 
andKoren);  B.  6'arsi'a?-osari'a(afterFewkes);  C,  Monocaulispendula(a,fter  Agassiz); 
D,  Clava  leptostyla;  E,  Parypha  crocea;  F,  Podocoryne  mirabilis  (after  Agassiz). 

rockweed)  and  Hydractinia  *  (on  shells  inhabited  by  hermit  crabs) 
which  have  sporosacs.  Indeed  the  genera  Corymorpha  *  and  Mono- 
caulis  *  are  only  differentiated  by  the  existence  of  medusae  in  the  former 
and  of  sporosacs  in  the  latter.  In  the  forms  with  alternation  of  genera- 
tions different  names  are  applied  to  the  hydroid  and  medusan  stages  as 
follows  : 

HYDROID.  MEDUSA. 

Pennaria.  Globiceps. 

/Syncoryne.  Sarsia. 

Bougainvillea.  Hippocrene,  Margelis. 

Gemmelaria.  Gemmaria. 

Podocoryne.  Dysmorphosa. 


C(ELENTERATA. 


Other  common  genera  in  American  waters  are,  of  hydroids,  besides 
those  mentioned,  Eudendrium,  Tubularia,  and  Thamnocnida;  of  medusae, 
Tiaris,  Turritopsis,  Dipurena,  Lizzia,  Nemopsis,  and  Hydrichthis. 

Order  IV.  Campanulariae  =  Leptomedusae  (Calyphoblastea). 

These  forms  are  readily  distinguished  from  the  last  by  the  fact  that 
they  are  always  colonial  and  possess  hydrotheeae,  the  medusas  always  being 
LeptomedussB  (p.  239).  A  peculiarity  of  the  group  is  the  existence  of 
gonothecae,  closed  perisarcal  envelopes,  inside  which  the  gonophores  arise 
from  the  blastostyle,  a  specialized  polyp,  without  mouth  or  tentacles 
(fig.  174,  /).  The  typical  Campanulariae  produce  medusae,  while  some 
forms,  like  Thaumantia  *  and  JEquoria  *  have  no  hydroid  stage,  and 
on  the  other  hand  Sertularia  *  and  Plumularia  *  have  no  medusa  stage. 


FIG.   186.— American    Campanularians.    (After  Verrill.)       A,  Clytia  noliformis ;  Br 
Calycellasyringa;  C,  Obelia  dichotoma  ,'  Z>,  Opercularella  pumila. 

Other  common  genera,  Clytia,*  Dipliasia*  and  Aglaophetiia*  among 
hydroids;  Obelia,*  Tima*  RUegmatodes*  among  medusae.  Possibly  the 
fossil  group  of  GRAPTOLITES  belongs  near  here.  Only  the  perisarc  is 
known,  and  this  is  composed  of  hydrothecae,  in  which  it  is  supposed  the 
hydranths  occurred. 

Order  V.  Trachymedusae. 

These  medusae,  mostly  from  warmer  seas,  have  no  hydroid  stage.  The 
characters  are  given  on  p.  239,  Trachynema,  Liriope*(ftg.  183),  and  Cam- 
panella  in  our  own  waters,  Geryonia,  etc.,  in  Europe. 

Order  VI.  Narcomedusae. 

In  addition  to  the  characters  on  p.  239  may  be  added  the  fact  that  the 
tentacles  arise  from  the  outside  above  the  rim  of  the  bell.  Cunocantha  * 
(fig.  183),  and  Cunina  *  in  our  warmer  waters,  jEgina  in  Europe. 


7.  HTDROZOA  :  SIPHONOPHORA. 


243 


si) 


Order  VII.  Siphonophora. 

The  Siphonophora  are  among  the  most  beautiful  of  pelagic 
animals,  some  transparent,  some  brightly  colored.  Each  (fig,. 
187)  consists  of  a  colony  of  individ- 
uals springing  from  a  common  C03- 
nosarcal  tube  which  is  strongly  mus- 
cular and  contains  a  central  canal 
lined  with  entoderm  by  which  the 
members  of  the  colony  receive  their 
nourishment.  At  one  end  the  tube 
is  usually  closed  by  a  float  filled  with 
air,  the  pneumataphore,  which  acts 
as  a  hydrostatic  apparatus,  and  keeps 
the  colony  vertical  in  the  water. 

The  individuals,  springing  from 
the  coenosarcal  axis,  perform  differ- 
ent functions  and  hence  have  differ- 
ent structures.  Close  behind  the 
float  commonly  come  several  swim- 
ming bells  (nectocalyces)  which  re- 
tain of  medusal  structures  only  those 
(bell,  velum)  necessary  for  swimming 
and  those  (ring  and  radial  canals) 
for  the  distribution  of  nourishment 
received  from  the  common  tube. 
Then  come,  scattered  through  the 
colony,  the  covering  scales,  for  pro- 
tection, firm  gelatinous  plates  which 
have  lost  the  ring  canal,  the  muscles, 
and  the  bell  shape  of  the  medusae. 
Food  is  taken  by  wide-mouthed  feed- 
ing tubes  (In/}  which  may  be  com- 
pared  to  polyps  (fig.  57)  or  the  m 
nubrium  of  a  medusa.  They  digest 
the  food  by  means  of  large  masses  of 
glands  ('  liver  bands  ')  and  convey  it  calyx);  st'  stalk' 
by  the  central  tube  to  all  the  members  of  the  colony.  At  the^ 
base  are  long  muscular  tentacles  (t)  from  which  small  lateral 
threads  depend,  each  ending  in  a  brightly  colored  swelling,  the*, 
nettle  head,  composed  of  large,  closely  packed  nettle  cells.  These- 
are  the  cause  of  the  nettling  produced  by  the  siphonophores,  which, 
in  many  is  so  severe  as  to  be  feared  by  man.  The  '  feelers ' 


244 


CCELENTERATA. 


recall  mouthless  polyps  and  manubria;  they  are  very  sensitive  and 
mobile  and,  while  tactile,  apparently  in  some  cases  are  digestive 
organs.  Latest  to  develop  in  the  colony  are  the  sexual  bells. 
They  are  usually  brightly  colored  and  resemble  small  mouthless 


B3iM 


FIG.  188.— Stephalia  coronata.    (After  Haeckel,  from  Lanp.)    A,  in  section;  au,  canal 
to  float;  fca,  canal  system  of  stalk;  o,  mouth  ;  other  letters  as  in  fig.  188. 

Anthomedusae  without  tentacles.  They  but  rarely  (Chrysomitra) 
separate  from  the  colony,  but  usually  persist  as  more  or  less  reduced 
sporosacs. 

From  this  it  follows  that  the  Siphonophora  afford  fine  examples 
of  division  of  labor  and  of  the  consequent  polymorphism  of  indi- 
viduals. This  can  indeed  be  carried  so  far  that  many  convey  the 
impression  of  being  individuals  with  a  multiplicity  of  organs.  The 
Siphonophora  are  all  marine,  and  occur  most  abundantly  in  trop- 
ical seas. 

Sub  Order  I.  PHYSOPHOR^E  (Physonectee).  Float  present,  but 
small ;  next  a  large  series  of  swimming  bells,  and  then  the  other  members 
•of  the  colony.  Physopliora,  Agalmia,  Nanomia*  (fig.  189). 

Sub  Order  II.  CALYCOPHOR^E  (Calyconectse).  Float  lacking  ;  one 
or  two  large  swimming  bells  ;  the  other  individuals  in  groups  which  fre- 
quently separate  before  becoming  mature,  and  were  once  regarded,  under 
the  name  Eudoxia,  as  distinct  animals.  Praya,  Diphyes*  (fig.  189),  in 
warmer  seas. 

Sub  Order  III.  CYSTOKECT^E.  Float  greatly  enlarged  ;  the  creno- 
sarcal  tube  reduced,  the  individuals  (no  covering  scales  nor  swimming 


II.   SCTPHOZOA. 


245 


bells)  being  attached  to  the  under  side  of  the  float.  Physalia,  the  Portu- 
guese man-of-war,  occurs  as  far  north  as  New  England.  It  is  brightly 
colored,  and,  sitting  high  on  the  water,  is  driven  about  by  the  wind.  It 
stings  very  severely. 

Sub  Order  IV.     DISCONANTH^E.     Float  a  flattened  disc  with  con- 
centric air  chambers;  the  manubrium  projects  from  the  centre  of  the  lower 


FIG.  189.— American  siphon ophores.    A,  Nanomia  earn.   (After  A.  Agassiz.)  B.  Velella 
meridionalis.    (After  Fewkes.)   C,  Diphyes  praya.    (After  Fewkes.) 

surface  of  the  float.  Porpita*  with  circular  disc.  Velella*  (fig.  189),  the 
paper  sailor,  has  a  triangular  '  sail '  on  the  disc.  Both  are  tropical  and 
subtropical. 

Class  II.  Scyphozoa  (Scyphomedusae). 

The  Scyphozoa  parallel  the  Hydrozoa  in  that  they  frequently 
have  an  alternation  of  generations.     The  asexual  generation  is  the 


FIG.  190.  FIG.  191. 

FIG.  190.-Scyphostoma  of  Aurelia  aurita.  (From  Korschelt-Heider.)  fc,  perisarc  cup ; 
pb  proboscis;  s,  stalk;  t,  gastral  folds;  tr.  ectodermal  funnels. 

Mo.  191.— bection  of  Scyphostoma.  (From  Hatschek.)  gr,  gastric  pouches;  s,  gas- 
tric folds ;  SOT,  muscles. 

scyphopolyp  or  scyphostoma,  the   sexual  an  acraspedote  medusa. 
In  contrast  to  the  Hydrozoa  the  asexual  stage  plays  a  subordinate 


246  CCELENTERATA. 

role;  it  is  closely  similar,  even  in  the  most  different  species,  and 
can  even  be  lost  (Pelagia),  while  the  medusae  are  always  well  devel- 
oped and  present  great  variety  of  form. 

The  scyphostoma  (figs.  190,  191)  recalls  superficially  Hydra, 
ibut  is  distinguished  externally  by  a  small  perisarcal  cup  in  which 
the  aboral  end  is  placed.  Internally  there  are  four  longitudinal 
folds  projecting  into  the  gastral  cavity  and  extending  from  the 
margin  of  the  mouth  to  the  opposite  pole.  These  septa  or  tceniola 
appear  in  cross-section  as  small  folds  of  entoderm  supported  by  a 
process  of  the  supporting  layer.  They  are  important  morpholog- 
ically, since  in  budding  they  produce  the  gastral  tentacles  ( pliacellce) 
of  the  medusae.  Further,  they  are  the  first  appearance  of  the  septal 
system,  so  strongly  developed  in  the  Anthozoa. 

The  acraspedote  medusae  are  large  forms   (four  inches  to  four 

feet  or  more  in  diameter)  with  an 
arched  umbrella  often  of  almost 
cartilaginous  consistency.  They 
are  distinguished  from  the  craspe- 
dotes  externally  by  notches  in  the 
margin  of  the  umbrella,  dividing 
the  periphery  into  lobes.  In  the 
common  forms  at  least  eight  lobes 
occur  (figs.  192,  193),  each  notched 
at  its  tip,  and  in  the  notch  the 
sensory  pedicels  bearing  both  ears 
and  eyes  and  covered  by  a  lappet. 

FTG.     192.  — Ephyra     of     Cotylorhiza.    Ill    SOHie    (fig.    193,   /,   //)    the    Sen- 
(After  Glaus.)    gt,  gastral  tentacles  i    i          <•  11  11- 

(phacellee);    rfc,    marginal    (sensory)  SOry  lobes  follow  each  other,    but  111 

others  the  intermediate  region  is 

also  notched,  the  sensory  pedicels  then  being  found  only  on  careful 
search  (fig.  194).  Tentacles,  when  present,  spring  from  the 
notches  of  the  intermediate  region. 

The  sensory  pedicels  predicate  the  position  of  eight  principal 
radii,  of  which  four  are  called  the  perradii,  the  four  alternating 
with  them  the  interradii.  Adradii  are  radii  lying  between  the 
principal  radii. 

The  lobing  of  the  umbrella  influences  all  other  structures. 
There  is  no  velum  (hence  these  are  called  Acraspedia),  its  place 
being  taken  by  a  thick  muscular  mass  (fig.  86,  m)  on  the  sub- 
umbrellar  surface.  Instead  of  a  nerve  ring  there  are  eight  nerve 
centres  connected  with  the  sensory  pedicels.  Each  of  these 
pedicels  (fig.  195)  is  a  modified  tentacle  with  an  entodermal 


11.   SCYPUOZOA. 

t  I     11 


247 


— H 


FIG.  193.— Ulmaris  prototypus.  (From  Hatschek.)  7,  radii  of  first  order  (perradii); 
//,  radii  of  second  order  (interradii);  /,  marginal  lobes;  n  oral  lobes  (cut  away  on 
right  side);  ^,  tentacles  (adradial);  the  gonads  (right  side)  are  interradial. 


FIG.  194.— Polyclonia  frondnsa  *  and   one  of  its  branching  oral  lobes,  showing  the 
closed  grooves  (s).    (After  Agassiz  ) 


248  CCELENTERATA. 

axis  and  an  outer  layer  of  ectoderm.  The  entoderm  forms  an 
otolith  sac  at  the  tip,  while  the  ectoderm  furnishes  a  nervous  cushion 
of  ganglion  cells  and  fibres  and  usually  a  simple  eye  spot.  Less 
evident  is  the  effect  of  the  lobing  on  the  internal  organs.  The 
gastrovascular  system  begins  with  a  quadrate  or  X-shaped  mouth 
(fig.  193).  The  perradial  angles  of  the  mouth  are  usually  produced 
into  long  curtain-like  oral  tentacles  of  great  use  in  the  capture  of 
food.  The  '  stomach/  which  begins  just  inside  the  mouth,  gives  off 
four  interradial  (i.e.,  alternating  with  the  corners  of  the  mouth) 


FIG.  195.— Sense  organs  of  Aurelia  aurita.    (After  Schewiakoff.)    ec,  ectoderm;  en, 
•     entoderm;  gv,  gastrovascular  space ;  w,  supporting  layer ;  o,  cup  eye  ;  oc,  pigment 
eye ;  ot,  otolith  sac  ;  rg,  olfactory  groove. 

pouches,  the  gastrogenital  pockets.  The  epithelium  of  these 
pouches  produces  on  the  one  hand  a  group  of  small  gastral  tentacles 
(phacellse),  each  extremely  mobile  and  supported  by  an  axis  of 
mesogloea;  on  the  other  plaited  folds  of  the  gonads,  these  being,  in 
contrast  to  the  Hydrozoa,  of  entodermal  origin.  In  this  the 
Scyphomedusae  show  relationships  to  the  Anthozoa.  From  the 
central  digestive  sac  arise  the  peripheral  portions.  These  consist 
in  the  larval  medusae  (Ephyra  stage,  fig.  192)  of  eight  radial  canals 
to  the  sensory  pedicels,  and  in  most  adult  medusae  of  these  same 
pouches  and  eight  others,  adradial  in  position,  to  the  tentacles.  In 
some  this  primitive  arrangement  is  complicated  by  an  extensive 
network  of  tubes  (fig.  193). 

In  the  species  with  an  alternation  of  generations  the  egg  pro- 
duces a  ciliated  larva  (fig.  196)  which  attaches  itself  and  develops 
into  a  scyphostoma.  This  scyphostoma  is  always  capable  of  ter- 


//.   SCYPHOZOA. 


minal,  and  often  of  lateral,  budding.  The  lateral  buds  always 
produce  new  scyphostomae,  the  terminal,  medusae.  In  the  latter 
the  scyphostoma  develops  into  a  strobila,  becoming  divided  by 
circular  constrictions  into  a  series  of  saucer-like  discs,  the  young 
jelly-fish.  As  the  successive  discs  become  ready  they  separate 
from  the  pile  and  swim  away  as  '  ephyrae/  At  first  the  ephyrae 
(fig.  192)  have  only  four  gastral  tentacles,  parts  of  the  gastral 
folds  of  the  scyphostoma  (p.  246);  they  lack  marginal  tentacleSj 


FIG.  196.— Development  of  Aurelia  aurita,  (From  Hatschek.)  First  row,  growth  of 
planula  to  scyphostoma;  below,  strobilation  (separation  of  ephyree):  left,  oral  view 
of  scyphostoma ;  right,  two  ephyree. 

but  have  the  eight  lobes  and  the  corresponding  sense  pedicels. 
Since  the  ephyrae  differ  markedly  from  the  adult  medusae  and  only 
gradually  change  into  the  sexual  form,  the  alternation  of  genera- 
tions is  complicated  by  a  metamorphosis.  This  metamorphosis 
persists  in  some  cases  (Pelagia  noctilu-ca)  where  the  alternation  of 
generations  is  suppressed;  the  egg  develops  directly  into  an 
ephyra,  which  becomes  transformed  into  the  adult  jelly-fish.  On 
the  other  hand  no  case  is  known  where  the  medusa  generation  is 
dropped  out  and  the  scyphostoma  give  rise  sexually  to  other  scy- 
phostomae. 


250 


C(ELENTERATA. 


Some  forms  differ  from  the  foregoing  description  in  structure  and  ap- 
parently in  development.  Some  have  only  four  sen- 
sory bodies,  the  places  of  the  other  four  being  taken 
by  tentacles.  In  these  cases  the  sensory  organs 
lie  (Peromedusae)  in  the  same  radii  (i.e.,  interradii) 
as  the  sexual  organs  or  (Cubomedusae)  the  sense 
organs  are  perradial.  Lastly,  some  have  no  sensory 
organs,  their  place  being  either  taken  by  tentacles  or 
left  vacant  (Stauromedusae).  This  shows  that  tenta- 
cles can  replace  sensory  pedicels,  and  since  they  have 
essentially  the  same  structure,  they,  like  the  cordylii 
•of  the  Trachymedusae,  are  modified  tentacles. 
Order  I.  Stauromedusae  (Calycozoa). 

The  best  known  forms  are  the  Lucernariae 
(fig.  197),  whose  exumbrellar  surface  is  drawn  out 
into  a  stalk  by  which  the  animals  are  attached.  The 
disc  is  drawn  out  into  eight  lobes,  each  with  a  cluster  of  knobbed  tentacles. 
.Several  species,  dark  green  in  color,  occur  in  New  England  waters.  The 
Tesseridce  (unknown  in  America)  are  free-swimming. 


Fia.  197.  —  Halyclystus 
auricularia.*  (After 
Clark.) 


Order  II.  Peromedusae. 

Cup-shaped  medusae  with  four  interradial  sense  bodies. 
,sea  forms.     Pericolpa,  Periphylla  in  the  Gulf  Stream. 


Mostly  high 


Order  III.  Cubomedusae. 

Sense  organs  perradial  in  position.  Occurring  in  tropical  and  semi- 
tropical  seas.  Charybdea  (fig.  198).  Development 
unknown. 

Order  IV.  Discomedusae. 

These  are  the  most  abundant  and  richest  in  spe- 
cies of  Scyphomedusae  and  hence  have  served  as  the 
basis  of  the  foregoing  account.  The  order  is  subdi- 
vided according  to  the  characters  of  the  mouth. 

(1)  CANNOSTOMJS,    mouth   triangular    without    oral 
tentacles;   shape  and   other   features  of  the  ephyra 
retained  in  the  adult.     Nausithoe  albida  (fig.  86)  of 
Europe   is   noticeable  because   its  scyphopolyp,  de- 
scribed as  Stephanocyplius  mirabilis,  is  parasitic  in 
sponges.     Liner ges  and  Atolla  in  the  Gulf  Stream. 

(2)  SEM^OSTOMJE,  mouth  X-shaped  with  long  fringed 
and  folded  arms  at  the  corners.     Aurelia  flamdula  * 
and  Cyanea  arctica*  common  in  New  England,  the 
latter,  the  'blue  jelly,'  often  very  large;  disc  7  feet  in 
diameter,    tentacles    extending  a  hundred   feet  or 
more.     Pelagia  *  in  our  warmer  waters.     (3)  RHIZO- 
STOME^E.four  oral  arms,  these  brancheddichotomously. 

19?-,~  cjiarybdea  -phe  !nouth  and  grooves  on  the  arms  closed  by  union 

ipialis.*      (From  * 

ihek.)  of  their  edges  so  that  many  small  sucking  stomata 


FIG. 

marsi 
Hatsc 


III.   ANTHOZOA. 


251 


remain  through  which  nourishment  is  taken.     Stomolophus*  and  Polyclo- 
-nia  frondosa*  (fig.  194)  on  coral  banks  in  our  warmer  seas. 

Class  III.  Anthozoa  (Actinozoa). 

The  Actinozoa,  including  the  sea  anemones,  sea  pens,  and  corals, 
are  exclusively  marine.  With,  few  exceptions  they  are  sessile  and 
form  colonies,  often  of  enormous  size.  In  this  as  in  appearance 
(fig.  199)  they  resemble  the  hydroid  polyps.  They  have  a  pedal 


FIG.  199. — Antheomorpha  elegans.    s,  s,  sagittal  plane. 

disc,  column,  tentacles,  and  peristome  with  central  mouth.  They 
are  distinguished  by  their  greater  completeness  in  histological  and 
organological  differentiation.  The  Anthozoan  polyp  has  a  well- 
developed  mesoglcea,  the  supporting  layer  of  the  hydroid  being 
here  a  layer  of  connective  tissue  with  numerous  cells,  giving  the 
animals  a  tough  fleshy  consistency.  Still  more  important  as  points 
of  distinction  are  the  presence  of  an  oesophagus  and  septae  bearing 
mesenterial  filaments  and  gonads. 

The  mouth  lies  in  the  centre  of  the  peristome,  and  in  shape  is 
usually  oval  or  slit-like.  Hence  there  is  a  biradial  symmetry — of 
importance  in  the  architectonic  of  the  polyp — for  there  is  a  sagittal 
axis  (fig.  199,  s,  s)  passing  in  the  long  axis  of  the  mouth  and  a 
transverse  axis  at  right  angles  to  it.  From  the  mouth  the  oesopha- 
gus hangs  down  into  the  body  as  a  flattened  tube  and  opens  at  its 
lower  end  into  the  wide  gastro vascular  cavity.  In  its  development 
this  oesophagus  is  an  inflected  part  of  the  peristome  and  hence  lined 
with  ectoderm,  and  its  lower  end  alone  can  be  compared  with  the 
mouth  of  the  hydrozoan  (fig.  200). 

The  03sophagus  is  held  in  position  by  radial  partitions,  the 
septa,  which  stretch  from  base,  column,  and  peristome  to  the 


252 


C(ELENTERA  TA. 


oesophagus,  dividing  the  peripheral  part  of  the  gastral  space  into 
small  pockets,  the  radial  chambers,  connected  below  the  end  of 
the  oesophagus  with  the  central  part.  Above,  these  chambers  con- 
tinue into  the  tentacles.  The  tentacles  therefore  are  outgrowths 
from  the  radial  chambers  and  usually  are  equal  in  number  to  them. 
Besides  the  complete  or  primary  septa  which  reach  the  oesoph- 


FlG.  200.— Stereogram  of  an  Anthozoan  (orig.).  In  the  cut  edges  the  ectoderm  white, 
the  entoderm  blocked,  the  supporting  layer  black.  The  septa  show  the  septal 
muscles,  and  the  communication  of  the  interseptal  chambers  with  the  oesophagus 
is  seen. 

agus,  there  may  be  others  incomplete  in  that  they  do  not  reach 
the  oesophagus  and  belonging  to  secondary,  tertiary  or  other  series 
(fig.  203). 

The  septa  support  a  number  of  important  organs :  the  mesen* 
terial  filaments,  gonads,  and  muscles.  The  mesenterial  filaments 
are  thick  strands  of  epithelium  rich  in  glands  and  nettle  cells, 
fastened  like  a  hem  on  the  edge  of  the  septa.  Since  they  are 
much  longer  than  the  peristomial-pedal  length  of  the  septa,  they 
cause  these  latter  to  wrinkle  and  fold,  thus  strikingly  resembling 


III.   ANTHOZOA. 


253 


the  mesenteries  of  the  mammals.  Lower  down,  in  some  species, 
the  filaments  become  free  and  form  long  threads,  acontia,  rich  in 
nettle  cells  which  are  protruded  for  defence  either  through  the 
mouth  or  pores  (cinclides)  in  the  column.  The  gonads — only 
exceptionally  hermaphroditic — lie  inside  the  mesenterial  threads 
as  band-like  folded  thickenings  (fig.  201,  7^3).  They  arise  as  in 
the  Scyphomedusae  from  the  entoderm,  but  early  migrate  into  the 


FIG.  201. 


FIG.  202 


FIG.  201.— Sections  of  Cereus  spmostts,  showing   complete   and   incomplete   septa. 

u,  acontia;  6,  mesenterial  filament;  c,  septal  stoma;  g,  gonads;  ft1,  septa  of  first 

order  with  gonads;  /i2— /i4,  incomplete  septa  of  second  to  fourth  order;  t1— *4, 

corresponding  tentacles. 
FIG.  202. — Section  of  septum  of  Edioardsia  tuberculnta.    ek,  ectoderm;  en,  entoderm; 

me,  supporting  layer;  mf,  septal  muscle;  o,  ovary  ;  u,  mesenterial  filament. 

mesoglcea  of  the  septum  (fig.  202,  o).  The  eggs,  when  ripe, 
escape  into  the  gastrovascular  cavity  by  dehiscence.  The  young 
leave  the  parent  at  various  stages  of  development,  sometimes  as 
planulae  (fig.  206,  A),  sometimes  as  young  with  tentacles. 

The  muscles  are  very  important,  morphologically.  Muscles 
and  nerves  occur  in  both  ectoderm  and  entoderm;  but  while  the 
nerves  are  best  developed  in  the  ectoderm,  forming  especially  a 


CCELENTERA  TA . 


thick  subepithelial  sheet  of  fibres  and  ganglion  cells  in  the  pori- 
stome,  the  muscles  of  the  ectoderm  are  weakly  developed  and  are 
confined  to  the  peristome  and  the  tentacles.  The  entodermal 
musculature  is  much  stronger.  At  the  oral  end  of  the  column  is 
usually  a  strong  circular  (sphincter)  muscle  which  by  its  contrac- 
tion can  draw  the  top  of  the  column  over  the  peristome.  The 
septa  also  bear  muscles,  on  one  side  running  transversely,  on  the 
other  longitudinally,  the  latter  alone  being  strongly  developed  and 
producing  marked  ridges  (fig.  202)  on  the  septa. 

In  the  Hexacoralla  the  septa  are  arranged  in  pairs,  not  only  in  being 
close  to  each  other,  but  in  having  similar  faces  turned  towards  each  other. 
The  rule  is  (fig.  203)  that  in  each  pair  the  sides  bearing  muscle  ridges  are 
turned  towards  each  other,  but  in  two  pairs  lying  m  the  sagittal  axis  these 
muscles  are  turned  outward.  From  these  relations  these  septa  are  called 
directives.  It  is  however  to  be  noted  that  in  our  common  anemone,  Me- 
tridium,  occasionally  three,  more  frequently  but  one  pair  of  directives 
occur.  The  paired  condition  of  the  septa  allows  the  recognition  of  two 
kinds  of  radial  chambers;  between  the  two  of  a  pair  is  an  intraseptal, 


FIG.  203. — Transverse  section  of  actinian  (Adamsia  diaphana)  AB,  plane  of  symme- 
try, a  second  lies  at  right  angles.    I-IV,  septa  of  four  orders. 

between  two  pairs  an  interseptal  chamber.  New  septa  only  appear  in 
the  interseptal  chambers.  At  one  time  all  Hexactiuians  have  but  six  septa, 
a  pair  of  directives  and,  right  and  left,  four  lateral  septa.  With  growth, 
other  septa  of  a  secondary  order  may  appear  in  the  interseptal  areas,  giving 
six  of  these.  And  so  with  septa  of  the  tertiary  order.  Irregularities  how- 
ever occur,  and  forms  are  found  which  have  abandoned  this  sexfold  plan 


177.   ANTHOZOA. 


255 


and  have  assumed  a  plan  of  four  or  ten,  but  without  altering  the  primitive 
conditions. 

In  the  Octocoralla  (fig.  204)  the  conditions 
are  simpler,  only  eight  septa  being  developed. 
These  are  disposed  equally  on  either  side  of  the 
oesophagus  and  may  have  (most  octocorallans) 
all  their  muscles  towards  one  end,  or  (Edwardsia, 
fig.  205,  IV)  may  have  the  muscles  of  one  pair 
reversed.  It  is  to  be  noted  that  hexactinians 
pass  through  an  Edwardsia  stage.  In  Cerianthus 
new  septa  are  always  added  at  one  end  of  the  sag- 
ittal axis  (fig  205,  II),  while  in  the  extinct  Tetra- 
coralla  (fig.  205,  I),  so  far  as  one  may  judge  from 
the  hard  parts,  the  septa  have  an  arrangement 
with  four  as  the  basis. 


FIG.  204.—  Transverse  sec- 
tion of  an  Octocprallan 
(Alcyonium).  x,  siphono- 
glyphe;  1-4,  septa  of  one 
side,  wi 


with  their  muscles 
on  one  side,  symmetrical 
with  those  of  the  other 
side. 


IV 


FIG.  205. — Arrangement  of  septa  in  various  Actinozoa.    I,  Tetracoralla ;   II,  Cerian- 
thus; III,  Octocoralla;  IV,  Edwardsia. 

By  far  the  greater  part  of  the  Anthozoa  reproduce  by  budding 
as  well  as  by  eggs.  Only  rarely  do  the  buds  separate,  but  generally 
they  remain  connected  with  the  mother  to  form  a  colony  of  hun- 
dreds or  thousands  of  individuals.  These  are  connected  by  an 
extensive  coenenchym  or  coenosarc,  consisting  largely  of  mesoglcea, 
but  having  an  outer  coat  of  ectoderm  and  penetrated  by  a  system 
of  branching  and  anastomosing  entodermal  canals  (fig.  206).  On 
disturbance  the  polyps  can  quickly  retract  themselves  into  the 
coenosarc. 

The  colonial  Anthozoa  have  almost  invariably  a  skeleton, 
secreted  by  the  ectoderm  and  consisting  either  of  calcic  carbonate 
or  of  an  organic  horn-like  substance.  Sometimes  the  horn  and 
lime  alternate.  One  recognizes  an  axial  and  a  cortical  substance. 
The  axial  skeleton  is  confined  to  the  deeper  portions  of  the  coenosarc, 
while  the  cortical  portions  are  formed  by  the  polyps  themselves 
and  to  a  large  extent  (figs.  207,  208)  repeat  their  complicated 
structure.  Except  in  a  few  forms  (Fungia)  a  theca  is  present ; 
this  is  a  calcareous  cup,  and  from  this  usually  extend  inward 
calcareous  partitions  called,  in  distinction  to  the  fleshy-  or  sarco- 
septa,  the  sclerosepta. 


25C 


C(ELENTERATA. 


The  theca  arises  by  a  fusion  of  sclerosepta.     If  this  fusion  takes  place 
some  distance  inside  the  peripheral  ends  of  the  sclerosepta,  the  distal  ends 


e: 


FIG.  206.— Corallium  rubrum,  red  coral.  (After  Lacaze  Duthiers.)  A,  ciliated  young; 
.B,  young  colony ;  C,  part  of  colony  with  polyps  in  extension  (a)  and  contrac- 
tion (c);  d,  coenosarc;  Z>,  stereogram  of  a  branch;  6,  c,  partly  and  completely  re- 
tracted polyps;  rf,  coenosarc;  e,  skeletal  axis  exposed;  /',  /,  larger  and  smaller 
coenosarcal  canals;  m,  mesenterial  filaments;  s,  oesophagus;  <,  retracted  tentacles; 
A,  greatly,  #,  C,  D,  slightly  enlarged. 

of  these  project  on  the  outer  surface  as  costae.  Still  outside  these  may  be 
a  second  cup,  the  epitheca.  In  the  centre  may  occur  a  large  calcareous 
•column  or  several  smaller  ones,  the  columella  (fig.  208).  Pali  are  small 


III.   ANTHOZOA. 


257 


free  particles  between  the  inner  ends  of  the  sclerosepta  and  the  columella, 
while  synapticulce  are  small  projections  connecting  the  septa.  As  the 
polyps  grow  they  build  the  theca3  higher  and  higher  and  consequently  draw 


Fio.  208. 

FIG.  207. — Sclerophyllia  mnrgariticola.    (After  Klunzinger.) 

FIG.  208.— Section  of  coral  of  Caryophyllia  cyathus.    (After  Koch.)    Outside  the  theca, 
septa  (I-XII)  of  first  and  second  order,  their  pali  and,  in  centre,  columella. 

out  from  the  deeper  portions,  which  may  become  cut  off  by  horizontal  parti- 
tions, the  tabulee.  Such  tabulae  occur  in  some  Madreporaria,  Octocorallans, 
and  Millepores  (p.  241)  which  were  formerly  united  in  a  group  Tabulatae. 


FIG.  209. 


Fio.  210. 


FIG.  209.— Diagrammatic  section  of  the  flesh  and  coral  of  a  hexacorallan  ;  above  the 

line  the  section  passes  through  the  oesophagus,  s;  below  the  line  it  is  lower  down ; 

7-,  directives  :  coral  black. 
FIG.  210.— Diagram  of  the  relations  of  the  coral  to  the  polyp.  (After  Koch.)  Ectoderm 

lined,  mesogloea  black,  entoderm  dotted,  coral  white,    a,  theca;  b,  mesenteries; 

c,  costae ;  d,  basal  plate ;  e,  external  wall ;  /,  sclerosepta. 

It  was  once  thought  that  the  coral  was  a  calcined  portion  of  the  soft 
parts  and  hence  that  sclerosepta  were  hardened  sarcosepta  etc.  This  has 
been  disproved.  The  sclerosepta  are  formed  in  the  radial  chambers  between 


258 


CCELENTERATA. 


the  sarcosepta,  and  the  theca  inside  and  at  some  distance  from  the  col- 
umn, the  outer  surface  of  which  secretes  only  the  inconstant  epitheca 
(fig.  209).  From  the  above  it  would  appear  that  the  sclerosepta  correspond 
in  number  to  the  sarcosepta,  but  this  is  not  always  the  case.  Thus  the 
Helioporidae,  which  on  the  grounds  of  the  skeleton  were  regarded  as  Hex- 
acoralla,  are  shown  by  the  soft  parts  to  be  undoubted  Octocoralla. 

By  means  of  their  skeletons  the  Anthozoa  produce  large  accumulations 
of  carbonate  of  lime,  the  well-known  coral  reefs,  on  the  bottom  of  the  sea. 
These  are  formed  by  many  species,  the  Madreporaria  playing  the  most 
important  role.  When  the  reef  reaches  the  surface  it  produces  an  island, 
the  most  noteworthy  form  being  the  atoll,  a  ring-like  structure  with  a 
central  lagoon.  The  origin  of  these  atolls,  as  well  as  that  of  fringing  and 
barrier  reefs,  was  for  a  long  time  explained  by  Darwin's  and  Dana's  theory 
of  coral  reefs.  Later  investigations,  notably  those  of  Mr.  Agassiz,  afford 
another  explanation. 

Order  I.  Tetracoralla  (Rugosa). 

Extinct  forms  from  the  paleozoic  rocks  with  the  parts  arranged 
in  fours  (fig.  211).  The  present  tendency  is  to  regard  them  as  modi- 
fied Hexacoralla. 

Order  II.  Octocoralla  (Alcyonaria). 

These  forms,  which  have  eight  single  septa,  are  externally  re- 
cognizable by  their  feathered  tentacles,  eight  in  number  (fig.  206). 


B 


FIG.  211. 


FIG.  212. 


FIG.  211.— Diagram  of  septa  in  a  tetracorallan.    (Orig.) 

FIG.    212.— Three    stages   in    development  of   Renilla   reniformis.     (After  Wilson.) 

A,  cleavage  of  egg  ;  B,  planula  ;   C,  development  of  oesophagus  ;  ec,  ectoderm  ; 

en,  entoderm;  r/i,  mesogkea  ;  o,  oesophagus. 

They  occur  in  all  seas  from  near  the  shore  to  great  depths.  In 
development  there  is  a  planula  (fig.  212)  in  which  the  oesophagus 
arises  as  a  solid  ingrowth  which  becomes  perforated  later.  The 
eight  septa  arise  simultaneously.  Usually  colonies  are  formed  by 
budding  and  a  polymorphism  may  occur,  some  individuals  which 
have  reduced  septa  and  lack  tentacles,  taking  in  water  for  the 
colony.  Many  are  phosphorescent. 


///.    ANTHOZOA:  HEX  A  COR  ALL  A. 


259 


In  the  ALCYONIIDSE  (Alcyonium,*  Anthomastus)  an  axial  skeleton  is 
lacking,  but  the  flesh  contains  numerous  calcareous  particles,  the  scleroder- 
mites.  The  sea  pens,  PENNATULID.E,  have  the  basal  part  buried  in  the 
mud,  the  rest,  expanded  like  a  disc  or  feather,  bears  the  polyps.  An  axial 
skeleton  usually  occurs  in  the  stalk.  Pennatula,*  colder  waters ;  Renilla,* 
warmer  seas.  The  GORGONIID^E  (sea  fans,  sea  whips)  have  an  axis  of  more 
firmness,  which  may  be  calcareous,  and  the  colony  may  branch  and  the 
branches  anastomose.  Here  belong,  besides  many  tropical  genera  whose 
names  end  in  <•  gorgiaj  Primnoa*  of  our  colder  waters;  Isis  of  tropical 
seas,  with  skeleton  of  alternating  calcareous  and  horny  parts,  and  the  pre- 
cious coral  (Corallium  rubrum,  fig.  206)  of  the  Mediterranean,  the  fishing 
for  which  at  Naples  amounts  yearly  to  half  a  million  dollars.  In  the  TUBI- 
PORID^E,  or  organ-pipe  corals,  the  separate  polyps  are  enclosed  in  parallel 
tubes  united  at  intervals  by  horizontal  plates.  The  Helioporce  were  long 
regarded  as  Hexacoralla  because  of  their  massive  skeletons  with  six  sclero- 
septa.  The  paleozoic  Syringopora  belongs  near  Tubipora,  while  the 
FAVOSITIDJE  resemble  the  Alcyoniidse. 

Order  III.  Hexacoralla  (Zoantharia). 

The  simple  tubular  tentacles  are  highly  characteristic  of  the 
Hexacoralla,  as  is  the  arrangement  of  the  paired  septa  in  sixes  as 
described  above.  Yet  there  are  exceptions  to  this  rule.  On  the 
one  hand  is  Edwardsia  (common  in  our  colder  waters),  in  which 
there  are  sixteen  or  more  tentacles  and  only  eight  septa  (fig.  205), 
but  which  exhibits  a  condition  through  which  the  young  actinians 
pass  ;  on  the  other  hand  in  the  Zoantharia,  Cerianthise,  and 
Antipatharia  the  rule  of  six  has  undergone  extensive  modification. 
Sub  Order  I.  ACTINARIA  (Malacoderma).  The  sea-anemones  are 
mostly  solitary,  without  skeleton;  with  numerous  septa  and  tentacles. 

They  occur  in  all  seas  from  tide  marks 
to  the  greatest  depth.  A  few  are 
free,  but  most  are  sessile.  Except  the 
colonial  Zoanthese  all  can  creep  by 
the  pedal  disc.  Represented  in  our 
seas  by  Metridium,  Bunodes,  Sagar- 
tia,  Biddium  (parasitic  on  Cyanea) 
Halcampa,  etc.  The  Zoantheae  have 
two  kinds  of  alternating  mesenteries 
and  the  individuals  of  the  colonies 
are  usually  incrusted  with  foreign 
matter.  Epizoanthus  lives  symbi- 
otically  with  hermit  crabs  (fig.  113). 

Sub  Order  II.  ANTIPATHARIA. 
Six  pairs  of  septa  and  six  (Antipathes) 
or  twenty-four  ( Gerardia)  simple  ten- 

FIG.    213.— American    sea-anemones.     A, 

ides  (after  stimp-  tacles;  colony  with  a  black  horny  axis 


son,  B,  Biddium  parasiticunt  (after 
Verrill),  C,  Bunodes  sttlla  (after  Ver- 
rill). 


10   calcareous  skeleton, 
late  the  Gorgonids. 


SimU- 


260  CWLEXTERATA. 

Sub  Order  III.  MADREPORARIA.  This  group,  the  richest  in  species  of 
any,  is  characterized  by  the  great  development  of  the  skeleton.  Theca, 
septa,  and  usually  columella  and  synapticuli  are  present,  and  frequently 
costaB  as  well.  Solitary  forms  are  few.  Usually  they  form  colonies,  fre- 
quently of  thousands  of  individuals,  bound  together  by  a  coenenchym 
extending  from  polyp  to  polyp  over  the  surface  of  the  coral.  A  colony 


FIG.  214.  Fio.  215. 

FIG.  214.— Astrmigia  danae*  ;  five  polyps  in  various  stages  of  expansion. 
FIG.  215.— Coeloria  arabica.    (After  Klunzinger.) 


FIG.  216.  FIG.  217. 

TIG.  216.— Cladocora  ccespitosa.    (After  Heider.)    Relations  of  coral  and  flesh. 
FIG.  217.— Favia  cavernosa.    (After  Klunzinger.) 

arises  from  a  single  animal  by  continued  fission  or  budding.  When  the 
division  is  not  complete  the  animals  may  form  long  series  with  numerous 
mouths  but  with  the  other  parts  united,  the  result  being  that  the  surface 
of  the  coral  is  marked  by  long  winding  grooves — incompletely  separated 
theca — with  sclerosepta,  as  in  the  brain  corals  (fig.  215). 

Since  but  little  is  known  of  the  soft  parts,  the  classification  of  the  Mad- 
reporaria  is  based  upon  the  coral.  Three  sections  of  the  sub  order  are  recog- 
nized. (1)  APOROSA,  with  compact  skeleton.  Some,  like  Caryophyllia 


IV.    CTENOPHORA.  261 

(fig.  208)  and  Sclerophylla  (fig.  207)  are  solitary.  Others,  like  Oculina,* 
branch,  and  still  others  form  compact  masses.  Astrangia  danae  (fig.  214), 
the  only  true  coral  in  New  England;  Astrcea,  the  brain  corals  (Cceloria, 
fig.  215,  Diploria,  Manicina);  Cladocora  (fig.  216),  Favia  (fig.  217). 


FIG  218.— Madrepora  erythrcea.     (After  Klunzinger.) 

(2)  FUNGI ACEA,  or  mushroom  corals,  with  no  outer  wall  to  the  coral.  Some 
are  colonial,  others  (Fungia)  are  solitary.  A  sort  of  strobilation  in  de- 
velopment. (3)  POROSA,  with  skeleton  porous  like  a  fine  sponge.  Madre- 
pora* deer's-horn  coral  (fig.  218),  Porites,  Astroides. 

Class  IV.  Ctenophora. 

The  Ctenophores  excel  all  marine  animals,  even  the  medusae, 
in  transparency  and  delicacy  of  tissues;  many  are  so  soft  that  a 
strong  current  tears  them,  and  no  attempts  to  preserve  them  have 
been  successful.  The  body  is  almost  always  biradially  symmetrical; 
i.e.,  is  divided  by  both  sagittal  and  transverse  planes  into  sym- 
metrical halves.  Since  the  longitudinal  axis  is  usually  longer  than 
the  others,  which  are  generally  equal,  the  body  is  usually  oval  or 
pear-shaped.  In  Cesium  the  sagittal  axis  is  greatly  longer,  giving 
the  animal  the  form  of  a  band,  whence  the  name  '  Venus  girdle/ 

The  bulk  of  the  animal  is  composed  of  a  soft  jelly  with  con- 
nective-tissue cells,  penetrated  in  every  direction  by  polynucleate 
muscle  cells  branched  at  their  ends  and  apparently  innervated  by 
special  nerve  cells.  On  the  outer  surface  is  a  layer  of  ectoderm, 
while  in  the  interior  is  a  system  of  branched  entodermal  canals. 

At  the  bottom  of  a  depression  (fig.  22 IB,  p)  at  the  aboral 
pole  is  a  thickened  patch  of  ectoderm,  the  sense  body,  which  has 
considerable  resemblance  to  an  otocyst  (fig.  222).  The  thick 
sensory  epithelium  forms  a  shallow  groove,  strong  hairs  which  rise 
from  the  edge  of  the  groove  arch  over  it,  enclosing  a  space  to  be 
compared  to  an  incomplete  vesicle.  In  the  centre  is  a  spherical 


262 


C(ELENTERATA. 


FIG.  221B. 


FIG.  219.— Swimming  plate  and  epithelial  cushion.    (After  Chun.) 

FIG.  220. — Hormiphora  plumosa.    (After  Chun.) 

FIG.  %%,l.—-PleurobracMarho<lodactyla.  (After  Chun.)  A,  aboral  pole;  #,  front,  C,  side 
view.  MM,  sagittal  axis ;  7T,  transverse  axis  ;  c.adr,  radial  vessel  ;  c.ir,  inter- 
radial  vessel;  c.pr,  right  and  left  gastrovascular  trunks;  ex,  opening  of  funnel 
vessel ;  <;,  subcostal  vessel ;  m,  '  stomach ' ;  mq,  paragastric  canals ;  ?i,  ciliated 
grooves  ;  DC,  sense  body  ;  o,  mouth  ;  01%  ovary  ;  p,  pole-plate  ;  r.r3,  rows  of  combs  ; 
sc/i,  tentacular  pouch  ;  scfcs,  its  aperture  ;  sp,  testes  ;  /&,  basis  of  tentacle  ;  tg.  sc7i, 
tentacular  canal;  ti\  funnel;  trg,  funnel  canal. 


IV.  CTENOPHORA. 


263 


mass  of  otoliths,  supported  on  four  bundles  of  S-shaped  agglutinate 
cilia.  From  these  bundles  of  cilia  eight  bands  of  thickened  epi- 
thelium, at  first  in  pairs  (fig.  223,  ws),  later  diverging,  pass  to  the 
oral  pole  (fig.  221,  r).  These  meridional  bands  (so  called  from 
their  course)  consist  in  part  of  ciliated  epithelium,  in  part  of  the 
characteristic  '  combs '  which  are  the  locomotor  organs,  and  which 
must  be  regarded  as  transverse  rows  of  long  agglutinated  cilia. 
The  combs  (fig.  219)  arise  from  thick  epithelial  ridges,  transverse 
to  the  meridional  bands,  and  are  so  far  apart  that  the  free  edges 


FIG. 


FIG.  223. 


FIG.  222.— Section  of  sense  body  of  Cnllianira.  A,  through  the  centre  ;  7?,  excentric  ; 
d,  roof  of  sensory  groove ;  /,  support  of  otoliths,  o  ;  p,  pigment  cell ;  se,  sensory 

FIG.  223.— Aboral  pole  of  Callianira  bialata.  (From  Lang.)  /,  supports  of  otoliths, 
o;  PP,  pole  plate;  sfc,  sense  body;  to",  openings  of  gastral  funnels;  ics,  ciliated 
bands- 

of  one  comb  overlap  the  base  of  the  next  like  shingles.  In  conse- 
quence of  their  fibrous  structure  the  combs  are  strongly  iridescent 
and  in  motion  cause  a  beautiful  play  of  metallic  red,  blue,  and  green 
over  the  meridional  bands.  These  combs  act  like  oars  and  row 
the  body  about.  Since  the  combs  begin  some  distance  from  the 
aboral  pole,  they  are  connected  with  it  by  means  of  ciliated  grooves 
following  the  line  of  the  meridional  bands.  Experiment  shows 
that  the  sense  body  is  an  organ  of  equilibration  and  of  correlating 
the  action  of  the  different  rows  of  combs. 

The  ectoderm  gives  origin  to  two  more  important  organs,  two 
pole  fields  and  two  tentacles.  The  pole  fields  (fig.  221,  p;  223,  pp) 
are  two  epithelial  patches  extending  a  short  distance  in  the 
sagittal  axis  from  the  sense  body  and  possibly  are  olfactory  or 
taste  organs.  The  tentacles  arise,  in  the  transverse  axis,  from  the 


264  C(EL  ENTERA  TA. 

bottom  of  deep  tentacular  sacs,  from  which  they  project  as  long 
cords  with  numerous  lateral  branches,  and  into  which  they  may  be 
retracted.  Tentacles  and  branches  contain  an  axial  muscle,  while 
the  ectodermal  coating  consists  largely  of  adhesive  cells.  These 
are  spherical  bodies  (fig.  224)  covered  with  a  very  sticky  granular 
secretion,  and,  like  a  Vorticella,  supported  on  the 
end  of  a  spiral  stalk  muscle.  These  are  used  in 
capturing  prey. 

The  ectoderm  also  forms  part  of  the  gastrovascu- 
lar  system.  It  turns  inward  at  the  mouth — situated 
at  the  lower  end  of  the  chief  axis — and  lines  the 
large  space  commonly  called  stomach  (fig.  221, 
m)  but  which  corresponds  to  the  oesophagus  of  the 
Actinozoa.  At  the  aboral  end  of  this  stomach 
begin  the  true  entodermal  portions,  the  so-called 
FIG  224 —Adhesive  funnels>  an(l  from  them  the  canals  distributed 
phora°f  (Aner  through  ^ne  j6^  t°  *ne  various  organs.  Two 
Samassa.)  (rarely  four)  funnel  canals  run  to  the  aboral  pole 

and  empty  (fig.  223,  to)  near  the  sense  body;  a  second  pair,  the 
paragastric  canals  (fig.  221 B,  mg),  which  run  parallel  to  the 
03sophagus,  end  blindly.  The  perradial  canals  (c.pr)  proceed  out- 
ward from  the  funnel,  and  besides  giving  off  a  canal  to  the  tentacle 
(tg)  each  divides  dichotomously  twice,  first  into  interradial  and 
then  into  adradial  canals,  each  of  these  last  connecting  with  a 
meridional  vessel  running  just  beneath  a  row  of  combs,  nourishing 
them  as  well  as  the  gonads.  The  gonads  consist  of  two  bands,  one 
male,  the  other  female,  running  in  that  wall  of  the  meridional  ves- 
sel nearest  to  the  combs.  In  spite  of  their  position  they  are 
apparently  ectodermal  in  origin. 

These  gonads  are  regular  in  distribution,  those  of  two  vessels 
which  are  nearest  each  other  being  of  the  same  sex.  The  eggs  and 
sperm  pass  out  through  the  gastrovascular  system. 

The  few  species  of  the  group  are  divided  into  the  TENTACULATA, 
with  tentacles,  and  the  NUDA,  without.  To  the  first  belong  the  CYDIP- 
PID^E,  with  pear-shaped  bodies  (Pleurobrachia*  on  our  coast,  fig.  222),  and 
Hormiphora  (fig.  221);  the  LOBAT^E  (Mnemiopsis,*  Bolina*),  with  lobes; 
and  the  band-like  CESTID^E  (Cesium,  the  Venus  girdle)  of  the  warmer 
seas.  The  BEROID.E  (Beroe,  Idyia*},  with  wide  mouth,  belong  to  the 
Nuda.  The  small  creeping  forms,  Cceloplana  and  Ctenoplana,  are  supposed 
by  some  to  form  a  transition  to  the  Turbellaria. 


SUMMARY  OF  IMPORTANT  FACTS.  265 

Summary  of  Important  Facts. 

1.  The  CCELENTERATA  (together  with  the  Echinoderma) 
were  formerly  called  Radiata  because  in  most  a  radial  form  of 
structure  is  present;  in  the  higher  groups  this  can  be  transformed 
into  biradial  or  even  bilateral  symmetry. 

2.  The  Coelenterata  are  sometimes  called  Zoophyta  (animal 
plants),  from  their  appearance  and  their  attachment.     In  many 
the  resemblance  is  heightened  by  their  formation  of  plant-like 
colonies  by  fission  and  budding. 

3.  The  name  Coelenterata  was  chosen  because  they  have  but 
one  system  of  cavities,  a  simple  or  ramified  digestive  sac,  repre- 
senting at  once  the  alimentary  tract  and  the  as  yet  undifferen- 
tiated  body  cavity. 

4.  This  ccelenteric  apparatus  is  called  the  gastrovascular  sys- 
tem because  its  branches  distribute  nourishment  to  all  parts  and  so 
perform  the  function  of  blood  vessels. 

5.  The  reproduction  is  either  sexual  or  asexual,  very  frequently 
cyclical  (alternation  of  generations). 

6.  The  animals  are  provided  with  nerves,  muscles,  and  sense 
organs  and  possess  marked  sensibility  and  mobility. 

7.  Especially  characteristic  are  the  tentacles  and  small  nettling 
organs,  the  cnidse,  in  special  cells. 

8.  Nearly  all  histological  differentiation  proceeds  from  ectoderm 
or  entoderm,  since  the  mesoderm  (mesoglcea)  plays  but  a  subordi- 
nate role  and  usually  functions  only  as  a  support. 

9.  Four  classes — Hydrozoa,  Scyphozoa,  Anthozoa,  and  Cteno- 
phora  are  recognized. 

10.  In   HYDKOZOA   and    SCYPHOZOA   there    are   usually  two 
alternating  generations,  the  sessile  asexual  polyp  and  the  free- 
swimming  sexual  medusa. 

11.  The  hydroid  polyp  and  the  craspedote  medusa  are  charac- 
teristic of  the  HYDROZOA. 

12.  The  hydroid  polyp  is  a  two-layered  sac  of  ectoderm  and 
entoderm,  a  supporting  layer  and  a  circle  of  tentacles.     In  the 
colonial  forms  there  is  usually  a  cuticular  envelope,  the  perisarc, 
secreted  by  the  ectoderm. 

13.  The  craspedote  medusa  is  bell-shaped,  with  smooth  bell 
margin,  its  aperture  partially  closed  by  a  diaphragm-like  velum; 
the  gonads  are  ectodermal. 

14.  The  medusae  arise  by  lateral  budding  from  the  hydroid. 

15.  If  the  medusa  remain  attached  to  the  parent  as  a  sporosac, 


266  C(ELENTERATA. 

alternation  of  generations  is  replaced  by  polymorphism;   it  can 
entirely  disappear  with  the  total  loss  of  either  hydroid  or  medusa. 

16.  The  scyphostoma  and  the  acraspedote  medusa  are  typical 
of  the  SCYPHOZOA. 

17.  The  scyphostoma  differs  markedly  from  the  hydroid  polyp 
in  the  presence  of  four  longitudinal  gastric  folds  or  septa  (taeniolge). 

18.  The  acraspedote  medusa  lacks  a  velum,  has  a  lobed  umbrella 
edge,  gastral  tentacles  (phacellse),  and  entodermal  gonads. 

19.  The  medusa  arises  from  the  polyp  by  terminal  budding 
(strobilation). 

20.  Alternation  of  generations  rarely  is  lost,  and  then  only  by 
suppression  of  the  scyphostoma. 

21.  The  ANTHOZOA  have  only  one  form,  the  coral  polyp;  it  is 
distinguished  from  the  hydroid  polyp  by  the  ectodermal  oesophagus, 
the   radial   septa    reaching   the    oesophagus;    the   well-developed 
mesoglcea  and  the  gonads  which,  arising  from  the  entoderm,  early 
migrate  into  the  mesoglcea. 

22k.   Most  Anthozoa  are  colonial  and  produce  a  skeleton  usually 
of  calcic  carbonate,  but  sometimes  of  l  horny '  substance. 

23.  The  skeleton  may  be  either  axial  or  it  may  extend  over  the 
individual  polyps  (cortical  skeleton). 

24.  The  living  Anthozoa  are  divided  according  to  the  number 
of  septa  into  Octocoralla  and  Hexacoralla.     To  these  are  added 
the  fossil  Tetracoralla. 

25.  The  Hexacoralla  have  numerous  tubular  tentacles  and  six, 
or  a  multiple  of  six,  pairs  of  septa. 

26.  The  Octocoralla  have  eight  single  septa  and  eight  feathered 
tentacles. 

27.  The  CTENOPHOKA  are  always  free-swimming  and  have  a 
large  mesoderm  with  numerous  muscle  cells. 

28.  Nettle  cells  are  absent,  and  are  replaced  by  adhesive  cells. 

29.  Most    characteristic    are   the    eight   meridional   rows   of 
'  combs '  whose  motions  are  controlled  by  a  common  organ,  the 
sense  body,  constructed  like  an  otocyst. 

30.  The  digestive  tract  consists  of  an  ectodermal  oesophagus 
and  a  branching  system  of  entodermal  vessels. 


PL  A  TUELMINTHES. 


267 


PHYLUM   IV.    PLATHELMINTHES   (FLATWORMS). 

This  group  is  well  characterized  by  the  name.  AVith  few 
exceptions  (rhabdocceles,  many  trematodes)  the  nearly  flat  ventral 
surface  and  the  slightly  arched  back  are  closely  approximate  and 
pass  with  a  more  or  less  sharp  margin  into  each  other.  In  many 
cases  the  ventral  surface  is  distinguished  by  its  lighter  color.  In 
all  the  bilaterally  symmetrical  body  is  composed  of  a  solid  paren- 
chyma, a  mass  of  connective  tissue  traversed  by  muscle  fibres,  in 
which  the  various  organs — alimentary  tract,  nerves,  excretory  and 
reproductive  organs — are  imbedded.  In  the  lower  forms  the  di- 
gestive system  is  markedly  like  that  of  the  co3lenterates  ( Actinozoa, 
Ctenophora)  in  that  there  is  but  a  single  opening  and  this  leads  by 
an  ectodermal  oesophagus  (stomodaeum)  to  the  interior.  In  para- 
sites the  digestive  tract  may  be  lost.  The  skin  is  a  one-layered 
epithelium,  sometimes  ciliated,  sometimes  protected  by  a  thick 
cuticula.  Inside  this  comes  a  muscular  layer  (fig.  225)  in  which 


FIG.  225.— Transverse  section  fright  half)  of  a  Planarian.  ri,  vitellaria;  dv,  dorso- 
ventral  muscle  fibres;  e,  ectodermal  epithelium  with  cilia;  {/,  gastric  diverticula; 
7),  testicular  follicles  ;  lm,  longitudinal  muscles  (dots,  in  section) ;  n,  lateral  nerve 
cord. 

longitudinal  muscles  are  always  present,  and  in  addition  frequently 
circular  and  oblique  muscles,  as  well  as  those  passing  from  dorsal 
to  ventral  surfaces.  The  nervous  system  (fig.  228)  consists  of  a 
pair  of  ganglia  (' brain')  in  front  of  (i.e.,  above)  the  oesophagus 
and  longitudinal  nerves  leading  backwards  from  it.  The  excretory 
organs  (fig.  226)  are  composed  of  a  series  of  tubes,  the  protone- 
phridia  or  «  water- vascular  system/  which  branch  and  ramify  the 
parenchyma.  In  most  the  sexes  are  united  in  one  individual  and 
the  reproductive  organs  take  up  considerable  space.  There  is  a 
small  paired  or  unpaired  ovary  and  vitellaria,  usually  paired  and 
branched.  The  eggs  arise  in  the  ovary,  and  to  these  are  added 
nourishment  in  the  shape  of  cells  (abortive  ova)  rich  in  yolk  from 


268 


PL  A  THELMINTHES. 


the  vitellaria.  At  the  point  where  oviducts  and  yolk  ducts  unite 
a  single  egg  cell  together  with  several  yolk  cells  are  united  into 
an  oval  body — the  compound  egg — protected  "by  a  shell  secreted 
by  special  glands  (fig.  227,  A).  This  forms  only  an  apparent 
exception  to  the  rule  that  the  egg  is  but  a  single  cell,  for  the 
development  shows  that  only  the  egg  cell  takes  a  direct  part  in  the 


FIG.  226. 


FIG.  227. 


FIG.  226.— Excretory  system  of  Cercaria.  (After  Albert  Lang.)  ft,  limb  of  bladder  ; 
b',  same  with  urinary  concretions;  cc,  collecting  canal;  cs,  canals  of  ventral 
sucker;  cv,  collecting  vacuole;  e,  eye;  ep,  excretory  pore;  J,  lumen  of  tail;  os,  oral 
sucker  ;  vs,  ventral  sucker. 

FIG.  227.— Eggs  of  Dtatomum  nodulosum.  (After  Schauinsland.)  A,  before  develop- 
ment; U,  later,  the  yolk  broken  down,  d,  yolk  cells ;  ei,  egg  cell ;  eh,  ectoderm  ; 
en,  entoderm ;  p,  pigment  spot. 

formation  of  the  embryo  and  is  the  true  ovum,  while  the  yolk  cells- 
break  down  and  furnish  food  to  the  growing  embryo  (fig.  227,  B). 

Class  I.  Turbellaria. 

The  Turbellaria  are  small,  only  a  few  being  measured  by  inches, 
while  many  are  almost  microscopic  in  size.  The  name  Turbellaria 
has  reference  to  the  currents  produced  by  the  ciliated  ectoderm 
which  covers  the  body,  the  cilia  arising  from  the  single  layer  of 
columnar  epithelial  cells  (fig.  58).  This  ectoderm  serves  at  once 
for  motion  and  for  respiration.  Most  species  are  aquatic  (fresh 
water  or  marine),  only  a  few  land  planarians  living  in  moist  earth. 
In  the  water  they  either  creep  slowly  over  stones  or  plants  on 
their  ventral  surface,  or  they  swim  freely.  In  swimming  the  larger 
species  progress  by  undulations  of  the  body,  the  smaller  by  means 
of  the  cilia. 


/.    TURBELLAR1A. 


269 


The  alimentary  canal  (fig.  228)  consists  only  of  oesophagus 
(pharynx)  and  mesenteron,  the  latter  terminating  blindly  since  no 
intestine  or  anus  is  present.  The  mouth  is  on  the  lower  surface, 
at  some  distance  from  the  anterior  end,  being  occasionally  in  the 
middle  or  even  behind  the  middle  of  the  body  (fig.  231).  It 
leads  into  the  muscular  oesophagus,  which  is  frequently  enclosed 
in  a  special  sheath  and  then  can  be  protruded  like  a  proboscis. 


FIG.  228. 


FIG.  229. 


FlG.  228.— Digestive  and  nervous  systems  of  Syncoelidium  pellucidum.  (After 
Wheeler.)  a,  alimentary  tract ;  6,  brain ;  In,  longitudinal  (ventral)  nerves ;  m, 
marginal  nerve ;  pi,  longitudinal  nerve  of  pharynx ;  pi\  ring  nerve  of  pharynx ; 
<n,  transverse  nerve ;  w,  uterine  ostium. 

FlG.  228.—Pulychcerus  caudatus.    (After  Mark.) 

The  mesenteron,  of  entodermal  origin,  varies  greatly  in  shape,  its 
modifications  being  made  the  basis  of  division  of  the  class  into 
orders.  In  the  Polycladidea  there  is  a  central  portion  from  which 
numerous  branched  caeca  are  given  off;  in  the  Tricladidea  there 
are  three  main  trunks,  each  with  lateral  caecal  diverticula;  while  in 
the  Ehabdocoelida  the  digestive  tract  is  a  simple  rod-like  sac,  in 
some  cases  ( Acoela)  without  internal  cavity.  The  supra-oesophageal 
ganglia  always  lie  at  the  anterior  end  of  the  body,  which  is  most 
sensitive,  and  may  be  produced  into  feeler-like  processes.  This 


270  PLATHELMINTHES. 

region  usually  bears  two  or  more  simple  eyes,  and  in  a  few  a  single 
otocyst. 

In  many  Turbellaria  nettle  cells,  like  those  of  the  Ccelenterata, 
occur  in  the  skin.  Much  more  common  are  the  rhabdites,  small 
rods  which  arise  in  epithelial  cells  which  sometimes  project  like 
glands  into  the  mesoderm.  Those  rhabdites  occur  in  the  shiny 
tracks  which  the  animals  leave  in  creeping. 

The  hermaphroditic  sexual  organs  (fig.  73)  and  the  excretory 
system  vary  considerably  in  the  separate  orders  and  families.  The 
eggs  are  usually  very  large  and  are  fastened  by  a  stalk  to  water 
plants.  Many  species  form  a  sort  of  cocoon,  containing  a  few  eggs 
and  numerous  yolk  cells.  In  the  marine  species  a  free-swimming 
larva  (fig.  230)  with  lobe-like  processes  may  hatch  from  the  egg. 


-En 


FIG.  230.— Larva   of  Stylochus  pilidium.    (From  Korschelt-Heider,  after  Gotte.)    D, 
enteron  ;  En,  remains  of  entoderm  cells  ;  -S,  oesophagus. 

This  larva,  by  a  metamorphosis,  is  converted  into  the  creeping 
adult.  Not  infrequently  besides  the  sexual  asexual  reproduction 
occurs.  The  Microstomidge  and  some  Planarice  are  capable  of 
transverse  division,  and,  when  well  nourished,  by  rapidly  repeated 
divisions  will  form  chains  of  individuals  arranged  in  a  row,  separa- 
tion taking  place  gradually.  For  each  posterior  individual  a  new 
brain  and  a  new  oesophagus  are  formed  (fig.  58).  The  Turbellaria 
possess  the  power,  to  a  marked  degree,  of  reproducing  lost  parts, 
which  makes  them  favorites  for  regeneration  experiments. 

In  a  few  Turbellaria  there  is  a  noteworthy  condition  of  the  digestive 
organs.  The  pharynx  connects  with  an  entodermal  syncitium,  a  proto- 
plasmic mass,  without  lumen,  containing  nuclei  in  which,  as  in  the  pro- 
toplasm of  a  protozoan,  the  food  is  digested.  This  entoderm  is  hardly 


//.    TREMATODA. 


271 


marked  off  from  the  mesoderm,  but  it  is  a  question  whether  these  l  Accela  ' 
are  primitive  or  degenerate. 

Order  I.  Polycladidea. 

Marine  species  of  considerable  size,  in  which  the  digestive  caeca  spring 
from  a  central  chamber.     Species  of  Leptoplana  and 
Stylochus  on  our  shores.     Thysanozoon,  Europe. 

Order  II.  Tricladidea. 

Alimentary  canal  with  three  main  trunks,  an  anterior 
unpaired  and  a  pair  of  posterior  branches,  arising  from 
the  pharynx.  These  trunks  bear  lateral  caecal  branches. 
Among  the  marine  genera  are  Bdelloura  *  and  SyncoR- 
lidlum  *  (fig.  229)  (parasitic  on  Limulus),  Gunda,*  Poly- 
clicerus  *  (fig.  229);  in  fresh-water  occur  Dendrocoelum* 
Planaria*  and  Polyscelis*  Phagocata  *  with  divided 
pharynx.  The  land  planarians  (Bipaliiun,*  10  or  12 
inches  long)  are  tropical,  but  have  been  introduced 
into  greenhouses  in  various  parts  of  the  country. 

Order  III.  Rhabdoccelida. 

Small,  even  microscopic,  in  size,  and  recalling  in 
habits  and  appearance  the  Infusoria;  alimentary  canal 
rod-like,  without  branches.  Monops*  and  Monoscelis* 
marine.  The  fresh-water  MICROSTOMID.E  reproduce 
almost  exclusively  by  fission,  so  that  sexual  individuals 
are  rare. 

Class  II.  Trematoda. 

These  are  exclusively  parasitic,  some  living  on  the  skin  or 
gills  (ectoparasites)  or  in  the  interior  of  other  animals  (entopara- 
sites).  In  structure  they  are  closest  to  the  triclad  Turbellaria, 
from  which  they  are  especially  distinguished  by  characters  the 
direct  result  of  their  parasitic  life.  Thus  they  have  lost  the  cilia 
or  these  only  appear  in  the  aquatic  larval  stages.  On  the  other 
hand  they  are  armed  with  structures  derived  from  the  skin  — 
suckers  and  hooks — for  adhesion  to  the  host.  The  suckers  are 
shallow  pits  of  columnar  epithelium,  lined  with  cuticle  and  fur- 
nished with  a  thick  muscular  layer  which  by  its  contraction  increases 
the  lumen  of  the  sucker,  the  edges  of  which  are  closely  applied  to 
the  host.  At  least  one  such  sucker  is  present;  if  but  one  or  two 
(entoparasites),  one  is  at  the  anterior  end  (oral  sucker)  and  sur- 
rounds the  mouth,  while  a  second  larger  sucker  may  occur  near  the 
month  (fig.  232),  but  may  be  (Amphistomum)  at  the  posterior  end. 
In  the  ectoparasites  there  are  a  pair  of  anterior  suckers  near  the 
mouth;  at  the  posterior  end  a  single  sucker,  or  a  number  of  suck- 
ers or  hooks  or  both  on  a  sucking  disc  (fig.  234). 


FlG.231.— Gundn  loba- 
ta.  (After  Schmidt.) 
<7,  cerebral  ganglia, 
with  eye  spots ; 
o,  mouth  (entrance 
to  long  pharynx); 
p,  genital  pore  with 
male  organs  be- 
hind, female  in 
fron  t . 


272 


PL  A  THELMINTHES. 


Other  results  of  parasitism,  are  the  weak  development  of  sense 
organs  and  brain  and  a  tendency  to  development  of  accessory 
ganglia  near  the  adhesive  organs.  Eye  spots  (two  to  four)  occur 
occasionally  in  the  ectoparasitic  species  and  in  the  larvae  of  the 
entoparasitic,  rarely  in  their  adult  condition.  The  alimentary  tract 
is  forked  (fig.  233)  and  occasionally  (fig.  232)  has  dendritic  blind 
sacs.  To  parasitism  may  also  be  attributed  the  great  development 
of  the  sexual  organs,  which  at  maturity  fill  a  great  part  of  the  body. 
Their  features  maybe  seen  in  fig.  233.  Two  vasa  deferentia  pass  for- 


s..i 


FIG.  232.  FIG.  233. 

FIG.  233.— Distomum  hepaticum,  liver  fluke.  (From  Boas.)  ?n,  cseca  of  <«,  limbs  of 
digestive  tract ;  Sj.s2,  anterior  and  posterior  suckers. 

FIG.  233.— Distomum  Innceolatum.  c,  cirrus,  beneath  it  the  opening  of  the  oviduct ;  d, 
vitellaria,  the  ducts  leading  to  the  shell  gland  ;  g,  ganglion;  /i,  testes  with  ducts 
to  cirrus  ;  I,  Laurer's  canal ;  o,  ovary,  the  shell  gland  behind  it ;  s',  s",  anterior 
and  median  suckers,  the  pharynx  and  the  bifurcated  digestive  tract  leading 
from  s';  u,  uterus ;  w,  terminal  vesicle  of  water-vascular  (excretory)  system. 

ward  from  the  testes  (A),  unite  and  form  a  seminal  vesicle.  The 
terminal  portion  of  the  united  ducts  can  be  protruded  as  a  penis 
or  cirrus,  armed  with  retrorse  hooks.  It  is  usually  enclosed  in  a 
( cirrus  pouch/  The  ovary  (o)  is  very  small  and  produces  small 
eggs,  deficient  in  yolk;  hence  the  vitellaria  (d)  are  well  developed. 
The  ducts  from  these  unite  with  the  oviduct,  producing  the  uterus 
(u),  which  receives  the  eggs,  is  much  convoluted,  and  empties  beside 
(in  some  species  in  a  common  antrum  with)  the  male  sexual 
opening.  The  first  part  of  the  uterus  is  called  the  ootype  because 
here  the  eggs  and  yolk  cells  are  formed  into  eggs  (fig.  227)  and 


//.    TEEM  AT  OD  A:  POLTSTOME^E. 


273 


enclosed  in  a  shell  with  a  lid  or  cover.  A  second  duct — Laurer's 
canal— goes  from  the  oviduct  to  the  dorsal  surface.  In  many 
Polystomeae  the  canal  is  double  (fig.  234,  sw)  and  is  connected 
with  copulation,  but  in  the  Distomes  it  is  rudimentary  or  lacking 
and  the  opportunities  are  present  for 
self-impregnation.  Many  Trematoda 
have  a  third  canal — the  vitello-intes- 
tmal  duct — leading  to  the  digestive 
tract. 

The  Trematodes  fall  into  two  great 
groups,  the  Polystomeae,  largely  ecto- 
parasites, and  the  Distomeae,  exclu- 
sively entoparasitic,  the  distinctions  in 
parasitism  being  correlated  with  differ- 
ences in  structure. 

Order  I.  Polystomese 
(Monogenea,  Heterocotylea). 

Most  Polystomes  live  on  aquatic 
animals — usually  fish,  rarely  Crustacea, 
where  they  attach  themselves  especially 
to  the  thin-skinned  and  richly  vascular 
gills.  Since  as  ectoparasites  they  are 
exposed  to  more  dangers,  their  adhesive 
organs  are  stronger  than  in  the  ento- 
parasites.  So  while  the  anterior  suckers 
are  weakly  developed,  in  some  cases 
absent,  the  hinder  end  bears  sometimes 
only  a  single  sucker,  but  usually  a  large 
adhesive  disc  armed  with  many  suckers 
and  hooks.  The  transfer  of  Polystomes 
from  one  host  to  another  presents  few 
difficulties  and  hence  the  life  history  is 
without  complications.  The  stalked 
eggs  are  attached  near  the  mother  and 
produce  larvae,  which  soon  after  hatch- 
ing have  the  adult  form  (hence  the 
name  Monogenea). 

The  American  species  have  been  scarcely  touched,  hence  most  of  our 
knowledge  is  of  European  species.  Gyrodactylus,  parasitic  on  the  gills  of 
the  carp,  is  interesting,  since  it  brings  forth  living  young  which,  even 
before  birth,  produce  a  new  generation  in  their  interior.  More  striking  is 
Diplozoon  paradoxum  (gills  of  Cyprinoids),  which  owes  its  name  to  the 


234.  —  Polystonnim 
rimum.  (After  Zeller.)  Above 
two  individuals  in  copulation  ; 
below  a  single  animal  enlarged, 
d,  digestive  tract,  distended 
with  blood;  rfgr,  yolk  duct;  dst, 
vitellarium;  gp,  genital  pore:  h, 
testicular  vesicles ;  m,  mouth; 
p/j,  pharynx ;  ov,  ovary ;  s?«, 
openings  of  the  paired  vaginae  ; 
M,  uterus;  v,  vaginae;  rd,  vas 
deferens;  x,  vitello-intestinal 
canal. 


274 


PL  A  THELMINTHE8. 


fact  that,  at  the  time  of  sexual  maturity,  two  individuals  become  fused 
like  Siamese  twins  (fig.  109).  The  young,  described  under  the  name 
Diporpa,  escape  from  the  eggs  and  only  unite  later.  Each  has  a  ventral 
sucker  and  a  dorsal  papilla.  When  they  unite  each  of  the  pair  seizes  the 
papilla  of  the  other  with  the  sucker,  and  then  the  two  grow  together  so 
that  the  male  opening  of  one  comes  opposite  the  female  opening  of  the 
other.  Polystomum  integerrimum  of  the  frog  (fig.  234)  affords  a  transition 


FIG.  235.— A,  Polystomum  hassalli*  (after  Goto),  from  bladder  of  mud-turtle. 
B,  Acnnthocotyle  verrilli*  (after  Goto),  from  skate.  C,  Gyrodactylus  elegans  (after 
von  Nordmann). 

to  entoparasitism.  At  first  it  lives  on  gills  of  the  tadpole,  but  at  the  time 
of  metamorphosis  it  is  forced  to  leave  this  place  and  p<iss,  by  way  of 
the  alimentary  canal,  to  the  urinary  bladder.  The  TEMNOCEPHALID.E  of 
warmer  regions  are  partially  ciliated,  and  have  from  four  to  twelve 
anterior  tentacles  and  a  posterior  sucker.  They  are  parasitic  on  Crustacea, 
molluscs,  and  turtles,  and  are  regarded  by  some  as  a  distinct  order. 
American  genera  of  PolystomeaB  are  Epibdella,  Polystomum,  Tristoma, 
Sphyranura,  Microcotyle. 

Order  II.  Distomeae  (Digenea). 

The  entoparasitic  Trematodes  occur  largely  in  the  digestive 
tract  and  its  appendages ;  more  rarely  in  blood-vessels,  urogenital 
organs,  and  ccelom  of  vertebrates  and  other  animals.  As  inhabitants 
of  the  dark  they  have,  with  few  exceptions,  lost  the  eyes,  which 


//.    TREMATODA:  DISTOME/E. 


'275 


appear  in  larval  life,  and  not  always  then.  Since  not  exposed  to 
danger  of  being  pulled  from  the  host,  they  possess  either  the  oral 
sucker  alone  (Monostomum)  or  this  and  a  second  ventral  sucker, 


FIG.  236  -Development  of  D.stomum  hepaticum.  (From  Korschelt-Heider  after  Leuck- 
art )  A  young  larva;  £,  sporocyst  from  the  lung  of  Limncea;  ( ,,  older  sporocvst 
with  redue;  Z>,  redia  which  has  produced  rediae  internally  ;  E,  redia  with  cer- 
cariee;  F,  cercaria  :  G.  encysted  Distomum.  ^j,  eye  spot;  Z>,  digestive  tract;  Z>r, 

open- 


and  only  rarely  other  attaching  apparatus.  They  are  markedly 
separated  from  the  Polystomes  by  their  life  history.  The  alterna- 
tion of  hosts  necessitated  by  the  endoparasitic  life  is  complicated 
by  an  alternation  of  generations  (better  heterogony,  p.  145)  with 


276  PLATHELMINTIIES. 

metamorphosis.       To   illustrate   this   the    history    of   Distomum 
hepatwum  of  the  sheep  is  chosen  (fig.  236). 

The  eggs  leave  the  maternal  uterus  before  embryonic  develop- 
ment is  begun,  pass  down  the  bile  ducts  and  thence  by  the  intes- 
tine to  the  exterior.  They  must  come  into  water  and  remain  here 
awhile  before  the  ciliated  larva  (<  miracidium/  A)  escapes  by  a 
lifting  of  the  lid  of  the  shell.  This  larva  bores  its  way  into  a 
small  snail  (sp.  of  Limncea),  where  it  grows  into  a  '  sporocyst'  (B). 
The  sporocyst,  a  muscular  sac  with  protonephridia  but  lacking  all 
other  organs,  produces  in  its  interior  eggs  which  develop  into  a 
second  reproductive  sac,  the  '  redia '  (D).  These  are  distinguished 
from  the  sporocysts  by  the  possession  of  pharynx  and  a  tubular 
intestine  as  well  as  a  birth-opening  for  the  escape  of  the  young  pro- 
duced inside.  According  to  the  season  these  young  are  either 
'cercariae'  (F)t  or  several  generations  of  rediae  may  follow  before 
the  cercariae  appear.  The  cercarias  are  adapted  for  an  aquatic  life, 
since  each  has,  besides  the  characteristic  organs  of  a  Distomum 
(genitalia  excepted),  a  strongly  vibratile  tail.  The  cercariae  escape 
from  the  snail,  swim  about  in  the  water  until  the  tail  drops  off, 
•  when .  they  encyst  on  water  plants.  When  these  encysted  young 
are  eaten  by  sheep  along  with  the  vegetation,  infection  follows. 

In  general  it  can  only  be  said  of  the  life  history  of  other  Trematoda 
that  the  miracidia  must  penetrate  a  mollusc,  and  that  the  different  species 
have  many  modifications  :  (1)  Ordinarily  development  begins  in  the  ma- 
ternal uterus.  (2)  Many  miracidia  are  naked  or  only  partly  ciliated.  (3) 
In  many  species  the  miracidia  only  hatch  when  the  egg  is  taken  into  the 
stomach  of  a  snail  along  with  food.  (4)  Very  frequently  the  cercaria 
passes  from  the  water  into  a  new  host  (mollusc,  arthropod,  or  vertebrate) 
and  becomes  encysted  here.  In  such  cases  there  are  three  hosts  in  the 
cycle.  (5)  On  the  other  hand  the  history  may  be  simplified,  as  when  the 
sporocyst  in  the  snail  produces  directly  *  cercariae  without  tails  '  (i.e.,  small 
Distoma),  which  only  need  to  be  eaten  by  the  definitive  host  to  reach  the 
sexually  mature  condition.  (6)  It  is  doubtful  if  the  sporocyst  may  be 
omitted  and  the  miracidia  develop  directly  into  redia. 

As  the  adjacent  scheme  shows,  the  typical  development  is  distributed 
among  three  hosts  by  the  intercalation  of  a  second  aquatic  interval.  It 
consists  of  two  generations  ;  one  extends  from  the  fertilized  egg  to  the 
sporocyst,  the  second  begins  with  the  unfertilized  egg  of  the  latter  and  de- 
velops, through  the  cercaria  and  the  encysted  Distomum,  into  the  sexually 
mature  individual.  There  is  no  sexual  reproduction  by  fission  or  budding, 
rather  an  alternation  of  sexual  and  parthogenetic  generations  or  heter- 
ogony.  Columns  a  and  c  show  how  the  history  maybe  simplified  and  com- 
plicated. 

Best  known  of  the  Distomeae  are  the  following:  Distomum  (Fasciolaria) 
hepaticum,  the  liver  fluke  (fig.  232),  about  the  size  and  shape  of  a  pump- 


II.    TREMATODA:   DISTONE^I. 


277 


kin-seed.     It  lives  in  the  bile-ducts  of  sheep,  cows,  pigs,  etc.,  and  rarely 
(twenty  known  cases)  of  man.     It  stops  up  the  ducts  and  causes  a  disease 

DEVELOPMENT   OF   DISTOME^E. 
(a)  Simple  (ft)  Ordinary  (c)  Complicated 


* 


Larva 

Water 

fl  f 

Larva 

Water 

-    I 

Larva 

Water 

o 

1 

.2    ! 
ta  •{ 

Sporocyst 

Host  I 

Mollusc 

V 

C 

Sporocyst, 
perhaps 
also  redia 

Host  I 
Mollusc 

Gener 

Sporocyst 

Host  I 
Mollusc 

|^ 

Redia 

M 

«.2 

I-H 

Cercaria 

Water 

_f 

Cercaria 

Water 

Encysted 
Distomum 

Hoet  I 

d 

.2 
g-t 

Encysted 
Diatomum 

Host  II 

. 

Encysted 
Distomum. 

Host  II 

Sexually 
Mature 

Host  II 

3 

O 

Sexually 
Mature 

Host  III 

9 

d 

Sexually 
Mature 

Host  III 

Distomum 

Distomum 

1 

Distomum 

known   as   'liver  rot,'  generally    resulting    in  death.    The    history    as 
described  above  shows  why  sheep  pastured  in  moist  places  are  subject  to 
tho  disease,  and  why  wet  seasons  are  times  of  epidemics.     Thus  in  the 
rainy  year  of  1830  about  one  and  a  half  mil- 
lions of  sheep   were  killed  in  England  ;   in 
1812,  300,000  in  the  neighborhood  of  Aries, 
France.     This  species   is   frequently   accom- 
panied by  D.  lanceolatum,  less  than  half  an 
inch  in  length  (fig.  233). 

Bilharziahcematobia  is  a  human  parasite, 
most  common  in  hot  climates,  and  especially 
so  among  the  Fellahin  of  Egypt.  The  sexes 
are  separate.  The  male,  half  an  inch  long,  by 
inrolling  of  the  ventral  side  (fig.  237)  forms 
an  incomplete  canal  (canalis  gynsecophorus) 
in  which  the  more  slender  female  usually  lies. 
These  united  worms  occur  in  the  portal  vein 
and  connected  vessels.  They  follow  these 
vessels  in  either  direction  and  lay  their  eggs 
in  the  mucous  membrane  of  the  ureters  and 
urinary  bladder,  as  well  as  in  liver  and  intes- 
tine. The  suppurative  sores  of  the  urinary 
tract  cause  albuminuria  or,  by  hemor- 
hage,  haematuria.  Diagnostic  of  the  dis- 
ease is  the  presence  of  the  eggs,  each  with  a 
spine,  in  the  urine.  Several  other  species 
occur  in  man,  among  them  D.  carnosum*  and  Z>.  westermanni*  in 


Fio.  237. — Bilharzia  hcematobia. 
Female  in  the  gynaecophoral 
canal  (c)  of  the  male;  s',  s", 
anterior  and  posterior  suckers. 


278  PLATHELMINTHES. 

America.  Other  species  occur  encysted  in  man,  two  (D.  ophthalmobius  and 
Monostomum  lentis)  in  the  capsule  of  the  lens  and  in  the  lens  itself.  The 
genus  Amphistomum  is  common  in  the  intestine  of  Ungulates,  one  species, 
A.  hominis,  occurring  in  man.  With  few  exceptions  the  adult  stages  of 
all  Distomes  occur  in  vertebrates,  the  larval  stages  in  molluscs.  Aquatic 
birds  are  very  apt  to  be  infested  with  them,  and  "  it  may  be  of  interest  to 
gourmets  to  know  that  the  trail  of  a  woodcock  largely  consists  of  distomic 
Trematodes." 

Class  III.  Cestoda. 

The  majority  of  the  cestodes,  and  especially  those  of  the  human 
intestine,  are  distinguished  from  the  similarly  entoparasitic  trema- 
todes  in  a  striking  manner.  But  the  boundaries  between  the  two 
groups  disappear  in  certain  forms  like  Archigetes,  Caryophyllceus, 
and  AmpUilina,  parasitic  in  lower  vertebrates  or  invertebrates  and 
which  are  now  assigned  to  the  trematodes,  now  to  the  cestodes. 
The  most  important  character  of  the  cestodes  is  that  as  a  result  of 
their  parasitic  life  they  have  lost  the  last  traces  of  an  alimentary 
canal,  and  are  nourished  by  the  juices  or  the  partially  digested 
food  of  the  host,  since  the  fluid  nourishment  is  taken  in  through 
the  skin  into  the  body  parenchyma.  It  is  a  disputed  question 
whether  the  cuticula  of  the  surface  is  penetrated  with  pores  for 
this  purpose. 

Two  other  characters  are  so  striking  that  they  are  among  the 
first  thought  of.  (1)  The  differentiation  of  two  developmental 
stages,  the  bladder  worm,  or  cysticercus,  living  chiefly  in  paren- 
chymatous  organs  (muscles,  liver,  brain),  and  the  sexually  mature 
animal,  living  as  a  parasite  in  the  alimentary  tract;  (2)  the  division 
of  the  body  of  the  adult  into  different  parts,  the  head  or  scolex, 
and  following  this  a  series  of  joints  or  proglottids.  Since  this  last 
feature  holds  for  all  human  tapeworms  and  hence  for  the  best 
known  species,  the  following  description  begins  with  these  typical 
forms. 

The  sexually  mature  tapeworm  or  strobila  (fig.  238)  consists  of 
a  single  scolex  in  front,  and  behind  this  follow  in  a  single  row  the 
proglottids.  The  number  of  these  last  varies  from  smaller  forms 
(Tcenia  ccliinococcus,  fig.  252)  with  three  or  four  to  several  hun- 
dreds or  even  several  thousands,  a  fact  which  speaks  for  the  enor- 
mous size  of  some  species.  The  proglottids  are  derivatives  of  the 
scolex,  from  the  hinder  end  of  which  they  become  separated  by  a 
kind  of  budding.  This  explains  the  well-known  fact  that  the  body 
is  not  rid  of  the  tapeworm,  so  long  as  the  head  remains  in  the 
host.  It  also  explains  the  peculiar  shape  of  the  worm,  which  is 


III.    CESTODA. 


279 


almost  thread-like  in  front,  increasing  posteriorly  to  a  broad  band, 
whence  the  common  name.  At  first  the  proglottids  are  small; 
they  increase  by  individual  nourishment  to  considerable  size,  and 


FIG.  238. 


FIG.  239. 


FIG.  238.— Tcenia  saginata.  (From  Boas,  after  Leuckart.)  Head  with  series  of  pro- 
glottids taken  from  various  regions  of  the  strobila. 

FIG.  239. — Nervous  system  of  Monezia.  (After  Tower.)  a,  suckers  ;  e,  excretory  tubes; 
p,  cerebral  ganglia.  Nerves  black. 

separate  from  the  hinder  end  of  the  chain  and  live  separately  when 
a  certain  measure  of  development  is  reached.  For  example,  the 
young  proglottids  of  the  human  tapeworm,  Tcenia  solium,  are  0.5 


280  PLATHELMINTHES. 

mm.  broad  and  0.01  mm.  long;  the  ripe  proglottids  at  the  end  are 
elongate  oval,  5  mm.  broad  and  12  mm.  (half  an  inch)  long. 

Head  and  proglottids  have  certain  common  characters.  Their 
connective-tissue  parenchyma  contains  numerous  spherical  con- 
cretions of  lime,  and  consists  of  cortical  and  medullary  sub- 
stance. The  first  contains  to  a  marked  degree  the  muscles,  the 
latter  the  other  organs.  Nerves  and  water-vascular  system  extend 
through  the  whole  length  of  the  worm.  In  the  head  is  the  paired 
cerebral  ganglion  of  the  flatworms  (fig.  239),  sometimes  fused  to  a 
single  mass  by  the  great  development  of  the  commissure  or  partially 
concealed  by  accessory  parts  connected  with  attachment  (fig. 
242).  From  the  brain  two  principal  nerves  run  backwards,  usually 
near  the  edges  of  the  proglottids  (fig.  244,  N).  The  water-vascu- 
lar (excretory)  system  begins  with  a  capillary  network  richly 
provided  with  flame  cells.  It  extends  through  head  and  proglottids ; 
usually  four  main  trunks  are  present,  two  being  less  developed  and 
it  is  possible  are  sometimes  absent.  The  two  chief  trunks  are  fre- 
quently connected  by  a  cross-trunk  on  the  hinder  margin  of  each 
proglottid  (fig.  244).  The  system  opens  on  the  posterior  edge  of 
the  last  proglottid,  but  accessory  mouths  may  occur  on  other 
proglottids. 

The  scolex  and  proglottids  are  distinguished  by  the  facts  that 
the  proglottids  contain  the  sexual  organs,  while  the  scolex  bears 
the  anchoring  apparatus,  for  the  latter  has,  besides  producing 
proglottids,  to  fasten  the  worm  in  the  intestines.  Most  important 
of  the  adhesive  organs  are  the  suckers  (acetabula) ;  less  important 
are  the  hooks,  which,  in  numbers,  are  either  arranged  in  a  circle  or 
are  borne  on  protrusible  and  retractile  probosces  (fig.  240—242). 

When  a  circle  of  hooks  is  present  it  is  on  the  anterior  end  and  is  moved 
by  a  special  apparatus,  the  rostellum.  This  is  a  plug  of  complexly  arranged 
muscles  (fig.  242)  which  can  arch  and  flatten  the  central  area.  In  many 
species  the  arching  is  increased  by  a  muscular  sheath,  the  flattening  by 
retractors.  Each  hook  has  its  point  outwards  and  its  base  with  two  roots, 
one  of  which  rests  on  the  rostellum;  the  protrusion  of  the  rostellum  forces 
the  points  outwards  into  the  mucous  membrane  of  the  intestine.  In  some 
Tcenice  without  the  circle  of  hooks  (T.  saginata)  the  rostellum  is  replaced 
by  a  sucker-like  depression.  Since  the  rostellum  arises  in  development 
from  a  similar  cup,  it  may  be  a  modified  apical  sucker,  but  it  is  doubtful 
how  far  comparisons  may  be  made  with  the  oral  sucker  or  the  alimentary 
tract  of  the  trematodes. 

The  sexual  organs  are  hermaphroditic  and  are  present  in  num- 
bers equal  to  those  of  the  proglottids,  so  that  these  were  formerly 


III.    CESTODA. 


281 


regarded  as  sexual  individuals  of  a  colony,  each  with  its  own 
reproductive  apparatus.  Two  types  must  be  recognized.  In  the 
one  the  presence  of  vitellaria  and  the  separate  openings  of  uterus 
and  vagina  recall  the  conditions  in  trematodes,  while  in  the  second 


FIG.  240. 


FIG.  242. 


FIG.  240.— Apical  view  of  head  of  Tcenin  solium.    (From  Hatschek.) 

FIG.  241.— Head  of   'Jetmrhuuchus  viridis.    (After  Wagner.)    Dissected  to  snow  the 

internal  parts  of  the  proboscides  (<>)  and  the  ganglion  (a). 
FIG.  242.— Schema  of  action  of  rostellum.    On  the  right  the  hooks  are  exserted  for 

adhesion,  on  the  left  retracted,    r,  rostellum;  s,  sheath;  /,  longitudinal  muscles. 


#2  od  sd  dg  u  ov 

FIG.  243.— Proglottis  of  Bothriocephalus  latus.  (After  Sommer.)  Right  only  vitel- 
larium,  left  only  testes.  shown,  cb,  cirrus  sheath  opening  with  the  vagina;  d<7, 
vitelline  duct:  dt,  vitellarium:  /i,  testes;  od,  oviduct;  ov,  ovary;  sd,  shell  gland;, 
tt,  uterus;  vci,  vagina;  vd,  vas  deferens  (dark-lined);  10,  excretory  canal. 

the  uterus  ends  blindly  and  the  vitellaria  are  modified  into  a  small 
albumen  gland.  Since  vagina  and  vas  deferens  almost  always  open 
together,  self -impregnation  is  possible.  Besides  cross-fertilization 
of  separate  proglottids  has  been  seen.  The  general  features  of  the 
two  types  may  be  made  out  from  figures  243  and  244,  reference 


282 


PL  A  THELMINTHES. 


Fia.  244.—  Proglottid  of  Tceiiia  sac/innta,  near  maturity.  (After  Sommer.)  rfo,  cirrus 
sheath;  d£,  vitellarium  ;  fc,  genital  pore;  JV,  nerve  cord;  Keph^  excretory  canal ; 
ov,  ovary;  rs,  receptaculum  seminis;  sdr,  shell  gland;  £,  testes;  it,  uterus;  vd,  vas 
deferens. 


FIG.  245.— Eggs  of  parasites  from  the  human  intestine,  enlarged  400  diameters.  (From 
Leuckart.)  a,  Ascaris  lumbricoides;  6,  c,  Oxi/uris  vermicularis ;  d,  Trichocephalus 
dispar;  e,  Dochmius  duodenalis;  f,  Dislomum  hepaticum;  g,  Dist.  lanceolatum;  h, 
Tcenia  solium;  i,  T.  saginata;  k,  Bothriocephalus  latus. 


777.    CESTODA. 


283 


being  made  to  the  description  of  the  organs  in  the  trematodes  (p. 


The  difference  in  the  sexual  apparatus  has  its  influence  on  the 
peculiarities  of  the  egg  (fig.  245).  In  BothriocepJialus  it  is  large 
(&),  has  a  tough  shell  with  a  lid,  and  encloses  a  small  egg  cell 
with  numerous  yolk  cells.  The  eggs  of  Tcenia  (h,  i)  are  small, 
with  a  layer  of  albumen  and  a  delicate  shell  which  is  lost  early.  It 
is  replaced  by  an  embryonic  shell,  a  radially  striped  envelope 
secreted  by  the  embryo  in  a  somewhat  advanced  stage.  It  is  in 
this  condition  that  one  usually  sees  Taenia  eggs. 

A  further  consequence  is  a  difference  in  development.  In  most 
Bothriocephalidse,  as  in  the  Trematoda,  the  egg  must  enter  the 
water  for  its  further  development.  Here  a  ciliated  oval  larva 
escapes  which  contains  a  six-hooked  larva  (oncosphcera,  fig.  2 


FIG.  246.— Development  of  Bothriocephalus  latus  (From  Leuckart.)  A,  ciliated 
larva ;  #,  same  with  escaping  six-hooked  larva ;  CY,  young  encysted  Bothrio- 
Qephalus. 

The  ciliated  envelope  is  temporary  and  is  cast  off  like  the  ciliated 
coat  of  the  trematode  larva.  The  six-hooked  larva  JD  some 
unknown  way  enters  a  fish,  becomes  encysted  (pleurocercoid)  in 
muscles  or  viscera,  and  changes  directly  into  the  head  of  a  Both- 
riocephalus. This  on  being  taken,  in  feeding,  into  the  intestine 
of  the  proper  host  develops  into  the  adult. 

The  longer  and  better  known  history  of  the  Tmnias  differs  con- 
siderably. The  distinctions  are  early  recognizable,  since  the  six- 
hooked  larva  lacks  the  ciliated  envelope  but  is  enclosed  in  its 
homologue,  the  embryonic  shell  already  alluded  to.  Since  this 
envelope  cannot  open  of  itself,  the  young  must  be  freed  from 
it  by  its  digestion  in  the  stomach  of  the  proper  intermediate  host. 
Thus  the  eggs  of  Tcenia  solium  must  pass  into  the  stomach  of  the 
pig  (they  are  taken  by  admixture  of  the  food  with  embryos  con- 
tained in  faecal  matter)  and  after  being  freed  from  their  shell  in 


284 


PL  A  THELMINTHES. 


the  stomach  the  larvae  with  their  six  hooks  bore  through  the  intes- 
tinal wall  and  migrate,  using  the  blood-vessels  in  their  course,  into 
the  muscles,  or  more  rarely  other  organs.  Here  they  develop 
into  bladder  worms  (cysticerci).  In  this  they  become  oval  and 
secrete  a  cyst  to  which,  as  a  foreign  body,  the  pig  adds  an  envelope 
of  connective  tissue.  The  cysticercus  blastema  grows  through 


Fio.  247.— Structure  and  development  of  the  cysticercus  (C.  cellulosae  of  Tcenia 
solium).  a, measly  meat,  natural  size;  below  an  escaped  cysticercus;  6,  cysticer- 
cus, with  exserted  scolex,  enlarged;  c-e,  development  of  the  scolex,  more  en- 
larged; c,  young  cysticercus  with  blastema  of  scolex  (above)  and  water- vascular 
net ;  d,  e,  different  stages  of  scolex  in  receptaculum,  the  cysticercal  wall  mostly 
removed. 

increase  of  cells,  but  more  by  the  infiltration  of  serous  fluid,  so 
that  it  becomes  distended  into  a  delicate  translucent  vesicle.  So 
abundant  can  this  be  that  in  T.  solium  the  microscopically  small 
embryo  can  grow  in  three  or  four  months  to  the  size  of  a  bean  or 
pea;  in  other  species  as  large  as  a  hen's  egg.  By  invagination  the 
wall  of  the  bladder  produces  the  blastema  of  the  scolex  (fig.  247,  c). 
This  has  at  first  a  sac-like  shape,  but  soon  increases  in  length,  its 
growth  being  confined  by  an  envelope,  the  receptaculum  (d),  so 
that  it  is  bent. 

At  the  apex  of  this  blind  sac  arises  the  characteristic  armature 
of  the  scolex  which  makes  it  possible  to  say  what  tapeworm  will 
come  from  the  cysticercus.  Thus  in  T.  solium  there  are  four 
suckers  and  a  crown  of  hooks.  These  parts  are  at  first  inverted 


III.    CESTODA.  285 

and  only  come  to  their  definitive  position  on  the  outside  of  the 
scolex  when  the  latter  is  protruded  as  one  would  turn  out  the 
finger  of  a  glove.  The  further  development  follows  when  the 
cysticercus  is  taken  into  the  stomach  of  the  new  host.  When 
man,  for  instance,  eats  infected  ('measly')  pork,  the cysticerci  are 
freed  by  action  of  the  digestive  juices  and  later  the  scolex  is 
everted.  The  embryo  passes  to  the  intestine,  becomes  attached 
and,  surrounded  by  nourishment,  begins  to  grow,  the  bladder 
remaining  attached  to  the  hinder  end,  and  soon  the  formation  of 
proglottids  begins  in  the  middle  piece  connecting  the  bladder  with 
the  scolex.  So  rapid  is  the  growth  that  in  ten  or  twelve  weeks 
Tcenia  solium  begins  to  set  proglottids  free. 

In  cases  where  the  bladder  reaches  a  considerable  size  it  has  the  power 
•of  producing  more  than  a  single  scolex.  The  bladder  of  Coennms  cerebralis, 
which  lives  in  the  brain  of  sheep,  produces  hundreds  of  scolices.  The  num- 
ber is  even  greater  in  Tcenia  echinococcus,  in  which  the  bladder  increases 
by  budding  for  some  time,  and  by  the  formation  of  numerous  daughter 
bladders  produces  marked  tumors  in  the  liver  of  man  and  domestic  animals, 
before  the  formation  of  scolices  begins.  In  the  interior  of  each  daughter 
vesicle  appear  a  number  of  brood  vesicles,  each  of  which  produces  numbers 
of  scolices,  so  that  from  a  single  six-hooked  embryo  thousands  of  scolices 
can  arise  (fig.  253).  This  extreme  case  stands  in  contrast  to  others  which 
connect  with  the  development  of  Bothriocephalus,  in  which  the  cysticercus 
is  replaced  by  a  cysticercoid  (fig.  248).  Here  there  is  no  infiltration  and 
the  scolex  is  closely  enclosed  by  an  envelope  comparable  to  the  bladder 
wall. 

All  of  this  is  of  importance  in  the  correct  conception  of  the  development 
of  a  tapeworm,  which  was  earlier  believed  to  be  a  complicated  alternation 
of  generations;  the  bladder  to  be  a  stage  which  by  endogenous  budding 
produced  scolices;  the  scolex,  in  turn,  a  stage  which  by  terminal  budding 
produced  the  sexual  animals,  the  proglottids,  and  the  tapeworm  itself  a 
•chain  of  individuals,  a  strobila.  This  view,  so  easy  to  learn,  so  easily  ex- 
plaining the  development,  contains  two  errors.  The  bladder  is  not  an  inde- 
pendent generation,  but  only  the  precocious  hinder  end  of  the  scolex.  The 
tapeworm  is  not  a  colony,  but  a  single  animal;  the  proglottids  are  not  in- 
dividuals, but  specialized  parts  of  a  single  whole.  This  view  is  confirmed 
by  a  comparison  with  other  forms.  The  Caryophylla3ida3  (fig.  249)  are 
single  bodies,  the  anterior  end  elongate  and  taking  the  place  of  the  scolex, 
while  the  broader  hinder  part  contains  a  single  hermaphroditic  apparatus. 
In  the  Ligulida3  the  body  is  still  unjoiuted,  but  has  increased  in  length  and 
contains  numerous  sets  of  sexual  organs.  This  duplication  of  the  repro- 
ductive apparatus  explains  the  appearance  of  proglottids. 

Family  1.  CARYOPHYLL^ID^:  (Cestodaria).  Cestodes  without  ace- 
tabula,  simple  sexual  apparatus,  scolex  and  proglottis  not  differentiated. 
Distinguished  from  tremat9des  by  absence  of  digestive  tract.  Larval  stages 
in  invertebrates,  adults  nearly  always  in  fishes.  Caryophyllceus  (fig.  249) 


286 


PL  A  TIIELMINTIIES. 


in  the  intestine  of  cyprinoids;   AmpMlina  in  body  cavity  of  sturgeon; 
Areliigetes  in  annelids  (Samuris). 

Family  2.  LIGULID.E.  No  acetabula;  numerous  sexua.l  organs,  but  no 
proglottids.  The  immature  stages  in  the  body  cavity  of  fishes,  the  adults, 
in  the  intestine  of  birds.  Ligtila. 


FIG.  248 


FIG.  350.  FIG. 

FIG.  248  — Cysticercoid  in  invaginated  and/extended  condition  from  Arion  ater* 
(From  Hatschek.) 

FIG.  249.—Ca1ryophyllceusmutabilis.  (After  M.  Schultze.)  df,  vasdeferens;  dv,  vitelline 
duct;  fc,  scolex;  ov,  ovaries;  ps,  penis;  v.s,  vagina  with  receptaculum  seminis;  f, 
testes;  nt,  uterus;  vi\  yitellarium  ;  v.s,  yesicula  seminalis.  The  connexion  of 
vagina  with  the  crossing  point  of  genital  duct,  vitelline  duct,  and  uterus  is 
lacking  in  the  figure. 

FIG.  250. —  Tapeworms  of  fishes.  (After  Linton.)  A,  Echinobothrium  variabile  *;  B, 
Rynchobothrium  bisulcatum  *  ;  C\  Tetrabotkrium* 

Family  3.  TETRARHYNCHID.E.  With  scolex  and  proglottids,  the  head 
with  four  protrusible  hooked  probosces  (fig.  241).  Immature  and  mature 
stages  in  fishes.  Tetrarhynchus,  Rynchobothrium.* 

Family  4.  TETRAPHYLLID^E.  Head  with  four  very  mobile  suckers,  often 
armed  with  hooks.  Echinobothriiun  *  (fig.  250),  Acanthobothrium.* 


III.    CESTODA. 


287 


Family  5.  BOTHRIOCEPHALID.E  Scolex  and  proglottids  present;  head 
spatulate  with  two  sucking  groves  on  the  narrower  sides.  Most  interesting 
is  Bothriocephalus  latus  (fig.  251),  the  largest  tapeworm  which  occurs  in 
the  human  intestine  (also  dogs  and  cats),  and  which  may  reach  a  length  of 
forty  feet  and  consist  of  over  four  thousand  proglottids.  As  has  been  out- 
lined above,  the  pleurocercoid  occurs  in  fishes,  and  man  acquires  the  parasite 
by  eating  uncooked  fish.  It  is  especially  abundant  in  Russia,  the  eastern 
provinces  of  Prussia,  and  in  Switzerland.  It  is  rare  in  America  and  occurs 
most  frequently  in  immigrants.  Other  species  occur  in  man  in  Greenland 
(R  cordatus)  and  China  (B.  mansoni). 


FIG.  251.— Head  and  ripe  proglottids  of  Bothriocephnlus  Intus,  the  head  showing  the 
sucker  at  the  angle,  the  proglottids  the  marking  produced  by  the  uterus. 

Family  6.  T.ENIAD.E.  With  scolex  and  separable  proglottids;  thescolex 
always  bears  four  suckers  and  in  many  a  ro- 
stellum  with  a  circle  of  hooks  (fig.  252).  In 
the  proglottids  the  vitellarium  is  replaced  by 
an  albumen  gland;  the  uterus  is  caecal,  and  the 
genital  pore  occurs  usually  laterally  in  the 
proglottids,  alternating  right  and  left,  rarely 
only  on  one  side  (Hymenolepis,  Anoplocepha- 
lus).  It  is  rarely  doubled  in  a  proglottid 
(Dipylidium,  Moniezia).  Intermediate  stage  a 
cysticercus  or  cysticercoid.  The  human  tape- 
worms are  grouped  here  together,  but  are  sub- 
divided accordingly  as  the  sexual  animal  or 
the  cysticercus  has  been  found  in  man. 

A.  Tcenice  sexually  mature  in  the  human 
intestine.  Most  noticeable  are  Tcenia  solium 
and  T.  saginata,  the  differences  between 
which  are  shown  in  fig.  252  and  the  follow- 
ing table.  It  is  to  be  noticed  that,  in  spite  of 
the  lack  of  hooks,  the  stronger  suckers  render 
T.  saginata  more  difficult  to  expel.  Tcenia  FIG.  252.— Head  and  ripe  pro- 
solium,  as  the  table  shows,  is  not  rare  in  the  f ^  ?'  ioiiunT™  S"ff"1" 
cysticercus  stage  in  man  and  occurs  sometimes  in  places,  like  the  brain 
and  eyes,  where  it  causes  severe  injury.  These  cases  are  in  part  explained 
by  lack  of  cleanliness  in  the  food,  which  may  contain  eggs,  but  it  is  possible 
through  internal  infection  ;  pieces  of  the  worm  passing  the  pylorus  and 
entering  the  stomach,  where  they  are  digested,  setting  the  embryos  free. 


288 


PL  A  THELMINTHES. 


*3 

Length  (n}  of 

the  worm 

Character 

Occurrence 

Head 

2>£ 

Uterus 

and  (h)  of  the 

of 

of 

?  S 

ripe 
proglottids 

Cysticercus 

Cysticercus 

§ 

With  rostel- 
lum  and 
circle  of 
hooks 
(26  in  2  rows); 
4  weak 

1 

Each  side 
with  7-9  large 
branched 
pouches 

a.  10  feet, 
h.  9-11  mm. 
long,  6-7  mm. 
broad 

6-20  mm., 
with 
abundant 
fluid 

In  pigs,  occa- 
sionally in 
muscles, 
brain,  and 
eyes  of  man, 
rarely  in 

* 

suckers 

mammals 

Sterna 
saginata 

No  rostellum  ; 
no  hooks  ; 
4  strong 
suckers 

i 

Each  side 
with  20-30 
delicate  little 
branched 
pouches 

a.  20  to  25  feet 
and  more, 
b.  18-20  mm. 
long,  5-7 
mm.  broad 

4-8  mm., 
tough,  with 
little  fluid 

Cattle 

Many  other  Tcenice,  which  are  common  to  other  mammals,  occur  occa- 
sionally in  the  human  intestine.  In  mice  and  rats  occur  T.  (Hymenolepis} 
murina  and  T.  diminuta  (=  leptocephala).  The  first  (identical  with  T. 
nana)  has  recently  been  very  abundant  in  human  intestines  in  Italy.  The 
worm,  an  inch  or  two  long,  may  occur  in  thousands  and  cause  severe  in- 
jury. This  species  may  develop  without  an  intermediate  host ;  the  eggs 
taken  into  the  stomach  pass  the  cysticercoid  stage  in  its  walls  and  thence 
to  the  intestine  to  become  adult.  T.  diminuta  (=flavopunctata),  which 
has  insects  for  its  intermediate  host,  has  been  described  from  man.  Other 
species  occur  in  the  tropics. 

B.  Forms  passing  the  Cysticercus  stage  in  man.  Besides  the  Cysticercus 
cellulosae  of  T.  soliitm  that  of  T.  acanthotrias 
(possibly  identical  with  T.  solium)  has  been 
found  in  man.  More  frequent  and  of  more  im- 
portance to  the  physician  is  the  Cysticercus  of 
Tcenia  echinococcus  (fig.  253),  which  lives  as  an 
adult  in  the  dog,  and  is  easily  overlooked  on  ac- 
count of  its  size.  It  is  at  most  5  mm.  (£  inch)  long 
and  consists  of  a  scolex  and  three  or  four  pro- 
glottids. The  scolex  bears  four  suckers  and  hooks 
on  the  rostellura.  When  the  eggs  are  taken  into 
the  human  stomach,  as  may  easily  happen  by 
stroking  and  kissing  infected  dogs,  the  embryos 
are  set  free  and  wander  into  liver,  lungs,  brain, 
or  other  organs  and  produce  here  tumors  which, 
*n  tne  case  °^  *ne  ^ver'  may  weigh  ten  or  even 
Right  sexually  mature;  thirty  pounds.  This  extraordinary  size  is  ex- 
left  a  part  of  an  echmo-  •  •  «  i_  .»  .«  j.-  c  j 
coccus  with  two  brood  plained  by  the  formation  of  daughter  bladders 

licesUleS  and  their  8C°"  (echinococcus)  described  above.     Echinococci  are 
more  common  in  cattle,  sheep,  and  swine  than  in  man. 

Common  Tcenice  of  domestic  animals  are  in  the  horse  Anoplocephala 
plicata  (4  to  30  inches),  A.  perfoliata  (£  to  3  inches),  A.  mamillana  (i  to 


IV.   NEMERTINI. 


289 


2  inches);  in  ruminants,  Moniezia,  expansa  (usually  7  feet,  sometimes  30 
feet  or  more),  often  fatal,  M.  denticulata  (1  to  5  feet),  the  most  common 
tapeworm  of  cows;  in  dogs,  Tcenia  marginata  (cysticercus  in  sheep  and 
swine),  T.  serrata  (cysticercus  in  rabbits),  T.  echinococcus  (above),  T.  COB- 
nurus  (cysticercus  in  brain  of  sheep,  causing  the  disease  called  *  stag- 
gers'),  Dipylidium  cucumerina  (most  common,  larva  in  the  dog-louse, 
Trichodectes);  in  the  cat,  Tcenia  crassicollis  (cysticercus  in  mice).  Several 
species  occur  in  domestic  birds,  one  (Drepanidotcenia  infundibuliformis), 
causing  epidemics  among  chickens. 

Class  IV.  Nemertini. 

Most  nemerteans  are  of  appreciable  size,  some  reaching  a  length 
of  a  yard  or  more  (Linens  longissimus  90  feet  !),  and  yet  they  are 
so  contractile  that  a  specimen  of  our  Cerebratulus  lacteus,  which 
can  extend  itself  to  fifteen  feet,  can  retract  to  two.  Nemerteans 
are  rare  in  fresh  water  or  moist  earth,  but  are  most  abundant  in  the 
sea,  where  they  burrow  through  the  mud  or  lie  rolled  up  beneath 
stones.  Many  are  noticeable  for  their  bright  colors.  Their  system- 
atic position  is  a  problem.  Frequently  they  are  included  in  the 
Plat  helm  in  thes,  but  the  presence  of  an  anus,  of  distinct  vascular 
system,  and  the  higher  organization  in  other  respects  renders  such 
a  position  doubtful. 

Like  some  flatworms  they  have  a  solid  parenchyma  bounded 
externally  by  a  ciliated  ectoderm  rich  in  mucus  cells,  and  inside 
this  at  least  two  muscular  layers,  which,  when  but  two  are  pres- 
ent, are  an  outer  circular  and  an  inner  longitudinal  layer.  They 
differ  from  all  other  Plathelminthes  in  having  a  complete 


ps  pm 


FIG.  254. — Diagram  of  Nemertean  (orig.).  5,  brain;  c,  ciliated  pit;  d,  dorsal  nerve 
trunk ;  d/,  dorsal  blood-vessel ;  gc,  gastric  caeca ;  ?,  intestine ;  Z,  lateral  nerve 
trunk;  h\  lateral  blood-vessel;  p,  proboscis  retracted ;  pm,  proboscis  muscles; 
pn,  protonephridial  tube  ;  po,  its  opening  ;  ps,  cavity  of  proboscis  sheath. 

alimentary  tract,  beginning  with  a  ventral  anterior  mouth  and 
continuing  as  a  straight  tube,  with,  usually,  paired  diverticula,  to 
the  vent  at  the  posterior  end  of  the  body  (fig.  254). 

Especially  diagnostic  is  the  proboscis,  which  lies  dorsal  to  the 
alimentary  tract  and  usually  opens  separate  from  the  mouth.     The 


290 


PL  A  THELMINTHES. 


f 


•nepA 


proboscis  is  a  muscular  tube  closed  at 
one  end  and  at  rest  is  infolded  like 
the  finger  of  a  glove  inside  a  closed  sac, 
the  proboscis  sheath,  which  extends  far 
back  in  the  body.  Its  tip  is  bound  to 
the  posterior  end  of  the  sheath  by  a 
retractor  muscle.  By  contraction  of 
the  sheath  the  proboscis  is  everted, 
while  it  may  be  retracted  again  by  the 
muscle.  Nettle  cells  are  not  uncom- 
mon in  the  proboscis  wall,  while  in  some 
forms  (the  older  Enopla)  the  effective- 
ness of  the  organ  is  increased  by  the 
presence  of  a  dart-like  stylet  at  the 
tip  (reserve  stylets  occur  on  either  side, 
fig.  255),  and  at  the  base  of  the  stylet 
is  the  opening  of  a  poison  sac. 

The  blood-vascular  system  consists 
of  a  pair  of  lateral  tubes  connected  by 
transverse  loops,  and  in  most  forms  a 
third  tube  is  present  lying  between  the 
intestine  and  the  proboscis  sheath. 
The  excretory  system  consists  of  two 
tubes  lying  close  beside  the  lateral  blood- 


FIG.  255.  1 10.  256. 

FIG.  255.— Young  Tetrastemma  obscurum.  (From  Hatschek,  after  M.  Schultze.)  a,  anus; 
cc,  dorsal  commissure;  eg,  cerebral  ganglia;  /,  ciliated  grooves;  i,  digestive  tract: 
Iv,  lateral,  mt>,  dorsal  blood-vessel;  nep/i,  water-vascular  tubes;  nZ,  lateral  nerve; 
oc,  eye  spot;  or,  proboscis  pore;  r,  proboscis;  r,,  glandular  hinder  portion  of  pro- 
boscis; rm,  retractor  of  proboscis;  sf,  stylets:  *,  opening  of  excretory  system. 

FIG.  256.— Pilidium  larva.  (From  Lang,  after  Salensky.)  eg,  invaginations  which 
later  give  rise  to  the  nemertine  skin;  w,  oral  lobes;  rad,  archenteron;  r?i,  ring 
nerve;  sp,  apical  plate;  st,  eesophagus;  wTc,  ciliated  band. 


IV.   NEMERTINL  291 

vessels  and  connecting  with  branches  terminating  in  flame  cells, 
while  they  open  separately  to  the  exterior  by  one  or  several  open- 
ings. 

The  central  nervous  system  (in  some  forms  still  in  the  ectoderm) 
consists  of  a  supracesophageal  brain  of  a  paired  ganglia,  from  which 
nerves  run  to  the  proboscis  and  two  lateral  cords  united  on  the 
ventral  side  by  numerous  transverse  commissures.  Connected 
with  the  brain,  either  directly  or  by  means  of  a  short  nerve,  are  the 
cerebral  organs  or  ciliated  grooves,  pits  placed  on  the  sides  of  the 
head.  These,  formerly  regarded  as  respiratory,  are  now  considered 
sense  organs.  Tactile  organs  and  simple  eyes  are  widely  distrib- 
uted ;  otocysts  are  very  rare. 

As  a  rule  the  nemertines  are  dioecious,  the  gonads  forming  a  row 
of  lateral  sacs,  alternating  with  the  intestinal  blind  sacs  and  open- 
ing dorsally.  The  development  is  sometimes  direct,  but  usually 
a  metamorphosis  occurs  in  which  a  larva,  the  pilidium  (or  a 
reduced  form  of  it,  Desor's  larva),  appears.  The  pilidium  is  a 
gelatinous  helmet-shaped  larva  with  right  and  left  below  a  pair  of 
lappets  (fig.  256).  The  margins  of  lappets  and  helmet  are  ciliated, 
while  at  the  top  a  bundle  of  longer  cilia  project  from  a  thick- 
ened patch  of  ectoderm,  the  apical  plate,  which  apparently  func- 
tions as  a  central  nervous  organ.  Inside  is  the  simple  caecal  arc  li- 
en teron,  the  mouth  ( blast opore)  opening  between  the  lappets. 
By  a  complicated  process  of  growth  and  infolding  this  mesenteron 
becomes  enclosed  in  its  own  skin,  produced  from  four  inpushings 
(es)\  an  anus  is  formed,  and  at  the  time  of  metamorphosis  the 
worm  thus  produced  escapes  from  the  rest  of  the  pilidium,  which 
quickly  dies. 

Order  I.  Protonemertini. 

Nervous  system  outside  the  muscles;  no  stylets  in  the  proboscis;  mouth 
behind  brain.  Carinella* 

Order  II.   Mesonemertini. 

Nervous  system  in  the  muscles;  mouth  behind  brain;  no  stylets. 
Cephalothrix.* 

Order  III.  Metanemertini. 

Nervous  system  in  the  parenchyma  inside  the  muscles,  mouth  in 
front  of  brain;  proboscis  as  a  rule  with  stylets.  Geonemertes*  and  some 
species  of  Tetrastemma*  terrestrial.  Amphiporus*  (numerous  eyes), 
Nectonemertes*  Malacobdella,*  leech-like  with  posterior  sucker,  parasitic 
in  lamellibranchs. 


292  PLATHELM1NTHES. 

Order  IV.  Heteronemertini . 

Body  wall  with  several  muscular  layers,  the  nervous  system  in  the 
muscles;  mouth  behind  brain;  proboscis  unarmed.  Linens,*  Micrura*  and 
Cerebratulus  *  (Meckelia)  on  our  coast,  with  cerebral  organs.  Eupolia. 

Summary  of  Important  Facts. 

1.  The  PLATHELMINTHES  are  bilateral  animals  of  flattened 
form  whose  nervous  system  consists  of  a  supracesophageal  ganglion 
and  lateral  nerve  trunks;  the  excretory  system  of  branched  water- 
vascular  tubes  (protonephridia). 

2.  The  TURBELLARIA  are  the  most  primitive;  the  Trematoda 
and  Cestoda  ha^e  descended  from  them. 

3.  The  Turbellaria  are  ciliated  externally.     They  have  no  anus 
and  no  circulatory  system.     The  digestive  tract  consists  of  cctoder- 
mal  pharynx  and  entodermal  stomach,  the  latter  many-branched 
in  the  Polyclads,  with  three  main  branches  in  the  Triclads,  and 
rod-like  in  the  Rhabdocceles. 

4.  Polyclads  and  Triclads  are  often  united  under  the  name 
Dendroccela. 

5.  In  the  parasitic  TREMATODA  the  cilia  are  entirely  lost  or 
confined  to  the  larval  stages.     Hooks  and  suckers  are  present  for 
attachment  to  the  host;  several  in  the  ectoparasitic  forms;  only 
one  or  two  suckers  in  the  internal  parasites. 

6.  In  the  DistomicB  there  occur  heterogony  and  alternation  of 
hosts.     From  the  egg  arises  a  sporocyst,  always  parasitic  in  mol- 
luscs, from  the   parthenogenetic  eggs  of  .which   develop  cercarise 
which  become  encysted  Distomia3  in  the  second  host,  sexual  Di- 
stomiae  in  the  third. 

7.  Best    known   of   the   Distoma   are  D.    liepaticum   and   D. 
lanceolatum  (rare  in  man,  common  in  sheep)  and  D.  hcematobium 
in  the  portal  vein  of  man  in  warm  climates. 

8.  The   CESTODA  are   characterized   by  the  entire  absence  of 
digestive  tract,  and  usually  by  the  existence  of  scolex  and  pro- 
glottids. 

9.  The  scolex  is  the  organ  of  attachment,  and  as  such  is  pro- 
Tided  with  suckers  and  frequently  with  hooks.     It  also  produces 
the  proglottids  by  terminal  budding. 

10.  The  proglottids  contain  an  hermaphroditic  sexual  apparatus. 

11.  The  eggs  produce  a  six-hooked  embyro  which  must  pass 
into  an  intermediate  host.     This  is  accomplished  either  by  taking 
the  eggs  in  passively  with  the  food,  or  the  embryo  must  pass  into 
the  water,  where  it  infects  fishes. 


ROTIFERA.  293 

12.  The  embryo,  in  the  intermediate  host,  becomes  encysted 
and  changes  directly  to  a  scolex  (plenrocercoid)  or  into  a  bladder 
worm  (cysticercus)  which  produces  internally  one  or  more  scolices. 

13.  The  scolex  is  freed  from  its  cyst  when  taken  along  with 
food  into  the  stomach  of  the  proper  host,  and  then  acquires  the 
capacity  of  development  into  a  tapeworm. 

14.  In  man  occur  as  cysticerci  Tcenia  echinococcus   (adult  in 
dog)  and  T.  solium;  as  adults  Tcenia  solium  (cysticercus  in  pigs), 
T.   saginata    (cysticercus    in   cattle),  and    Bothrioceplialus    latus 
(pleurocercoid  in  fish). 

15.  The  NEMERTINI  are  distinguished  by  a  complete  alimentary 
canal  with  anus,  and  a  proboscis  dorsal  to  the  digestive  tract. 


PHYLUM  V.  ROTIFERA  (ROTATORIA). 

The  aquatic  wheel  animalcules,  or  Rotatoria,  are  among  the 
smallest  Metazoa,  and  can  be  distinguished  from  the  Infusoria, 
which  they  resemble  in  habits,  only  by  the  microscope.  The  body 
is  divisible  into  three  regions,  head,  trunk,  and  tail.  The  trunk 
is  covered  by  a  tough  cuticle  into  which  head  and  tail  can  be 


f 

FIG.  257.— Diagram  of  rotifer.  (After  Delage  et  Herouard.)  7>,  brain;  /c,  flame  cell; 
gy,  gastric  gland;  i,  intestine;  w,  mastax;  ov,  ovary;  pg^  pedal  gland;  pv,  pulsat- 
ing vesicle  of  excretory  system;  g,  stomach. 

retracted.  The  tail  or  <  foot '  is  often  composed  of  rings  which  can 
be  telescoped  into  each  other  and  which  by  their  superficial  resem- 
blance to  segmentation  formerly  led  to  the  association  of  the  roti- 
fers with  the  Arthropoda.  The  last  tail  ring  often  bears  a  pair  of 
pincer-like  stylets  which  together  with  adhesive  glands  enable  the 
animal  to  adhere  to  objects.  The  head  has  the  most  delicate 
cuticle  and  is  expanded  in  front  to  a  trochal  disc,  an  apparatus  of 
varying  appearance,  which  is  surrounded  by  a  ring  of  cilia  of  use  in 
swimming  and  also  in  directing  food  to  the  ventral  mouth.  The 


294 


ROTIFER  A. 


alimentary  canal  consists  of  oesophagus,  mastax  (chewing  stomach), 
glandular  stomach,  and  intestine;  all  except  the  mastax  cilated. 
The  mastax  bears  two  chitinous  jaws  (trophi),  which  in  life  are  in 
constant  motion  and  comminute  the  food.  The  cerebral  ganglion 
lies  above  the  oesophagus,  with  which  simple  eyes  and  peculiar  sense 
organs,  the  cervical  tentacles,  are  frequently  connected.  The 
usually  single  ovary  and  the  paired  protonephridia  empty  into  the 
posterior  part  of  the  alimentary  canal,  which  thus  becomes  cloacal 
in  character.  For  a  long  time  males  were  unknown  until  Dal- 
rymple  discovered  that  these  are  much  rarer  and  smaller  ( dwarf 


Fio.  258.—  Brachionus  urceolaris.  A,  female  with  four  eggs  in  various  stages ;  B,  male ; 
C,  'flame'  from  protonephridia,  greatly  enlarged;  b,  urinary  bladder;  c,  cloacal 
opening;  d,  gastric  glands;  0,  ganglion,  with  eye;  ft,  testis;  fr,  mastax;  TO, 
stomach;  o,  ovary;  p,  penis;  f,  tentacle;  10,  protonephridia. 

males/ and  that  they  have  a  much  simpler  structure  (fig.  258,  #). 
Usually  the  alimentary  tract  is  reduced  to  a  solid  cord  in  which  the 
testes  are  imbedded. 

The  Rotifers  have  two  kinds  of  eggs,  large  winter  eggs  enclosed 
in  a  thick  shell  and  smaller  thin-shelled  summer  eggs.  The  latter 
develop  parthenogenetically  and  by  their  numbers  and  rapid 
growth  subserve  the  distribution  of  the  species.  The  winter  eggs 
require  fertilization,  and  have  a  long  resting  period,  thus  serving 
to  tide  over  periods  of  cold  or  drought.  The  adult  animals  can 
withstand  a  certain  amount  of  desiccation;  and  they  are  often 
found  in  damp  moss  or  in  eave  troughs  in  a  sort  of  sleep  from 
which  they  are  awakened  by  water. 


CCELHELMINTHES. 


295 


In  structure  the  Rotifiers  are  much  like  the  trochophore  type  of 
embryo  of  annelids  and  molluscs  to  be  described  later.  They  must  hence 
be  regarded  as  extremely  primitive  forms,  connected  at  once  with  the 
ancestors  of  these  groups,  and,  as  shown  by  nervous  system  and  excre- 
tory organs,  with  the  flatworms  as  well.  Most  species  are  cosmopolitan 
and  inhabitants  of  fresh  water.  Many  species  in  America.  Near  the 
Rotifera  may  be  placed  the  fresh-water  GASTROTRICHA  (Ichthydium, 
Clmtonotus)  and  the  marine  ECHINODERID^,  forms  which  are  little 
understood. 


PHYLUM  VI.  CCELHELMINTHES. 

The  Coelhelminthes  are  distinguished  from  all  the  forms  which 
have  gone  before  by  the  presence  of  a  body  cavity,  separating  the 
outer  body  wall  from  the  intestine.  This  cavity  is  the  coelom,  but 
whether  it  be  homologous  in  different  groups,  e.g.  nematodes  and 
annelids,  is  not  settled.  The  body  muscles  are  developed  from  the 


FIG.  259.— Section  of  Ascari*  lumhricoides  through  the  pharyngeal  bulb;  beside  it  a 
bit  of  the  body  wall  more  enlarged,  c,  cuticle ;  d,  dorsal  line  ;  7i,  hypodermis ;  m, 
longitudinal  muscle;  n,  nucleus  of  muscle  cell;  p,  muscle  cell;  s,  lateral  line; 
r,  ventral  line  ;  u\  excretory  canal. 

outer  (parietal)  epithelial  wall  of  the  coelom  and  hence  are  <  epi- 
thelial muscle  cells '  (figs.  259,  260).  The  excretory  organs  con- 
nect the  body  cavity  with  the  outer  world  and  hence  are  nephridia 
(earlier  called  segm en tal  organs,  cf.  fig.  69).  Internally  they  begin 
with  a  ciliated  funnel,  the  nephrostome,  and  continue  as  long  coiled 
tubes  expanding  just  before  the  outer  end  to  a  kind  of  bladder. 
The  sexual  apparatus  is  simple.  The  gonads  (fig.  260,  o)  are 


296 


CCELHELMINTHES. 


specialized  parts  of  the  ccelomic  epithelium  and  their  products  are 
usually  carried  to  the  exterior  by  the  nephridia  (more  rarely  by 
special  ducts),  so  that  here,  as  in  vertebrates,  we  can  speak  of  a 
urogenital  system.  A  closed  blood  system  is  now  present,  now 


FIG.  260.— Transverse  section  of  Sagitta  bipunctata  and  a  bit  of  the  body  wall  more 
enlarged.  (After  O.  Hertwig.)  c,  coelonx;  dd,  entoderm;  d/,  splanchnic  meso- 
thelium;  e,  epidermis;  w,  somatic  mesoderm  (muscles  and  epithelium);  o,  ovary. 

absent.     Little  in  general   can   be  said  of  the  nervous  system: 
details  will  be  given  in  connexion  with  the  separate  classes. 


Class  I.  Chaetognathi. 

These  marine  forms,  a  half  to  two  inches  long,  perfectly  trans- 
parent, are  well  adapted  to  serve  as  an  introduction  to  the  ccelomate 
worms.  They  live  at  the  surface  of  the  sea,  preying  on  other  ani- 
mals, and  from  their  shapes  and  rapid  motions  deserve  the  name 
Sagitta — arrow — given  some  forms.  The  animals  swim  by  means 
of  horizontal  fins,  one  surrounding  the  tail  and  one  or  two  pairs 
on  the  sides  of  the  trunk  (fig.  261).  On  either  side  of  the  mouth 
is  a  lobe  bearing  strong  bristles  used  in  seizing  prey  (Chaeto- 
gnathi, bristle-jaw).  Internally  the  body  is  divided  into  three 
segments,  head,  trunk,  and  tail,  by  transverse  septa  which  divide 
the  coelom  into  corresponding  parts.  Each  segment  of  the  cce- 
lorn  again  is  divided  into  right  and  left  halves  by  a  mesentery 
(fig.  260),  supporting  the  straight  intestine,  running  lengthwise 
through  it.  The  intestine  terminates  at  the  anus  at  the  end  of  the 
trunk  segment. 

The  nervous  system  is  entirely  ectodermal.     In  the  head  is  a 


/.    CH^TOGNATBI. 


297 


pair  of  fused  cerebral  ganglia  (fig.  262),  in  the  trunk  segment  a 
large  ventral  ganglion,  and  these  are  connected 

"m  by  long  cesophageal  commissures.  Of  interest, 
because  characteristic  of  nematodes  and  many 

f  annelids,  are'  the  relations  of  the  musculature, 
which  consists  of  longitudinal  fibres  alone. 
The  body  cavity  is  lined  with  epithelium  (fig. 
260),  which,  so  far  as  it  abuts  against  the  ali- 
mentary tract,  is  called  splanchnic  (or  visceral) 
mesoderm;  that  on  the  side  of  the  ccelom  to- 
wards the  ectoderm  is  the  somatic  mesoderm. 
"  The  muscles  arise  from  the  latter  layer  and  are 
divided  into  four  fields,  right  and  left  dorsal, 
right  and  left  ventral.  The  sex  cells  also  arise 
from  the  epithelium  of  the  coalom,  the  eggs 
»r  in  the  trunk  segment,  the  sperm  in  the  tail. 
The  eggs  are  carried  to  the  exterior  by  special 
ducts.  The  sperm-forming  cells  early  lose 
their  connexion  with  the  epithelium,  fall  into 
,  the  ccelom,  where  they  develop  the  spermato- 
zoa. These  are  carried  out  by  canals  which 
by  their  relations  to  the  co3lom  recall  the  ne- 
phridia  of  the  annelids. 


171 


FIG.  261.  FIG.  262. 

FIG.  261.— Sagitta  hexaptera,  ventral  view.  (After  O.  Hertwig.)  a,  anus;  &<?,  ventral 
ganglion;  d,  intestine;  //,  fin;  /to,  testes;  m,  mouth;  ov,  ovary;  oud,  oviduct:  s?>, 
seminal  vesicle;  sc,  cecophageal  commissure;  sfl,  tail  fin;  s/,  sperm;  wo,  female 

FIG.  262.— Head  of  Sagittn  bipunctata,  dorsal  view.  (After  O.  Hertwig.)  an,  nerve 
to  ati,  eye;  g,  brain;  gh,  bristles;  ?-?i,  nerves  to  ro,  olfactory  organs;  sc, oesophageal 
commissure. 

The  development  of  Sagitta  is  significant  from  two  points  of  view. 
The  archenteron  (fig.  108)  is  divided  by  lateral  folds  into  an  unpaired 
middle  portion  and  two  paired  lateral  chambers  ;  the  first  is  the  defini- 
tive digestive  tract,  the  latter  the  anlagen  of  the  ccelomic  diverticula. 


298  CCELHELMINTHES. 

In  other  words  the  ccelom  is  an  outgrowth  from  the  archenteron,  i.e.  is  an 
enterocoele.  Second  :  The  gonads  are  derived  from  a  pair  of  cells  in  the 
primitive  entoderm,  which  later  are  carried  into  the  coelomic  walls.  Hence 
each  divides  into  anterior  and  posterior  cells,  the  anterior  developing 
into  the  ovary,  the  posterior  into  testes.  Hence  here  the  male  and  female 
sex  cells  are  beyond  doubt  descendants  of  a  common  mother  cell. 

The  few  species  of  Chaetognathi  are  arranged  in  two  or  three  genera, 
of  which  JSagitta,  represented  on  our  coasts  by  8.  elegans*  is  best  known. 
Spadella. 

Class  II.  Nemathelminthes. 

Like  the  flat  worms,  the  roundworms  are  characterized  by  their 
shape,  they  being  thread-like  or  cylindrical  animals  whose  form  is 
the  result  of  the  existence  of  a  body  cavity  in  which  the  viscera  are 
so  loosely  held  that  on  cutting  through  the  muscular  body  wall 
they  will  fall  out  (fig.  259).  Since  the  Nemathelminthes  share  this 
coelom  with  most  annelids,  the  distinction  between  the  two  rests 
largely  upon  negative  characters,  the  roundworms  lacking  the 
segmentation  of  the  body  cavity  and  the  corresponding  ringing  or 
annulation  of  the  body  wall.  To  the  Nemathelminthes  belong 
three  orders,  much  alike  in  habits  and  appearance  but  differing 
considerably  in  structure.  Of  these  the  most  important  are  the 

nematodes. 

Order  I.  Nematoda. 

The  nematoda  contain  numerous  species  of  thread-shaped 
worms  varying  from  0.001  to  1.0  metre  in  length,  many  of 
which,  through  their  wide  distribution  as  parasites  in  plants, 
animals,  and  man,  possess  special  interest.  The  outer  surface  is 
covered  by  a  tough  cuticle  secreted  by  the  underlying  hypodermis 
(fig.  259),  a  layer  corresponding  to  epithelium  and  cutis,  which  in 
cross-section  shows,  median  and  lateral,  four  thickenings,  the 
dorsal,  ventral,  and  lateral  lines.  In  the  lateral  lines  run  the  excre- 
tory vessels,  two  longitudinal  canals  which  are  united  near  the  head 
by  a  transverse  vessel  opening  on  the  ventral  surface  by  an  unpaired 
porus  excretorius  to  the  exterior.  They  are  related  to  the  coelom 
by  two  giant  cells  on  either  side  which  send  processes  into  the 
body  cavity.  These  lateral  and  median  lines  divide  the  muscles 
(here  only  longitudinal)  into  four  fields,  as  in  Chsetognaths.  These 
muscles  are  parts  of  the  somatic  epithelium,  a  layer  of  vesicular 
cells  which  by  their  size  (fig.  259)  so  encroach  upon  the  ccelom 
that  scarce  space  is  left  for  the  alimentary  canal  and  reproductive 
organs.  A  splanchnic  mesoderm  is  lacking. 

The  alimentary  canal  begins  with  a  terminal  mouth  and  ends 
with  the  anus,  which  is  ventral  and  in  front  of  the  end  of  the  body. 


//.   NEMATHELMINTHES :  NEMATODA. 


299 


The  mouth  connects  with  the  muscular  oesophagus,  which,  for 
sucking  purposes,  is  expanded  posteriorly  to  a  pharyngeal  bulb  and 
is  lined  throughout  with  a  cuticle.  From  this  point  to  the  anus 
the  stomach-intestine  is  usually  uniform  (fig.  263).  The  oesophagus 
is  surrounded  by  a  nervous  ring  which  sends  forward  and  back  a 


FIG.  263.  FIG.  264. 

FIG.  263.— Structure  of  young  female  Ascaris  (based  on  a  drawing  by  Leuckart). 
d,  intestine;  o,  ovary;  s,  lateral  line;  v,  ventral  line:  va,  vagina. 

FIG.  264.— Diagram  of  nervous  system  of  a  nematode.  (After  Butscbli.)  c,  commis- 
sures; d,  dorsal  nerve;  z,  infraoesophageal,  s,  supraoesophageal  part  of  nerve 
ring ;  v,  ventral  nerve. 

large  number  of  nerves,  those  in  the  mid-dorsal  and  ventral  lines 
being  strongest.  At  points  on  these  nerves  are  collections  of 
ganglion  cells,  but  a  formation  of  ganglia,  as  in  the  annelids,  does 
not  occur  (fig.  264). 

The  sexual  organs  of  these  rarely  hermaphroditic  forms  are 
very  simple.  Males  and  females  are  easily  distinguished  not  only 
by  the  copulatory  organs,  but  by  the  openings  of  the  genital  ducts. 
These,  in  the  male  (fig.  265),  are  in  the  end  of  the  alimentary 
canal,  which  hence  is  a  cloaca.  In  the  female  (fig.  263)  there  is  a 
special  genital  opening  on  the  ventral  surface  between  mouth  and 
anus,  the  position  varying  with  the  species.  In  general  the  struc- 


300  CCELHELMINTHES. 

ture  of  the  reproductive  organs  is  alike  in  both  sexes.  In  both, 
on  account  of  the  great  fertility,  these  are  long  tubes  coiled  for- 
ward and  back  and  ending  in  fine  threads  which  produce  eggs  or 
sperm  (ovaries,  testes),  while  the  rest  serves  as  seminal  vesicle,  or 
receptaculum  seminis,  and  ducts.  In  the  male  the  genital  tube  is 
always  single;  in  the  female  it  is  usually  double,  the  right  and 
left  halves  uniting  a  little  before  the  external  opening  (fig.  263, 
va).  Most  common  of  copulatory  organs  in  the  male  are  spicula, 
bent  spines,  which  lie  in  a  sheath  behind  the  vent  and  can  be 
protruded  through  the  cloacal  opening,  appropriate  muscles  caus- 
ing them  to  retract.  Besides  there  may  be  valves  to  right  and  left 
to  clasp  the  male,  or,  as  in  Trichina,  the  whole  cloaca  is  pro- 
trusible. 

Since  there  is  copulation,  the  eggs  are  fertilized  in  the  uterus, 
after  which  they  are  either  laid  or  retained  for  more  or  less  of  their 
development,  many,  like  Trichina,  being  viviparous.  The  post- 
embryonic  development  depends  largely  upon  the  mode  of  life. 
Free-living  species  grow  by  repeated  molts  without  much  change 
of  form.  In  many  Anguillulidae,  which  show  how  free  life  can  be 
transformed  into  parasitic,  there  is  an  alternation  of  generations 
(heterogony)  from  an  hermaphroditic  entoparasitic  to  a  free 
dioecious  generation.  The  occasional  suppression  of  the  free  gen- 
eration which  occurs  in  many  Anguillulids  leads  to  the  Strongylidse, 
where  the  offspring  of  the  parasitic  generation  can  live  free  for  a 
time  (rhabditis  larvae),  but  must  return  to  parasitism  to  undergo  a 
metamorphosis  and  become  sexually  mature.  The  free  life  is 
shortened  again  in  the  Ascaridae,  where  the  eggs  must  pass  to  the 
exterior  for  a  longer  or  shorter  time,  but  the  embryos  only  escape 
when  the  eggs  are  taken  into  another  host.  Lastly,  there  are 
species  like  Trichina  where  the  free  life  is  entirely  suppressed  and 
transportation  from  host  to  host  takes  place  in  the  encysted  con- 
dition passively  by  food. 

Family  1.  ANGUILLULIDS  ;  small  thread-like  nematodes  with  double 
pharyngeal  swelling  which  live  in  mud,  organic  fluids  or  plants,  rarely  in 
animals  ;  male  with  two  spicula.  Anguillula  aceti,  vinegar  eel,  2  mm. 
long,  in  vinegar  and  stale  paste.  Rhabditis  (Rhabdonema)  nigrovenosar 
not  1  mm.  long,  lives  in  mud  and  stands  in  heterogony  with  a  second 
form  which  lives  in  the  lung  of  frogs.  Strongyloides  intestinalis  of  the 
tropics,  but  which  has  recently  appeared  in  southern  Europe,  has  a  some- 
what similar  history,  the  adult  stage  being  reached  in  the  human  intes- 
tine. Here  also  belong  numerous  plant  parasites  of  which  Tylemhus 
tritici  and  Heterodera  schachti  demand  notice,  the  first  doing  great  dam- 
age to  wheat,  the  second  to  turnips  in  Europe.  Tylenchus  devastatrix 
attacks  rye  and  hyacinths. 


77.   NEMATHELMINTIIES:  NEMATODA. 


301 


Family  2.  ASCARHLE.  Mouth  surrounded  by  three  lips,  one  dorsal, 
two  ventral ;  males  with  two  spicules.  Besides  numerous  species  in  lower 
vertebrates  two  of  the  most  common  parasites  of  man,  the  human  round- 
worm  and  the  pin  worm,  belong  here.  The  former,  Ascaris  lumbricoides,  * 


FIG.  265. — Dorsal,  end,  and  ventral  views  of  head  and  hinder  end  of  male   Ascaris 
lumbricoides.    (From  Hatschek.) 

inhabits  the  small  intestine,  often  in  enormous  numbers.  The  females 
average  about  5-6  inches,  the  males  4  inches,  in  length.  The  ani- 
mals are  enormously  prolific,  a  female  containing  about  60,000,000  eggs. 
The  eggs  (fig.  246,  a)  are  easily  recognized.  Shortly  after  fertilization 
the  eggs  pass  out  of  the  intestine  with  the  faeces,  but  develop  without 
intermediate  host  if  in  the  course  of  two  or  three  months,  when  the  embryo 
has  developed,  they  are  taken  into  the  human  intestine.  The  development 
of  the  pinworm,  Oxyuris  vermicular  is,*  is  somewhat  similar  except  that 
the  embryos  are  developed  in  the  egg  at  the  time  of  oviposition,  and  hence 
after  a  shorter  stay  outside  the  body  are  capable  of  infection.  The  white 
worm,  not  half  an  inch  long,  lives  in  the  rectum,  especially  of  children, 
and  in  crawling  outside  the  anus  causes  intolerable  itching.  Ascaris 
mystax*  occurs  in  dogs  and  cats  (occasionally  in  man).  A.  megalo- 
cephala  *  (a  favorite  animal  for  cy tological  researches)  and  Oxyuris  equi  in 
the  horse.  These  do  little  harm.  On  the  other  hand  Heterakis  maculosa 
often  destroys  whole  flocks  of  pigeons. 

Family  3.  STRONGYLID.E.     These  are  readily  recognized  by  the  bursa 
of  the  male,  a  broadening  of  the  hinder  end  of  the  body  by  two  ring-like 


FIG,  266.— Anterior  end  of  AnTcylostomum  duodenale,  dorsal  and  side  views,  o,  inner, 
h,  outer  ventral  teeth ;  c,  dorsal  tooth  of  m,  mouth  capsule  ;  d,  stylet ;  e,  ventral 
ridge  ;  oe,  oesophagus. 

processes,  which  contains  two  spicula.  Frequent  but  not  constant  is  a 
widened  capsule  surrounded  by  papillae  at  the  mouth.  A  number  of 
species  of  Strongyltts  occur  in  domestic  animals.  Syngamus  trachea- 


302 


CCELHELMINTHES. 


Us*  half  to  three  quarters  of  an  inch  in  length,  the  male  and  female 
always  in  pairs,  cause  the  disease  known  as  'gapes  'in  fowl.  Ankylo- 
stomum  (Dochmius)  duodenale*(ftg.  266),  about  two  fifths  of  an  inch  in 
length,  lives  in  the  small  intestine  of  man,  causing  severe  loss  of  blood 
and  the  disease  known  as  Egyptian  chlorosis.  The  eggs  develop  in  mud 
and  moist  earth,  and  hence  people  who  drink  muddy  water  (Fellahin  of 
Egypt)  or  work  much  with  clay  (potters  and  brick-makers)  are  especially 
subject  to  infection.  It  was  first  known  in  Egypt,  caused  considerable 
trouble  during  the  building  of  the  St.  Gotthard  tunnel  in  Switzerland, 
and  now  is  common  in  Germany.  Recently  it  has  been  thought  that  the 

Ankylostoma  larvae  obtain  entrance  to- 
man through  the  skin,  as  in  bathing, 
etc. 

Family  4.  TRICHOTRACHELID.E.  These 
owe  their  common  name  of  '  hair  necks  ' 
to  the  fact  that  the  part  of  the  body 
which  contains  the  pharynx  is  hair-like 
and  elongate,  while  the  pharynx  itself 
traverses  a  peculiar  cord  of  cells.  Long- 
est known  of  the  family  is  Trichocepl  talus 
dispar*  of  man  (fig.  267),  about  an  inch 
or  an  inch  and  a  half  in  length,  which 
lives  with  its  neck  burrowed  like  a  cork- 
screw in  the  wall  of  the  intestine  near  the 
caecum.  Since  it  does  not  move,  it  causes 
little  injury.  Its  presence  can  be  recog- 
nized by  the  oval  brown  double-shelled 
eggs  (fig.  246,  d)  in  the  faeces. 

A  second  species,  Trichina  spiralis  * 
(figs.  268,  269),  is  much  smaller,  but  much 
more  dangerous.  Two  stages  are  to  be 
distinguished,  the  encysted  muscle  Tri- 
china and  the  sexually  mature  intestinal 
Trichina.  The  first  was  discovered  in  a 
human  body  in  1835;  the  latter  was  not 
known  until  much  later,  its  history  being 
worked  out  by  Leuckart,  Virchow,  and 
Zenker.  In  the  encysted  stage  it  occurs 
in  the  muscles  of  pigs,  rats,  mice,  man, 
rabbits,  guinea  pigs,  dogs,  etc.  (never  in 


FIG.  267. 


FIG.  269. 


FIG.  268. 


intestinal  wall.  (From  Leuck-  birds),  enclosed  in  an  oval  capsule  about 
FiG.ai268.  —  Trichina  spiralis,  male.  0.4  to  0.6  mm.  long  and  hence  recogniz- 

tesTe™  HatSCbek)'  ''*'  cloaca:  ''  able  by  a  practised  observer  with  the 
FIG.  269.—  Trichina  in  muscle.  (From  naked  eye.  They  are  more  easily  seen 

when  they  are  partially  calcified  and 

have  a  whitish  color.  Certainty  in  their  recognition  demands  a  low  power 
of  the  microscope.  In  the  capsule  is  coiled  up  the  worm,  about  1  mm. 
long,  which  is  not  yet  sexually  mature,  although  furnished  with  the 


//.   NEMATHELMINTHES :  NEMATODA. 


anlagen  of  sexual  organs.  To  attain  this  it  must  be  transported  into  the 
intestine  of  another  host,  ^hen,  for  instance,  man  eats  trichinosed  pork 
the  worms  are  freed  from  the  muscle  and  capsules  by  the  digestive 
fluids  and,  entering  the  small  intestine,  become  sexually  mature  in  a  few 
days.  The  female  (3-4  mm.  long,  the  male  1.5  mm.)  penetrates  into  the 
superficial  layer  of  the  intestinal  villi  and  in  course  of  a  month  gives  birth 
to  1500  (some  say  10,000)  living  young,  after  which  she  dies.  The  young, 
on  the  other  hand,  penetrate  the  lymph  vessels,  and  by  way  of  the 
thoracic  duct  are  carried  into  the  blood-vessels,  and  wander  from  the 
capillaries  into  the  muscles,  especially  those  which  are  much  worked,  like 
the  diaphragm,  eye  muscles,  and  muscles  of  the  neck,  and  which  conse- 
quently have  a  rich  blood  supply.  They  enter  the  sarcolemma  of  the 
muscle,  destroy  the  muscle  substance,  and  finally  become  enclosed  by  a 
capsule  secreted  by  the  host.  The  wandering  takes  place  about  the  second 
or  third  week  after  infection,  the  encystment  in  about  three  months.  A 
slight  infection  causes  disagreeable  symptoms ;  but  where  large  numbers 
obtain  entrance  the  cases  are  frequently  fatal.  The  worst  epidemic  known 
was  in  Emmersleben,  Saxony,  in  1884,  where  57  died  in  four  weeks  from 
infection  from  one  pig. 


FIG.  270.— Transverse  section  of  young  Gordius.  (After  von  Linstow.)  a,  hypoder. 
mis;  b,  muscular  layer;  c,  cuticle;  d,  parenchyma;  e,/,  muscles  and  mesenteries;  g, 
alimentary  canal ;  ft,  nervous  system. 

Family  5.  FILARIID.E.  These  are  extremely  elongate,  hair-like  worms. 
Their  best-known  representative  is  Dracunculus  medinensis,  the  guinea 
worm  (the  female  about  a  yard  long,  and  about  as  large  as  stout  packing 
twine),  which  produces  a  sickness  known  to  the  Greeks  as  dracontiasis.  It 
forms  abscesses  beneath  the  skin  in  which  the  worm  is  coiled  up.  The  em- 
bryos break  through  the  wall  of  the  mother  and  must  enter  the  water  and 
penetrate  a  small  crustacean,  Cyclops.  It  is  apparently  introduced  into 
the  human  system  by  swallowing  the  Crustacea  with  drinking  water.  The 
worm  has  recently  been  found  in  the  tropics  of  America. 

A  second  species  is  Filaria  sanguinis  hominis,  the  adults  of  which — 
3  to  6  inches  long — live  in  the  lymphatic  glands  of  man,  while  the  young 
escape  into  the  blood,  often  in  immense  numbers.  They  often  escape 


304: 


C(ELIIELMTNTHES. 


through  the  kidneys,  where  they  produce  serious  disturbance  (albuminuria, 
haematuria).  There  is  possibly  a  connexion  between  them  and  elephanti- 
asis. The  intermediate  host  is  apparently  the  mosquito.  As  yet  they  are 
known  only  in  the  tropics.  Other  species  occur  in  man  and  other  animals. 
Family  6.  MERMITHIDTE.  Elongate  nematodes  with  six  oral  papillae. 
They  live  in  the  body  cavity  of  insects  and  pass  into  damp  earth,  where  they 
become  sexually  mature.  They  share  with  the  Gordiacea  the  common 
name  '  hairworms.'  Mermis.* 

Order  II.  Gordiacea. 

The  hairworms  resemble  the  nematodes  in  general  appearance,  but 
differ  greatly  in  structure.  The  body  cavity 
has  both  splanchnic  and  somatic  epithelium ; 
the  intestine  is  supported  by  mesenteries;  there 
is  an  oesophageal  nerve  ring  and  unpaired  ven- 
tral nerve  cord,  and  the  female  genitalia  enter 
the  cloaca.  The  adults  live  in  water,  where 
they  lay  their  eggs;  the  larvaa  live  in  insects, 
there  being  in  some  cases  at  least  an  alterna- 
tion  of  hosts.  These  (and  the  Mermithida?)  are 
popularly  believed  to  be  horse  hairs  changed 
into  worms.  Gordius,*  Chordodes* 

Near  the  Gordiacea  must  be  mentioned  the 
marine  Nectonema,*  the  young  stages  of  which 
are  apparently  passed  in  the  mosquito. 

Order  III.  Acanthocephala. 

The  species  of  spine-headed  worms  live  in  the 
alimentary  canal  of  vertebrates.  In  appearance 
they  resemble  the  Ascaridae  (p.  301),  but  are  easily 
distinguished  by  the  proboscis,  which  may  be  re- 
tracted by  muscles  and  exserted  by  contraction  of 
the  muscular  body  wall.  This  proboscis  bores  into 
the  intestinal  wall  and  is  held  in  place  by  numer- 
ous retrorse  hooks  (fig.  271).  In  internal  anatomy 
the  entire  absence  of  an  alimentary  canal  marks 
them  off  from  Nematodes  and  Gordiacea,  as  also  the 
peculiar  structure  of  the  reproductive  organs  and 
a  closed  vascular  system  in  the  body  wall  which 
extends  into  two  sacs,  the  lemnis.cS,  lying  beside 
the  proboscis  sheath.  The  unpaired  ganglion  lies 
on  the  proboscis  sheath  between  the  lemnisci.  An 
intermediate  host  occurs  in  development,  the  larva 
living  in  an  arthropod.  Thus  the  larva  of  Ecliin- 
orhynchus  (Gigantorhychus)  gigas*  of  the  pig 
lives  in  the  larva  of  the  '  .Tune  bug'  (Melolontlia), 
that  of  E.  proteus  of  European  fresh-water  fishes 
in  Crustacea.  One  species,  E.  hominis,  is  ex- 
tremely rare  in  man. 


.-<* 


FIG.  271.— Male  Ectiinorhyn- 
ckus  angustatus.  (From 
Hatschek.)  b,  penis  sac ; 
de,  seminal  vesicle;  dr, 
glands;  gr,  ganglion;  /Jem- 
nisei;  lig,  ligament;  mim?, 
retractors  of  proboscis 
and  its  sheath ;  p,  penis ; 
r,  proboscis ;  rs,  proboscis 
sheath;  t,  testes;  vd,  vas 
deferens. 


777.    ANNELIDA. 


305 


Class  III.  Annelida. 

The  segmentation  of  the  body,  which  was  shown  in  a  slight  de- 
gree in  the  Chsetognathi,  reaches  its  highest  development  in  the 


Fio,  272.— Diagram  of  annelid  somites  (orig.).  am,  acicular  muscles;  c,  coelom;  cm, 
circular  muscles  ;  ev,  circular  blood-vessels;  d,  dorsal  blood-vessel;  /,  intestine  ; 
Im,  longitudinal  muscles;  rn,  mesentery;  ?i,  nerve  cord;  no,  nephridium;  ne,  nv% 
neuro-  and  notopodia,  forming  parapodium;  s,  septum;  so,  somatopleure ;  sp, 
splanchnopleure ;  £,  typhlosole. 

Annelids,  where  it  appears  both  in  the  outer  ringing  of  the  body 
and  in  the  arrangement  of  the  most  important  systems  of  organs — 
metameric  arrangement  of  excretory  organs,  nervous  system, blood- 


FIG.  273.— Trochophore  (Loven's  larva)  of  Polygordius.  (From  Hatschek.)  A,  anus; 
dLM,  dorsal  muscles;  ED,  hind  gut:  «/,  stomach;  J,,  intestine;  Mstr,  mesodermal 
band;  ?i,  nerves;  Neph,  protonephridia;  O,  mouth;  Oe,  oesophagus;  oeLM,  oesopha- 
geal  muscle;  SP,  apical  plate;  vLM,  ventral  muscle;  vLN,  lateral  nerve;  Wkr^  wkr, 
pre-  and  post-oral  zones  of  cilia;  WS,  apical  cilia;  wz,  adoral  cilia. 

vessels — internal  segmentation.  To  this  is  added  an  extraordinary 
increase  in  number  of  body  segments  (somites,  metameres),  which 
can  far  exceed  a  hundred.  We  can  thus  define  the  Annelids  as 


306 


CCELHELMINTHES. 


worms  with  ccelom  and  with  external  and  internal  segmentation. 
In  the  development  there  frequently  occurs  a  type  of  larva,  the 
trochophore,  which  must  be  referred  to  here,  since  it  is  of  great 
morphological  significance,  resembling  in  structure  the  rotifers 
and  recalling  the  larva  of  the  molluscs  and  to  a  less  extent  that  of 
the  echinoderms.  It  is  (fig.  273)  a  gelatinous  body  traversed  by  an 
alimentary  canal  with  fore-,  mid-,  and  hind-gut  regions.  At  first 
it  is  everywhere  ciliated,  but  with  advance  of  development  the 
cilia  become  restricted  to  certain  thickened  tracts  of  epithelium, 
the  ciliated  bands.  One  of  these,  the  preoral  band,  is  especially 
constant.  It  runs  circularly  ( Wkr)  around  the  body,  surrounding 
a  circular  prestomial  area,  in  the  centre  of  which  is  the  anlage  of 
the  cerebral  ganglia,  a  thickened  patch  (apical  plate)  of  ectoderm, 
often  bearing  a  tuft  of  cilia.  Other  ciliated  bands  (post-oral, 
perianal)  often  occur.  Of  internal  organs,  besides  numerous 
muscle  fibres,  the  most  noticeable  are  the  excretory  organs,  true 
protonephridia,  which  open  to  the  exterior  either  side  below.  The 
trochophore  in  some  respects  resembles  the  larvae  of  some  Turbel- 
laria  (fig.  231)  and  Nemertines  (fig.  256),  showing  that  the  annelids 
are  related  to  these  groups. 

The  above  account  applies  most  closely  to  the  Chsetopoda  and 
the  closely  related  Archianellida.  In  other  forms  one  or  more 
features  may  be  lacking — in  the  GephyraBa  segmentation  of  the 
body;  in  the  Hirudinei  most  of  the  coslom  and  the  trochophore. 
Yet  these  are  so  closely  related  that  they  must  be  included  under 

the  common  head;  the  missing 
characters  have  been  lost  during 
evolution. 


Sub  Class  I.    Chcvtopoda. 

These,  like  the  Nematoda,  are 
cylindrical  worms,  but  are  at  once 
distinguished  by  the  segmenta- 
tion. Deep  circular  constrictions 
(fig.  274)  bound  the  somites  ex- 
ternally. Internally  the  coelom 
is  divided  by  the  septa — delicate 
double  membranes  which  extend 
from  the  ectoderm  to  the  alimen- 
tary canal — into  as  many  cham- 
bers as  there  are  metameres,  while 
a  longitudinal  mesentery,  also  double,  separates  the  coelomic 


FIG.  274.— Earthworm,  side  view  and 
anterior  end  enlarged.  (After  Vogt 
and  Jung.)  1,  first  segment  with 
mouth  and  prostomium;  15,  male 
sexual  opening;  33-37,  clitellum. 


///.    ANNELIDA:  CH^ETOPODA.  307 

pouches  of  the  right  side  from  those  of  the  left  (figs.  275  and 
272).  The  alimentary  canal  also  shows  distinctions;  for  while  it 
differs  greatly  in  the  various  species,  it  has  constantly  a  terminal 
anus,  while  the  mouth  is  ventral  and  is  overhung  by  the  preoral 
segment,  the  prostomium. 

Nervous  system,  blood-vessels,  and  excretory  organs  are  influ- 
enced by  the  segmentation.  The  nervous  system  is  built  on  the 
ladder  plan.  It  begins  with  a  supraoesophageal  ganglion  ('  brain ') 
lying  in  the  prostomium,  from  which  the  cesophageal  commissures 
pass  around  the  oesophagus  to  form  the  ventral  chain,  which  con- 
sists of  as  many  pairs  of  ganglia,  united  by  longitudinal  commis- 


FIG.  275.— Anterior  end  of  Nate  elinguis.  7i,  cerebrum,  connected  by  commissure  with 
ventral  chain,  n\  dy,  contractile  dorsal,  vg,  ventral  blood-vessel;  w,  muscular 
layer  of  skin;  clb,  vb,  dorsal  and  ventral  chaetae;  d,  septa;  7f,  prostomium;  o, 
mouth. 

sures,  as  there  are  somites  present.  These  ganglia  of  the  ventral 
chain  are  closely  similar,  since  the  segmentation  of  the  body  is 
homonymous.  There  is  but  the  slightest  division  of  labor  among 
the  somites,  and  hence  they  differ  but  slightly  among  themselves. 
The  prostomium  always  bears  tactile  organs  and  frequently  eyes, 
which  in  many  marine  forms  are  highly  developed,  with  lens, 
vitreous  body,  and  retina.  Otocysts  are  rare,  but  occur  in  diverse 
species.  Ciliated  pits  (olfactory)  occur  on  the  head,  goblet  organs 
(taste)  on  head  and  trunk,  and,  lastly,  lateral  organs,  sensory  struc- 
tures of  unknown  function,  may  be  metamerically  arranged. 

The  blood-vessels  are  most  frequently  represented  by  two  main 
trunks  which  frequently  (as  in  earthworms)  contain  blood  colored 
red  by  haemoglobin.  One  trunk  runs  dorsal,  the  other  ventral, 
to  the  intestine,  the  two  being  connected  by  vessels  (figs.  272,  276) 
in  each  segment.  The  blood  passes  forward  in  the  dorsal  vessel, 
backwards  in  the  ventral.  It  is  propelled  by  contractile  portions 
of  the  vessels;  usually  the  dorsal  vessel  pulsates,  but,  as  in  the  earth- 
worms, certain  of  the  circular  vessels  in  the  anterior  part  of  the 


308 


CCELHELMINTIIES. 


body  may  function    as  hearts   (fig.    276,   c).     Rarely,   as  in  the 
Capitellidse,  circulatory  organs  may  be  lacking. 

The  excretory  organs  (nephridia)  were  formerly  known  as 
*  segmental  organs/  since  they  occur  in  pairs  in  each  segment. 
These  supplant  the  embryonic  protonephridia;  each  consists  of  an 
internal  ciliated  funnel,  the  nephrostome,  a  more  or  less  convo- 

dg     lg      a 


oe 


St       QC 


ds 


CO 


s  o   vd    pt    vg       p 

FIG.  276.— Anterior  end  of  Pontodrilus  marionis.  (After  Perrier.)  a,  vascular  arches; 
b,  ventral  nerve  chain;  c,  'hearts';  co,  oasophageal  commissure;  dg,  dorsal  blood- 
vessel; ds,  septa;  gc,  cerebrum;  Z,  retractors  of  pharynx;  lg,  lateral  blood-vessel: 
o,  ovary;  oe,  oesophagus;  p,  receptacula  seminis;  ph,  pharynx;  pt,  ciliated 
funnels  of  vas  deferens;  8,  nephridia;  vd,  vas  deferens. 


FIG.  277.— Schematic  cross-section  of  an  annelid.  (After  Lang.)  etc,  aciculum;  b, 
chsetae;  hrn,  ventral  nerve  cord:  dc,  dorsal  cirrus:  dp,  notopodium;  /c,  gill:  In?, 
longitudinal  muscles;  rnd,  digestive  tract;  ?ip,  nephridium;  oy,  ovary;  rm,  circu- 
lar muscles;  tm,  transverse  muscles;  fr,  nephrostome;  uc,  ventral  cirrus;  vd,  uv, 
dorsal  and  ventral  blood-vessels;  vp<  neuropodium. 

luted  tube,  and  the  external  opening  (fig.  69).  In  many  instances 
(Oligochaetes,  some  Polychaetes)  the  nephrostome  is  in  one  somite, 
the  external  opening  in  the  succeeding.  The  nephridia  also  usually 
serve  as  genital  ducts,  carrying  away  the  reproductive  cells,  which 


HI.   ANNELIDA:  CH^TOPODA. 


309 


always  arise  from  the  ccelomic  epithelium.  In  the  Oligochaeta 
(fig.  286),  besides  the  nephridia  in  the  genital  segments  special 
oviducts  and  vasa  deferentia  occur  which  are  usually  regarded  as 
modified  nephridia. 

Of  the  many  modifications  of  nephridia  only  a  few  can  be  noticed  here. 
Occasionally  there  may  be  more  than  one  pair  in  a  somite,  or  they  may 
have  more  than  one  nephrostome.  Again,  they  may  be  lacking  from  more 
or  fewer  somites.  In  many  Oligochsetes  they  may  empty  into  the  anterior 
or  posterior  ends  of  the  digestive  tract.  In  many  (Glycera,  Hesione, 
Nephthys,  Goniadd)  the  internal  ends  of  the  nephridia  are  branched, 
the  branches  being  closed  by  *  solenocytes,'  tubular  cells  bearing  an  internal 
bundle  of  cilia. 

In  many  marine  annelids  there  occurs  a  metamorphosis  in  which 
pelagic  larvae  occur.  These,  in  spite  of  their  many  modifications, 
are  comparable  with  the  <  Loven's  larva/  the  trochophore  already 


mes 


FIG.  278.— .4,  larva  of  Polygordiu*;  B,  same  changing  to  segmented  worm.    (After 
Hatscnek.)    a,  anus;  Ten,  excretory  organ;  mes,  segmented  mesoderm. 

described  (p.  306).  The  differences  largely  consist  of  modifica- 
tions of  the  ciliary  apparatus;  the  number  of  bands  may  be 
increased  (polytroche  larvae),  or  a  single  band  may  occur  at  the 
middle  (mesotroche)  or  at  the  end  (telotroche)  of  the  body.  The 
larva  becomes  a  segmented  worm  by  the  hinder  end  of  the  larva 
growing  out  and  dividing  into  segments  (fig.  278).  In  this 


310 


CCELIIELMINTHES. 


jointed  portion  the  coelom  arises  as  a  new  formation,  divided  from 
the  first  into  separate  chambers.  The  nephridia  also  arise  de 
novo,  independent  of  the  protonephridial  system,  which  is  often 
called  head  kidney  because  the  chief  part  of  the  trochophore  forms 
the  head  of  the  adult. 

The  fresh-water  annelids  develop  directly,  but  the  embryos  pos- 
sess a  reminiscence  of  an  earlier  larval  life  in  that  the  head  lobes  are 
very  apparent  and  contain  protonephridia,  which  leads  to  the  con- 
clusion that  these  animals  earlier  had  a  metamorphosis.  From  the 
resemblance  of  the  trochophore  to  the  Rotifera  the  farther  conclu- 
sion is  drawn  that  the  annelids  have  descended  from  Rotifer-like 
ancestors,  the  body  cavity,  nephridia,  blood-vessels,  and  ventral 
nerve  chain  being  new  formations. 

Besides  sexual  reproduction  many  fresh-water  and  marine 
species  may  reproduce  asexually,  this  being  rendered  possible  by 
the  great  homonymy  of  the  segmentation.  By  rapid  growth  at 
the  hinder  end  as  well  as  at  a  more  anterior  budding  zone  numer- 
ous somites  are  formed  which  separate  in  groups  from  the  parent 
to  form  young  worms.  In  some  cases  the  formation  of  new  somites 


FIG.  279.— Budding  in  Myrianida.    (After  Milne-Edwards.)    The  sequence  of  letters 
shows  the  ages  of  the  individuals. 

may  take  place  more  rapidly  than  the  separation,  the  result  being 
chains  of  worms  (fig.  279). 

By  a  combination  of  sexual  and  asexual  reproduction  a  typical  alter- 
nation of  generations  occurs,  the  origin  of  which  receives  light  from  the 
following  facts:  In  many  polychaBtes  which  reproduce  exclusively  by  the 
sexual  process  the  sexless  slowly-moving  young  (atoke)  at  sexual  ma- 
turity becomes  so  altered  in  appearance  as  to  have  been  described  under 
another  name.  It  becomes  very  active  in  its  movements,  and  the  hinder 


III.   ANNELIDA:  CH^ETOPODA. 


311 


somites,  which  contain  the  sexual  organs,  develop  special  bristles  and  para- 
podia  (fig.  284,  A).  Thus  many  species  of  Nereis  pass  into  the  *  Hetero- 
nereis '  stage.  In  other  Polychaetes  the  sexual  part  (epitoke)  separates 
from  the  sexless  atoke  portion  and  swims  freely,  while  the  atoke  produces 
new  epitokes.  In  the  Samoaii  Islands  Eunice  viridis  reproduces  in  this 
way,  the  epitokes  coming  to  the  surface  at  certain  times  in  incredible 
numbers,  forming  the  'palolo  worm,'  a  delicacy  in  the  Samoan  diet.  In 
still  other  species  the  epitoke  regenerates  the  head  and  thus  becomes  an 
independent  generation.  Syllis  and  Heterosyllis  are  thus  related.  The 
AutolytidaB  furnish  the  most  complication.  Here  the  atoke,  by  budding 
as  in  Myrianida,  fig.  279)  forms  chains  of  dimorphic  individuals  which 
later  separate.  The  individuals  of  male  chains  were  formerly  described  as 
*  Polybostriclms,'1  the  females  as  '  Sacconereis."1  This  same  homonymy  ex- 
plains the  regenerative  powers  of  many  worms.  Thus  if  certain  earth- 
worms be  cut  in  two,  they  will  continue  to  live  and  will  reproduce  the  lost 
parts. 

Another  important  character  of  the  Chaetopoda  is  the  posses- 
sion of  bristles  or  chaetae.  These  arise  in  special  follicles,  singly 
or  several  in  a  bunch,  of  which  usually  there  are  four — right  and 
left,  dorsal  or  lateral  and  ventral — in  each  somite.  Each  follicle 
(fig.  280)  is  a  sac  of  epithelium  opening  on  the  surface  and  having 
at  the  base  a  special  cell  for  the  development  of  each  bristle.  The 
developed  bristles  project  from  the  follicle  and,  moved  by  appro- 


FIG.  280.— Arrangement  of  a  bristle  in  an  Oligochaete.  (After  Vejdowski.)  e,  epithe- 
lium; rm,  iw,  circular  and  longitudinal  muscles;  w,  muscle  of  the  follicle;  b,, 
chaeta  follicle,  its  chaeta  in  function ;  ba,  follicle  for  replacement,  the  formative 
cell  at  its  base. 

priate  muscles,  form  small  levers  of  use  in  locomotion.     Their 
numbers,  shape,  and  support  are  of  much  systematic  importance. 

Order  I.   Polychaetse. 

The  Polychaetae  owe  their  name  to  the  fact  that  each  group  of 
"bristles  contains  many  chaetae;    but  more  important  is  that  the 


312 


C(ELHELMINTHES. 


bristles  of  each  side  are  supported  by  a  fleshy  outgrowth  of  the 
somite,  the  parapodium,  in  which  two  portions  corresponding  to  the 
bunches  of  bristles — dorsal,  notopodium;  ventral,  neuropodium 
— may  be  recognized  (fig.  281).  This  is  the  first  appearance  of 
A 


K 


FIG.  281.— A,  head  with  protruded  phaiynx;  B,  parapodium  of  Nereis  versipedata. 
(After  Ehlers.)    c,  cirri;  fc,  jaws;  I,  head  with  eyes;  p,  palpi;  *,  tentacles. 

true  appendages,  but  they  differ  from  those  of  Arthropoda  in 
that  they  are  not  jointed  to  the  body  nor  jointed  in  themselves. 
On  the  dorsal  surface  may  occur  diverse  outgrowths,  known,  accord- 


Fia.  282.— Ampkitrite  ornata*    (From  Verrill.) 


ing  to  position  or  function,  as  cirri,  elytra,  gills,  etc. ;  on  the  head, 
palpi  and  tentacles.  The  cirri  are  long  processes  on  the  parapodia, 
and  like  palpi  are  tactile  (fig.  281).  Elytra  are  thin  lamellse 
which  cover  the  back  like  shingles  and  thus  protect  the  body. 


///.   ANNELIDA:  CH^ETOPODA.  313 

Nearly  all  Polychsetes  are  dioecious  and  undergo  a  more  or  less  pro- 
nounced  metamorphosis  ;  with  few  exceptions  (Manyunkia  *  from  the 
Schuylkill,  Nereis  *  in  California)  they  are  marine.  They  are  usually 
divided  according  to  their  habits  into  fixed  (Sedentaria)  and  free  forms 
(Errantia),  but  this  classification  lacks  a  morphological  basis.  The  Seden- 
taria feed  on  vegetable  matter,  usually  form  tubes  of  leathery  organic 
substances,  in  which  foreign  matter  may  be  incorporated  or  which  may 
be  calcified.  The  worms  project  their  anterior  segments  from  the  tubes. 
The  Errantia  often  secrete  gelatinous  tubes  in  which  they  can  hide,  but 
they  never  lose  their  powers  of  locomotion,  and  from  time  to  time  leave  their 
retreats  and  swim  about  preying  on  other  animals.  Correlated  with  habits 
are  differences  in  structure.  In  the  Errautia  the  head  and  trunk  are  not 
very  different ;  the  anterior  part  of  the  alimentary  tract  can  be  protruded 
as  a  proboscis,  and,  corresponding  to  their  predaceous  habits,  is  often 
armed  with  strong  jaws  (fig.  281,  A).  In  the  Sedentaria  there  are  no  such 
pharyngeal  teeth,  but,  on  the  other  hand,  there  is  a  greater  difference 
between  anterior  and  posterior  somites.  In  the 
latter  the  parapodia  are  weakly  developed,  and 
this  part  resembles  the  OligochaBtes  in  ap- 
pearance. The  head  and  beginning  of  the 
trunk  (thorax)  are  richly  provided  with  gills 
and  tentacles  for  respiration  and  the  taking  of 
food  (fig.  282). 

Sub  Order  I.  ERRANTIA.  Predaceous 
annelids  with  strongly  armed  pharynx.  The 
EUNICID^E,  mostly  represented  on  our  shores  by 
small  species,  contains  some  species  a  yard  in 
length.  Diopatra,*  Nothria*  The  ALCIOPHXE 
are  transparent  pelagic  forms  with  large,  highly 
developed  eyes.  The  SYLLIDJS  usually  have 
three  long  tentacles  ;  Autolytus*  Myrianida* 

(p.  310).    The  POLYNOHXE  *  (Lepidonotus*  Poly-       projecting  at  the  sides. 
noe*  (fig.  283),  Aphrodite  aculeata,*  the  sea 

mouse,  6  inches  long)  are  bottom  forms  with  elytra  covering  the  back. 
NEREIDS  ;  Nereis  virens,*  the  clam  worm  of  all  northern  seas. 

Sub  Order  II.  SEDENTARIA  (Tubicola,  Cryptocephala).  These  forms 
cannot  wander  about  at  will,  but  live  in  their  tubes.  In  the  SABELLI- 
DJE  the  tube  is  membranous  and  there  is  a  crown  of  gills  ;  Myxicola* 
Chone,*  Manyurikia*  In  the  SERPULHLE  the  tube  is  calcified  and  is 
closed  by  an  operculum  on  one  of  the  gills.  Hydroides;  *  Spirorbis,* 
forming  coiled  tubes  on  seaweed  ;  Protula*  The  ARENICOLID2E,*  which 
burrow  in  sand,  have  gills  on  the  sides  of  the  body.  The  MALDANID^E 
(Clymene*  Axiothea*)  have  extremely  long  segments  and  build  tubes 
of  sand.  The  TEREBELLID^E  (Terebella*  Amphitrite  (fig.  282),  Thelepus*) 
have  numerous  filiform  tentacles  and  branched  gills  on  the  anterior  end. 

The  AECHIANELLID^],  which  lack  bristles  and  parapodia, 
must  be  placed  near  the  Polychaetae  and  are  usually  regarded  as  very 


314 


CCELHELMINTHES. 


primitive  forms  which  in  structure  and  development  (fig.  273)  are 
of  importance  in  the  phylogenesis  of  the  Annelids.    Polygordius.  * 
ABO 


FlG.  284.— New  England  Annelids.    A,  male  Autolytvs  ;  B,  Sternaspis  fossor  ;  C,  Cis- 
tenides  gouldii ;  Z>,  Clymene  torquata.    (From  Emerton  and  Verrill.) 

Order  II.  Oligochaetae. 

The  Oligochaetes  are  almost  as  preeminently  fresh-water  and 
terrestrial  forms  as  the  Polychaetes  are  marine.  They  are  in 
most  respects  simpler  than  their  marine  relatives,  apparently  the 
result  of  degeneration,  which  has  followed  from  the  more  simple 
conditions  of  existence.  Eyes  are  rudimentary  or  lacking,  they 
have  no  palpi,  cirri,  or  tentacles;  gills  are  rare,  but  most  striking 
is  the  absence  of  parapodia,  the  bristles  projecting  directly  from 
the  body  wall  (fig.  280).  The  chaetae  may  be  regularly  distributed 
around  each  somite  (Pericliceta)  or  gathered  on  the  sides  (Megas- 
colex)  or  arranged  in  four  bunches  so  that  in  the  animal  four 
longitudinal  rows  occur.  The  animals  are  hermaphroditic,  testes 
and  ovaries  lying  in  different  somites.  Usually  the  integument  in 
the  neighborhood  of  the  sexual  openings  is  thickened  by  the 


///.   ANNELIDA:  CH^TOPODA. 


315 


presence  of  numerous  glands,  forming  a  clitellum  (fig.  274),  which 
secretes  the  egg  cocoons.  The  clitellum  also  functions  in  copula- 
tion, secreting  bands  \vhich  hold  the  animals  together  so  that 
the  sperm  from  one  passes  into  the  receptaculum  seminis  of  the 
other.  After  impregation  the  eggs  are  usually  enclosed  in 
cocoons. 

Sub  Order  I.  LIMICOLA  (Microdrili. )  Mostly  fresh-water  forms. 
The  TUBIFICID.E,  in  consequence  of  the  red  blood,  when  present  in  large 
numbers  color  the  bottom  red.  They  quickly  retract  into  the  tubes  which 


FIG.  285.— Aulophorus  vagus,  in  tube  of  Pectinatella  statoblasts.    (After  Leidy.) 

they  form  in  the  mud.  Tubifex*  Petoscolex  ;  Clitellio  irroratus*  common 
on  our  seashores.  The  NAIDID^E  are  transparent  forms  living  on  water 
plants  which  reproduce  asexually  throughout  nearly  the  whole  year. 


\r-vd 


cti 


FIG.  286.— Sexual  organs  of  Lumbricus  herculeus  (after  Vogt  et  Jung);  the  seminal 
vesicles  of  the  right  side  cut  away,  hm,  ventral  nerve  cord;  foZ,  bi\  lateral  and  ven- 
tral rows  of  setse;  di,  septa ;  ftj,  fts,  testes,  enclosed  in  sperm  reservoir;  o,  ovaries ; 
ew,  oviducts ;  sbu,  sperm  reservoir  ;  sftj,  2,  3,  sperm  sacs  (seminal  vesicles) :  *t „ 
seminal  receptacles;  t,,  ts,  seminal  funnels  connected  with  the  vasa  deferentia; 
to,  funnels  of  oviducts;  rd,  vas  def erens. 

Z>ero*  and  Aulophorus*  have  gills  around  the  anus.  ENCHYTILEIDJE ; 
Distichopus,  Pachydrilus.  The  DISCODRILIDJE  (Bdellodrilus,  Nyzobdella) 
are  parasitic. 

Sub  Order  II.  TERRICOLA    (Macrodrili).     Here  belong  the  terrestrial 
forms,  the  earthworms,  our  species  of  moderate  size,  in  the  tropics  large 


316 


C(ELHELMINTHES. 


species  (Megascolex  australis  four  feet  long).  Our  species  belong  to 
Lumbricus*  Allobophora*;  Perichceta*  has  been  introduced  from  the 
tropics;  Diplocardia  *  with  double  dorsal  blood-vessel.  Most  species  agree 
in  habits;  they  burrow  through  the  earth,  swallowing  the  humus  and 
casting  the  indigestible  portions  on  the  surface.  They  loosen  the  soil  and 
are  continually  bringing  the  deeper  parts  to  the  surface,  and  thus  do  great 
good.  Contrary  to  oft-repeated  statements,  earthworms  occurred  in  our 
prairies  and  plains  when  first  broken  up  by  the  plow.  Details  of  the 
reproductive  organs  of  one  species  are  shown  in  fig.  286.  These  vary 
greatly  and  are  largely  used  in  classification. 

Sub  Class  II.  Gephyrcea. 

The  exclusively  marine  Gephyraea  are  distinguished  at  the  first 
glance  from  the  Chaetopoda  by  the 
entire  absence  of  segmentation.  The 
body  is  oval  or  spindle-shaped,  circular 
in  section.  The  mouth,  at  the  ex- 
treme anterior  end,  is  either  surround- 
ed by  a  circle  of  tentacles  (fig.  287} 
and  is  retracted  together  with  the 
anterior  end  of  the  body  by  internal 
retractor  muscles,  or  is  overhung  by  a 


FIG.  287.  FIG.  288. 

FIG.  287.— Anatomy  of  Phascalosoma  gouldi  (orig.).  a,  anus;  a,  anterior  retractors 
d,  digestive  tract;  g,  gonads;  m,  mouth;  n,  nephridia;  TIC,  ventral  nerve  cord 
pr,  posterior  retractors. 

FIG.  288.— Larva  (trochophore)  of  JSchiurus.    (After  Hatschek.)    a,  anus;  d,  intestine 
hw,  postoral  cilia;  fcra,  protonephridia;  m,  mouth;  mes,  mesoderm  bands  with  indi- 
cation of  segments;  n,  ventral  nerve  cord;  sc,  oesophageal  commissure;  sp,  apical 
plate;  vto,  preoral  ciliated  band. 


///.   ANNELIDA:   GEPHYR^A.  317 

dorsal  spatulate  preoral  lobe  or  *  proboscis '  which  may  be  several 
times  the  length  of  the  body  and  forked  at  its  tip  (fig.  289). 

Internal  segmentation  is  also  lost,  septa  being  entirely  lacking. 
The  nephridia  are  also  reduced  in  number,  at  most  but  three  pairs 
being  present,  and  in  some  but  a  single  unpaired  organ.  They 
are  sexual  ducts,  and  in  the  Chaetiferi  there  are  special  excretory 
organs  (fig.  289,  g]  covered  with  branching  canals  opening  to  the 
body  cavity  by  nephrostomes  and  emptying  into  the  intestine.  These 
resemble  somewhat  the  branchial  trees  of  the  holothurians  (infra), 
and  hence  these  animals  were  formerly  supposed  to  bridge  the  gap 
between  holothurians  and  annelids,  whence  the  name  (yetyvpa, 
bridge)  Gephyrsea.  The  vascular  and  nervous  systems  are  more 
like  those  of  other  annelids.  The  vascular  system  consists  of  a 
dorsal  and  usually  a  ventral  longitudinal  trunk;  the  nervous 
system  of  a  brain,  oesophageal  collar,  and  ventral  cord,  the  latter 
without  division  into  ganglia.  The  relations  of  the  Gephyrsea 
to  the  Chaetopoda  are  shown  by  the  development.  In  some 
(Chaetiferi)  there  is  a  trochophore  (fig.  288)  from  which  the  worm 
arises,  as  in  the  Chaetopoda,  by  growth  at  the  hinder  end;  this  at 
first  has  a  segmented  coalom  and  nervous  system,  the  metamerism 
being  lost  later. 

Order  I.  Chaetiferi  (Armata,  Echiuroidea). 

With  spatulate  preoral  lobe,  often  forked  at  the  tip;  at  least  a  pair  of 
ventral  setae;  a  trochophore  in  development.  Ecliiurus  pallasii*  in  our 
northern  waters,  Thalassema*  farther  south.  In  Bonellia  there  is  a 
marked  sexual  dimorphism  (fig.  289).  The  female  is  2  to  3  inches  long  and 
has  a  proboscis  8  to  12  inches  long.  The  male  is  only  1  mm.  long,  totally 
different  in  appearance,  and  lives  parasitically  in  the  oesophagus  of  the 
female  (fig.  289,  B). 

Order  II.  Inermes  (Achaeta,  Sipunculoidea). 

Distinguished  by  lack  of  chsetse,  the  mouth  surrounded  by  tentacles, 
and  the  dorsal  and  anterior  position  of  the  anus.  The  larva  is  a  modified 
trochophore  without  preoral  ciliated  band  and  without  segmentation;  only 
two,  sometimes  but  one,  nephridia.  Phascalosoma*  common  on  our 
shores.  Phascolion  strombi  *  builds  tubes  in  deserted  snail  shells.  Sipun- 
culus.* 

Order  III.  Priapuloidea. 

No  tentacles,  mouth  with  chitinous  teeth,  terminal  anus,  no  nephridia; 
two  protonephridia  united  with  the  sexual  organs  and  opening  either  side 
of  the  vent.  Development  unknown.  P?*iapulus,  Halicryptus. 


318 


C(ELHELMINTHES. 


Sub  Class  III.  Hirudinei  (Discophori). 

Three  points  of  external  structure  clearly  distinguish  the  leeches 
from  the  chsetopods.  First,  the  absence  of  bristles  (except  in 
Acanthobdella)  and  the  presence  of  two  suckers,  one  of  which  occurs 
on  the  posterior  ventral  surface  and  is  used  only  for  attachment 
and  locomotion,  the  other,  sometimes  scarcely  differentiated, 
A 

t 


FIG.  289.— Bonellia  viridis.  A,  female  (after  Huxley);  J?,  male  (after  Spengel).  c, 
cloaca;  if,  rudimentary  intestine;  0,  excretory  organ;  i,  intestine;  w,  muscles  sup- 
porting intestine;  s,  balls  of  spermatozoa  in  B,  in  A,  proboscis  (preoral  lobe);  w, 
single  segmental  organ,  functioning  as  oviduct;  vd,  nephridium  with  ciliated 
funnel  serving  as  vas  deferens. 

surrounds  the  mouth  and  is  used  in  sucking  the  food.  In  locomo- 
tion anterior  and  posterior  suckers  are  alternately  attached,  the 
body  being  looped  up  and  extended  like  that  of  a  '  span  worm. ' 
The  animals  can  also  swim  well  by  a  snake-like  motion  of  the 
whole  body. 

A  second  point  is  the  fine  ringing  of  the  body,  there  being 
usually  many  more  rings  than  somites,  the  primitive  segment  rings 
being  divided  by  secondary  constructions,  there  being  three,  five, 
or  even  eleven  rings  to  a  segment.  The  middle  or  one  of  the 
anterior  rings  is  often  distinguished  by  bearing  strongly  developed 
sense  organs.  As  in  the  earthworms,  certain  of  the  somites  at  the 
time  of  reproduction  may  develop  into  a  clitellum  which  secretes  the 
egg  cocoons. 


///.   ANNELIDA:  HIEUDINEL 


319 


A  third  character  is  the  marked  flattening  of  the  body  in  the 
dorso ventral  direction  (except  in  Ichthyobdellidae  and  a  few  other 
forms),  the  animals  thus  recalling  the  flatworms.  This  may  be 
the  result  of  the  very  slight  development  of  the  coelom.  In  most 
leeches  there  is  a  body  (parenchyma,  traversed  by  longi- 
tudinal,transverse  and  dorsoventral  muscles  in  which 
the  organs  are  immediately  imbedded  (fig.  290). 

The  alimentary  tract  is  provided  with  paired 
diverticula  (fig.  291),  varying  in  number  in  different 
species.  Between  the  last  and  largest  pair  of  these 
sacs  is  the  intestine,  which  opens  dorsal  to  the  pos- 
terior sucker.  The  jawed  and  jawless  leeches  show 
considerable  differences  in  the  pharyngeal  region. 


FIG.  290.— Transverse  section  of  Hirudo  medicinalis.  (From  Lang.)  dm,  Zm,  rm,  dorso- 
ventral, longitudinal,  and  circular  muscles;  vi,  vd,  w,  lateral,  dorsal,  and  ventral 
blood-vessels,  the  latter  surrounding  the  ventral  nerve  cord,  ni;  h,  testes;  vd,  vas 
de^erens;  md,  midgut;  np,  nephridial  tubule;  enp,  urinary  bladder. 

FIG.  291. — Digestive  tract  of  Hirudo  medicinalis.  (From  Lang.)  a,  oesophagus;  6,  in- 
testine; d,,  da,  gastric  diverticula. 

In  the  first  there  are  three  jaws  in  the  phaynx,  semicircular  chitin- 
ous  plates,  the  free  edge  of  each  armed  with  numerous  calcified 
teeth  (fig.  292).  To  these  are  attached  two  muscles,  one  to  retract 
them,  when  not  in  use,  into  pockets,  while  the  other  exserts  them 
and  rotates  them,  causing  a  triradiate  wound  from  which  the  blood 
flows.  This  bleeding  is  difficult  to  staunch,  since  glands  on  the 
lips  and  between  the  jaws  produce  a  secretion  which  hinders  the 
coagulation  of  the  blood.  In  the  jawless  leeches  a  sharp  conical 
process  arising  from  the  pharynx  can  be  protruded  from  the 
mouth,  and  serves  for  wounding  and  sucking.  The  vascular 
system  usually  contains  red  blood,  and  consists,  in  the  Gnatho- 
bdellidae,  of  four  longitudinal  trunks,  a  dorsal,  two  lateral,  and  a 


320 


C03LHELMINTHES. 


ventral,  the  latter  surrounding  the  ventral  nerve  cord.     These  are 
connected  by  a  complicated  system  of  capillaries. 

The  nervous  system  consists  of  brain  and  ventral  cord,  the  lat- 
ter containing  frequently  twenty-three  ganglia  (the  first  of  five 
fused,  the  last  of  seven).  Nerves  from  the  brain  go  to  the  eyes. 
Eight  and  left  of  the  ventral  cord  are  the  hermaphroditic  sexual 
organs.  In  Hirudo  medicinalis  (fig.  293)  there  are  nine  pairs  of 


FIG.  292. 


FIG 


FIG.  292.— Hirudo  medicinalis^  medicinal  leech.    (After  Leuckart.)    a,  anterior  end 

with  three  jaws  (fc);  6,  a  single  jaw  with  its  muscles. 
FIG.  293.— Nervous  system,  blood-vessels,  sexual  organs,  and  nephridia  of  a  leech, 

ventral  view,    ft,  testes;  hb,  urinary  bladder;  ly,  lateral  blood-vessel;  n,  ventral 

nerve  cord;  n/i,  epididymis;  ov,  ovary;    p,  penis;  sc,  iiephridia;  it,  uterus  and 

vagina;  vd,  vas  deferens;  vg,  ventral  blood-vessel. 


(7i),  the  ducts  of  which  unite  to  form  a  vas  deferens  on 
either  side  (vd).  These  pass  forward,  form  by  coiling  a  so-called 
epididymis  (nh)  and  empty  into  the  median  unpaired  penis  (p). 
In  the  space  between  the  epididymis  and  the  first  pair  of  testes 
are  the  ovaries  (ov)  and  oviducts  and  an  unpaired  vagina  (u).  The 
nephridia  (17  pairs  in  this  species)  are  complicated  and  are  pro- 
vided with  bladder-like  expansions. 

That  the  Hirudinei  are  true  annelids  and  not  segmented  Plathelminthes 
is  based  upon  the  view  that  their  coelom  is  reduced  by  ingrowth  of  paren- 
chyma and  altered  to  canals  connected  with  the  vascular  system.  At 
any  rate  the  ventral  and  lateral  vessels  are  to  be  regarded  as  remnants  of 
a  coelom.  In  Clepsine  there  are  the  dorsal  and  ventral  blood- vessels  of 
the  Chaetopoda  and  besides  four  longitudinal  coelomic  sinuses  connected 
by  transverse  anastomoses.  The  dorsal  sinus  encloses  the  dorsal  blood- 
vessel, the  ventral  many  of  the  viscera,  among  them  the  ventral  nerve 
cord.  This  is  also  to  be  regarded  as  ccelomic,  since  the  nephrostomes  con- 
nect with  it.  In  most  Hirudinei  a  canal  system  filled  with  blood  has 
arisen  from  the  coelom  and  blood-vessel,  and  in  Neplielis  is  in  part  lacunar 
in  character.  Further  relations  are  shown  by  Acanthobdella  peledina, 
parasitic  on  fishes.  This  has  both  blood-vessels  of  the  Oligochsetes,  a 


IV.  POLTZOA.  321 

body  cavity  divided  by  septa  and  chsetsd.  On  the  other  hand  it  is  leech- 
like  in  other  features;  two  suckers  and  sexual  apparatus  on  the  Hiru- 
dinean  pattern. 

Order  I.  Gnathobdellidae. 

The  jawed  leeches  include  Hirudo  medicinalis,  once  extensively  used 
for  blood-letting  but  now  little  employed.  Hcemadipsa  includes  land 
leeches,  one  of  the  terrors  of  travelers  in  the  tropics.  In  Nephelis  *  the 
jaws  are  soft.  Macrobdella  *  includes  our  largest  native  species. 

Order  II.  Rhynchobdellidae. 

Without  jaws.  The  CLEPSINID^E  mostly  feed  on  snails  and  fishes. 
Clepsine  *  in  our  waters.  Hcementaria  officinalis  of  Mexico  is  used  for 
blood-letting  ;  H.  ghiliani  of  South  America  is  poisonous.  The  ICHTHYO- 
BDELLID^E,  cylindrical,  occur  in  salt  and  fresh  water,  parasitic  on  fishes. 
Ichthyobdella,*  Pontobdella*  marine  ;  Piseicola,  fresh  water. 

Class  IV.  Polyzoa  (Bryozoa). 

In  external  appearance  the  Polyzoa  closely  resemble  the 
hydroids,  so  that  the  inexperienced  have  difficulty  in  distinguishing 
them.  Like  them  by  budding  they  form  colonies  which  are  either 
gelatinous  or  calcareous  incrusting  sheets  or  assume  a  more  bush- 
like  character.  Further  they  have  a  crown  of  ciliated  tentacles 
which  can  be  spread  out  or  quickly  retracted.  In  internal  charac- 
ters the  two  groups  are  greatly  different.  The  Polyzoa  have  a 
complete  alimentary  canal,  with  its  three  divisions,  which  is  bent 
upon  itself  so  that  the  anus  lies  near  the  mouth.  The  central  nerv- 
ous system  lies  between  mouth  and  anus,  and  the  single  pair  of 
nephridia  empty  by  a  common  opening.  Beyond  this  it  is  diffi- 
cult to  go,  since  the  two  groups  of  Polyzoa — Endoprocta  and  Ecto- 
procta — differ  so  widely  that  one  may  doubt  whether  they  belong 
together.  The  Entoprocta  have  no  coelom,  and  resemble  in  this 
respect  the  Rotifera;  the  Ectoprocta  are  true  Coelhelminthes  and 
by  way  of  Phoronis  show  resemblances  to  the  Sipunculoida  and  so 
to  the  Annelida. 

Sub  Class  I.  Entoprocta. 

The  single  individuals  of  the  Entoprocta  (fig.  294)  are  shaped 
like  a  wine-glass  and  are  placed  on  stalks  which  rise  from  (usually) 
creeping  stolons.  The  circle  of  tentacles,  arising  from  the  edge 
of  the  cup,  enclose  the  peristomial  area  in  which  are  both  mouth 
and  anus,  and  between  these  the  excretory  and  reproductive  organs 


322 


C(ELHELMINTHES. 


open.  The  space  between  the  horseshoe- shaped  intestine  and  the 
body  surface  is  completely  filled  by  a  pa- 
renchyma containing  muscle  cells,  and 
correspondingly  the  excretory  organs  are 
proton ephridia.  In  our  fresh-water  Urna- 
tilla  gracilis  *  these  organs  are  branched 
and  begin  with  flame  cells.  Pedicellina  * 
and  Loxosoma,  marine. 

Sub  Class  II.   Ectoprocta. 

In  the  Ectoprocta  there  is  a  spacious, 
often  ciliated,  coelom  between  the  alimen- 
tary canal  and  skin,  so  that  these  are 
separated  and  have  a  certain  amount  of 
independence  (fig.  295).  On  this  account 

FlO.  294. — Loxosoma  Rinqulare.  -,  .  ,.  j -i       -i       ,     -i     n 

(After  Nitsche.)   single  in-  nas    arisen    a    peculiar   method    (wholly 

dividual  in  optical  section.  •     :j   .c        -ri  11       •     11    \       £    i         i-" 

A,  rectum;  Ga,  ganglion;  incleiensiDle  morphologically)   oi  treating 
8tomaecshlne;r'tentacles;^them  as  two  individuals,  polypid,  the  in- 
testine and  tentacles;  cystid  the  rest,  especially  the  body  wall  and 
skeleton. 


FIG.  295,—  Flustra  membranacea  (after  Nitsche),  a  single  animal,  a,  anus;  etc,  ectocyst: 
en,  entocyst;  /,  funiculus;  0,  ganglion;  fc,  collar  which  permits  complete  retrac- 
tion; m,  stomach,  also  dermal  muscular  sac;  o,  oesophagus.  A^  avicularium;  //' 
vibracularium  of  Bugula.  (After  Claparede.) 

The  cystid  is  cup-shaped  or  saccular.  It  consists  of  an  endo- 
cyst — the  body  wall — and  an  ectocyst — a  cuticular  skeleton,  usually 
strongly  calcified,  secreted  by  the  ectoderm.  The  surface  of  the 


IV.   POLTZOA:  ECTOPROCTA.  323 

entocyst  is  always  covered  by  the  ectocyst  only  on  the  basis  and  side 
walls;  the  peripheral  end  remains  soft  and  forms  a  sort  of  collar 
into  which  the  tentacles  and  adjacent  parts  of  the  cystid  can 
be  retracted.  In  the  ectocyst  there  is,  as  will  be  seen,  a  larger  or 
smaller  opening  which  in  many  species  (Chilostomata)  can  be 
closed  by  a  lid  (operculum).  The  circle  of  tentacles  surrounds 
the  mouth  alone,  while  the  anus  is  outside  near  the  collar.  The 
strongly  bent  alimentary  canal  extends  into  the  cystid  and  is  bound 
at  its  hinder  end  by  a  cord,  the  funiculus,  to  the  base  of  the  cystid. 
Ganglion  and  nephridia  lie  between  the  mouth  and  anus.  The 
gonads  arise  from  the  epithelium  of  the  coelom,  the  testes  usually 
on  the  funiculus,  the  ovaries  on  the  wall  of  the  cystid. 

Hundreds  and  thousands  of  individuals  form  colonies  (fig.  297) 
in  which  cystid  abuts  against  cystid.  The  coelom  of  adjacent  cystids 
may  be  distinct  or  a  wide  communication  may  exist.  The  colonies 
grow  by  budding;  in  the  Gymnolaemata  a  part  of  a  cystid  becomes 
cut  off  as  a  daughter  cystid  in  which  the  polypid — alimentary  tract 
and  tentacles — arises  by  new  formation ;  or  (Phylactolaemata)  the  bud 
anlage  of  the  polypid  arises  before  the  first  appearance  of  the  cystid. 

Division  of  labor  or  polymorphism  is  common.  Besides  the 
animals  already  described,  which  are  primarily  for  nourishment, 
three  other  individuals  may  occur,  ovicells,  vibracularia,  and  avic- 
ularia.  All  three  are  cystids  which  have  lost  the  polypid.  The 
ovicells  are  round  capsules  which  serve  as  receptacles  for  the 
fertilized  eggs.  The  vibracularia  (fig.  295,  B)  are  long  tactile 
threads;  the  avicularia  (fig.  295,  A)  are  grasping  structures  of 
uncertain  function.  They  have  been  seen  to  seize  small  animals 
and  hold  them  until  decay  set  in.  It  is  possible  that  the  fragments- 
serve  as  food  for  the  polypids.  The  avicularia  have  the  form  of  a, 
bird's  head,  the  movable  lower  jaw  being  a  modified  operculum. 

Under  unfavorable  conditions  a  polypid  in  a  cystid  may  break  down, 
and  be  lacking  for  some  time  until  better  relations  cause  its  new  forma- 
tion. Besides  in  the  depopulated  cystids  there  may  appear  statoblasts, 
lens-shaped  many-celled  internal  buds  enveloped  in  a  firm  envelope  which 
form  a  resting  stage  for  the  preservation  and  distribution  of  the  species. 
Each  statoblast  is  surrounded  by  a  girdle  of  chambers  which  by  drying 
become  filled  with  air,  causing  the  statoblast  to  float  when  it  again  comes, 
into  water.  From  the  statoblast  a  smaller  polyzoon  escapes  which  de- 
velops a  new  colony.  The  statoblasts  are  adaptations  to  the  conditions 
of  fresh-water  life  and  occur  only  in  the  Phylactolaemata. 

Order  I.  Gymnolaemata  (Stelmatapoda). 

The  tentacles  in  a  ring  around  the  mouth.  The  numerous  species  are 
almost  exclusively  marine  and  are  abundant  on  every  coast.  In  the 


324 


CCELHSLMINTHES. 


•CHILOSTOMATA  the  cystids  can  be  closed  by  an  operculum  :  Gemmel- 
laria,*  Cellularia*  Buyula*  Flustra*  (fig.  295),  Eschara*  The 
€YCLOSTOMATA  have  tubular  cystids  without  an  operculum.  Crisia* 

-TT 


FIG.  396.— American  gymnolsematous  Polyzoa.  (After  Busk,  Hincks,  Norman,  and 
Verrill.)  A,  Tnbulipora  flabellaris,  young;  B,  Flustrella  hispida ;  C,  Eucratea 
chelata  ;  D,  Gemellaria  loricata  ;  E,  Kinetoskias  tniitti;  F,  Membranipora  spini- 
fera  ;  (?,  Porella  loevis  ;  H,  Lepralia  americana  :  /,  Cribillina  puiicturata. 

Ttibulipora,*  Hornera*  In  the  CTENOSTOMATA  the  cystid  is  more  cal- 
careous and  the  opening  is  closed  by  a  folded  membrane.  Alcyonidium,* 
Vesicularia,  Valkeria.*  Paludicella*  (fresh- water). 

Order  II.  Phylactolaemata  (Lophopoda). 
Tentacles  borne  on  «,  horseshoe-shaped  lopliophore  extending  on  either 


Fio.  297.— Small  colony  of  Lnphopus  crystallfnus  (after  Kraepelin),  with  young  and 
old,  some  extended,  others  more  or  less  retracted,    o,  statoblasts. 

side  of  the  mouth,  the   tentacles  on  its  margins.     All   are   fresh-water 
species.     Pectinatella,*  Lopliopus  (fig.  227),  Plumatella* 


V.   PHORONIDEA.     VI.   BRACHIOPODA.  325 

Class  V.  Phoronidea. 

The  single  genus  Phoronis*  occurs  on  our  eastern  shores. 
The  animal  was  first  placed  among  the  Chaetopoda  on  account  of 
its  worm-like  body  situated  in  a  chitinous  tube  like  many  sedentary 
annelids.  Then  it  was  placed  in  the  Polyzoa,  with  which  it  is 
more  nearly  related.  The  young,  described  as  Actinotrocha,  is  a 
modified  trochophore  with  the  mouth  overhung  by  a  large  hood 
and  the  postoral  band  of  cilia  drawn  out  into  a  series  of  fingers 
which  become  the  tentacles  of  the  adult;  the  anus  is  terminal. 
At  the  time  of  metamorphosis  this  larva  becomes  doubled  on  itself 
by  a  complicated  process,  so  that  the  alimentary  canal  is  U-shaped 
and  the  anus  is  near  the  mouth,  while  the  tentacles  are  borne  on  a 
horseshoe-shaped  basis  around  the  mouth. 

Class  VI.   Brachiopoda. 

On  account  of  the  bivalve  calcareous  shells  the  Brachiopoda 
were  long  regarded  as  molluscs,  but  later  the  fact  that  the  valves 
are  not  paired  as  in  the  lamellibranchs,  but  are  dorsal  and  ventral, 
that  the  nervous  system,  the  excretory  and  reproductive  organs, 
the  body  cavity,  and  the  development  resemble  those  of  the  annelids 
rather  than  those  of  the  molluscs,  led  to  their  recognition  as  a  dis- 
tinct class  allied  to  the  former  group. 

The  body  has  a  greatly  shortened  long  axis  (fig.  298)  and  in 
consequence  a  transversely  oval  visceral  sac.  In  most  a  stalk  (st)  for 


FIG.  298.— Anatomy  of  Rhynchonella  psittacea.  (After  Hancock.)  a',  left,  «',  right 
arm;  a,  opening  into  the  cavity  of  the  arm:  d,  intestine;  e,  blind  end  of  the  intes- 
tine ;  0,  stomach  with  liver;  m,  adductors  and  divaricators  of  shell;  o,  oesophagus; 
p',  p2,  dorsal  and  ventral  mantle  lobes;  sf,  stalk;  1, 2,  first  and  second  septum,  on 
the  second  a  nephrostome. 

attachment  arises  from  the  posterior  end.     From  the  anterior  side 
two  folds,  the  mantle  lobes,  extend  forwards  (/?),  one  ventral,  the 


326 


C(ELHELMINTHES. 


other  dorsal,  their  free  edges  bearing  bristles.  Each  mantle 
secretes  a  shell  largely  composed  of  carbonate  and  phosphate  of 
lime.  In  a  few  the  dorsal  and  ventral  shells  are  similar,  but 
usually  the  ventral  valve  (in  Crania  attached  directly  without  the 


FIG.  299.— Waldheimia  flavescens.  (From  Zittel.)  Shell  with  arms  and  muscles,  a,  arm 
with  fringed  border  (/i);  c,  c',  divaricators;  d,  adductors;  D,  hinge  process  (the 
vertical  line  shows  position  of  hinge). 

intervention  of  a  stalk)  is  more  strongly  arched  and  has  an  opening 
at  the  posterior  end  for  the  passage  of  the  stalk  (figs.  299,  300). 
The  flatter  dorsal  valve  frequently  bears  a  characteristic  feature  in 
the  skeleton  of  the  arms  (fig.  300)  which,  when  present,  has  greatly 


FIG.  300. — Waldheimia  flavescens.  (From  Zittel.)  A,  dorsal,  5,  ventral  valve;  a,  b,  c, 
impressions  of  muscular  insertions;  a,  adductors;  h",  adjusters  (stalk  muscles) ; 
r,  c',  divaricators;  s,  hinge  groove  of  upper  valve  in  which  the  tooth  (t)  of  the 
lower  valve  passes ;  Z,  support  of  arms;  d,  deltidium;  /,  foramen  for  stalk. 

different  expression.  Its  basis  consists  of  two  calcareous  rods 
which,  bilaterally  symmetrical,  project  downwards  from  the  dorsal 
valve.  These  may  be  connected  by  a  curved  transverse  band,  and 
from  their  ends  a  spiral  process  may  extend  on  either  side.  This 
apparatus  supports  the  spiral  arms.  When  closed  the  valves  com- 
pletely enclose  the  body.  When  they  open  the  gape  is  anterior, 


VI.   BEACHIOPODA.  327 

the  posterior  parts  remaining  in  contact.  At  this  part,  except  in 
the  Ecardines,  a  hinge  is  developed  just  in  front  of  the  posterior 
margin,  consisting  of  projections  (teeth)  in  the  ventral  valve  which 
fit  into  corresponding  grooves  in  the  dorsal.  Opening  and  closing 
the  valves  are,  contrary  to  what  occurs  in  Lamellibranchs,  active 
processes,  accomplished  by  appropriate  divaricator  and  adductor 
muscles  (fig.  299).  These  produce  scars  on  the  shell,  important  in 
the  study  of  fossil  forms. 

The  usually  spirally  coiled  arms,  which  lie  right  and  left  of  the 
mouth  and  which  give  the  name  to  the  class,  fill  most  of  the  shell. 
On  the  outer  side  of  the  spiral  axis  runs  a  longitudinal  groove 
which  reaches  to  the  tip  of  the  arms  and  is  bounded  by  a  row  of 
small  tentacles.  By  means  of  cilia  on  tentacles  and  groove  food  is 
brought  to  the  mouth.  These  arms  strongly  resemble  the  lopho- 
phore  of  a  phylactolaemate  Polyzoan,  which  only  needs  extension 
and  coiling  to  produce  this  condition.  In  development  the  arms 
of  the  Brachiopod  pass  through  a  lophophore  stage. 

In  the  body  there  is  a  body  cavity  which  extends  into  both, 
mantle  folds.  It  encloses  alimentary  tract,  gonads,  and  liver,  and 
is  divided  into  right  and  left  halves  by  a  dorsal  mesentery  support- 
ing the  intestine.  Each  half  in  turn  is  divided  by  incomplete 
septa  into  anterior,  middle,  and  posterior  divisions  recalling  those 
of  Sagitta  (p.  296).  If  the  arrangement  of  the  septa  is  not  so 
clear  as  in  that  form,  it  is  to  be  explained  by  the  shortening  of  the 
long  axis  and  the  twisting  of  the  alimentary  tract.  This  latter 
consists  of  oesophagus,  stomach,  which  receives  the  liver  ducts,  and 
intestine,  which  in  some  species  terminates  blindly. 

The  gonads  are  chiefly  in  the  mantle  lobes.  The  sexual  cells 
pass  outwards  through  the  nephridia,  which  begin  in  one  coelomic 
pouch  with  a  wide  nophrostome,  perforate  the  septum,  and  open 
to  the  exterior  in  the  next  somite.  Since  usually  there  are  two 
septa,  two  pairs  of  nephridia  may  occur,  but  one  is  usually  degen- 
erate. The  nervous  system  consists  of  an  cesophageal  ring  with 
weak  dorsal  ganglion,  which  sends  nerves  into  the  arms,  and  a 
stronger  ventral  mass  representing  the  ventral  chain.  The  heart 
lies  dorsal  to  the  stomach. 

In  development  the  brachiopods  recall  both  Sagitta  and  the  Annelida. 
They  resemble  Sagitta  in  that  in  Argiope  the  coelom  arises  by  out- 
growths from  the  archenteron,  divided  by  septa  into  three  pairs  of  pouches. 
They  are  annelid-like  in  the  form  of  larva  and  in  the  presence  of  chastae 
which  are  formed  in  separate  follicles.  In  an  earlier  period  of  the  earth 
brachiopods  were  so  numerous  in  species  and  individuals  that  they  are 
among  the  most  important  fossils  in  the  determination  of  geologic  horizons. 


328 


C(ELUELMINTHES. 


Now  there  are  but  few  species,  some  inhabitants  of  the  greatest  depths  of 
the  sea. 


FIG.  301.—  Development  of  brachiopod.  (After  Kowalevsky.)  A,  gastrula  with  early 
enterocrelic  pouches;  B,  closure  of  blastopore;  C,  coelom  separated,  body  annu- 
lated;  D,  cephalic  disc  and  mantle  developing,  the  latter  with  long  setae;  E,  at- 
tached embryo,  the  mantle  lobes  folded  over  cephalic  disc  (setae  omitted),  c, 
cephalic  disc;  d,  dorsal  lobe  of  mantle;  e,  enterocoele;  rn,  mantle;  v,  ventral  man- 
tle lobe. 

Order  I.  Ecar  dines. 

Hinge  absent;  valves  similar  when  the  stalk  passes  out  between  them 
(Lingula  *),  or  unequal  when  the  ventral  is  perfo- 
rated by  the  stalk  (Discina)  or  when  the  animal  is 
directly  attached  by  the  shell  (Crania}. 

Order  II.  Testicardines. 

Hinge  present,  valves  unequal,  the  ventral 
perforated  by  the  stalk;  anus  degenerate.  Rhyn- 
chonella,*  Terebratulina*  in  onr  colder  waters. 
Qpirtfer,  Orthis,  Pentamerus,  Atrypa,  important 
fossil  genera. 


FIG.  wrebratuiia  sep 
tentrionalig* 


Summary  of  Important  Facts. 

(1)  The  CCELHELMINTHES  are  characterized  by  a  well-developed  body 
cavity  (coelom). 

(2)  The  CH.ETOGNATHI  are  hermaphroditic  worms  with  three  pairs  of 
ccelemic  pouches,  with  fins,  and  bristle-like  jaws. 

(3)  The    NEMATODA   are   mostly  dioecious,  usually  parasitic  elongate 
worms,  with  cylindrical  unsegmented  body,  with  nerve  ring  (no  ganglia), 
paired  excretory  organs,  and  tubular  gonads. 

(4)  The  most  important  species  parasitic  in  man  are  Ascaris  lumbri- 
coides  in  the  small  intestine,  Oxyuris  vermicularis  in  the  large  intestine, 
the  blood-sucking  Ankylostoma  duodenalis,  and  the  notorious   Trichina 
spiralis.     In  hot  climates  occur  Filaria  sanguinis  Jwminis  and  Dracun- 
culns  medinensis. 

(5)  The  GORDIACEA  have  mesenteries  and  splanchnic  mesoderm;  they 
are  parasitic  in  insects. 

(6)  The  ACANTHOCEPHALI  lack  alimentary  tract,  have  a  spiny  proboscis 
and  a  very  complicated  reproductive  apparatus.     The  adults  are  parasitic 
in  vertebrates,  the  young  in  arthropods. 


ECHINODERMA.  329 

(7)  The  CH^TOPOD  ANNELIDS  have  segmented  bodies,  the  segmentation 
showing  itself  in  ringing  of  the  body  wall  and  in  the  separation  of  the 
coeloem  into  a  series  of  pouches  by  transverse  septa  and  the  metameric 
arrangements  of  blood-vessels,  ganglia,  and  excretory  organs. 

(8)  The   CH^ETOPODA  are  distinguished    from   other  annelids   by  the 
chaetaa  (usually  four  bunches  in  a  somite)  arising  in  special  follicles.     The 
chaetae  are  few  in  the  hermaphroditic  Oligochaetae,  numerous  and  borne  on 
special  parapodia  in  the  Polychaetae. 

(9)  The  GEPHYR^A  are  closely  related  to  the  Chaetopoda.    They  are 
saccular,  with  a  crown  of  tentacles  or  well-developed  preoral  lobe.     They 
have  largely  or  entirely  lost  the  segmentation.    Evidence  of  segmentation 
appears  in  some  cases  in  development  and  in  the  ventral  nerve  cord  and 
nephridia. 

(10)  The  HIRUDINEI  are  hermaphroditic  Annelida  which  lack  chaetae 
but  have  sucking  discs.     Their  flattened  bodies,  rudimentary  ccelom,  and 
rich  body  parenchyma  give  them  a  certain  similarity  to  the  Plathelmin- 
thes. 

(11)  The  Hirudinei  have  either  a  protrusible  pharynx  (Rhynchobdella) 
or  three  toothed  jaws  (Gnathobdella).     To  the  latter  belongs  the  medici- 
nal leech  (Hirudo  medicinalis). 

(12)  The  POLYZOA  are  like  the  Hydrozoa  in  being  colonial  and  having  a 
circumoral  crown  of  tentacles.     They  are  distinguished  by  the  complete 
alimentary  canal,  the  large  coelom,  and  the  ganglionic  nervous  system. 

(13)  The  PHORONIDEA  are  closely  like  the  Polyzoa. 

(14)  The  BRACHIOPODA  have  a  bivalve  shell,  the  valves  being  dorsal  and 
ventral. 

(15)  The  body  cavity  is  divided  by  two  septa  into  three  (paired)  cham- 
bers, of  which  one,  rarely  two,  are  provided  with  nephridia. 

(16)  Most  brachiopods  are  attached  by  means  of  a  stalk.     They  are 
divided  into  Ecardines,  without  a  hinge  and  with  anus,  and  Testicardines, 
with  a  hinge  and  no  anus. 


PHYLUM  V.  ECHINODERMA. 

The  Echinoderma  are  separated  from  most  other  animals  by 
their  radial  symmetry,  but  recall  in  this  respect  the  Ccelenterata, 
a  fact  which  led  to  their  inclusion  by  Cuvier  in  the  group 
'  Radiata/  a  view  of  their  relationships  which  was  set  aside  by 
Leuckart  on  account  of  their  different  structure,  especially  the 
presence  of  a  coelom.  In  fact  the  radial  symmetry  of  the  echino- 
clerms  has  a  different  value,  for  while  in  the  Coelenterata  the 
number  four  or  six  (apparently  derived  from  four)  is  fundamental, 
Echinoderma  are,  with  few  exceptions,  five-radiate.  Further,  the 
radial  symmetry  of  the  Coelenterata  is  primitive,  that  of  the 
Echinoderma,  as  development  shows,  is  derived  from  the  bilateral 


330 


ECHINODERMA 


type.     In   other   words,  the   echinoderms  have   descended    from 

bilateral,  possibly  worm-like,  ancestors. 

The  structure  of  the  integument  gives  these  animals  a  charac- 
teristic appearance.  In  the  mesoderm  under  the 
epithelium  calcareous  plates  arise,  forming  a  body 
armor  or  test,  and  since  these  are  usually  pro- 
duced into  spines,  they  have  given  the  name 
Echinoderma,  spine  skin,  to  the  group.  This 
mesodermal  skeleton  at  times  becomes  degenerate, 
as  in  the  Holothurians  (it  rarely  entirely  disappears 
as  in  Pelagothurid),  but  even  then  shows  itself 
as  spicules  and  '  wheels '  of  lime.  The  sphaeridia 
and  pedicellaria  (fig.  303) — not  always  present — 

Flia'ria!°3cioseddiand  are  characteristic  appendages  of  the  integument. 
°Pen  The  first  are  sense  organs;  the  latter  are  usually 

stalked  forceps-like  grasping  structures  with  calcareous  skeleton. 

In  life  they  are  active  and  apparently  either  clean  the  skin  or  are 

defensive. 

Certain  plates  possess  a  morphological  interest  since  they  appear  early 
in  many  larvae,  and  in  the  adults  of  different  classes  can  be  recognized  in 
similar  positions.  In  the  neighborhood  of  the  arms  are  five  basalia,  inter- 
radial  in  position,  farther  five  radialia  (*  apical  skeleton ')  and  five  inter- 
radial  '  oralia '  around  the  mouth. 

Not  less  characteristic  than  the  skeleton  is  the  ambulacral  (or 
water-vascular)  system  (fig.  304).  This  begins  usually  externally 

and  then  ordinarily  by  a  calcareous 
plate,  the  madreporite,  which  is 
perforated  with  fine  pores  and 
serves  for  the  entrance  of  sea  water. 
The  water  passes  into  a  canal 
which,  on  account  of  its  calcified 
walls  in  the  starfish  (fig.  305),  is 
called  the  stone  canal  and  leads 


FIG.  304. 


FIG.  305 


FIG.  304.— Water- vascular  system  of  starfish  (orig.).  a,  ampullae  ;  a/>,  ambulacra ;  c, 
radial  canal;  rn,  madreporite;  n,  radial  nerve;  p,  Polian  vesicle;  r,  ring  canal, 
beneath  it  the  nerve  ring;  s,  stone  canal;  f,  racemose  vesicle. 

FIG.  305  —Transverse  section  of  stone  canal  of  Astropecten  aurantiacus.  (After 
Ludwig.) 


ECHINODERMA.  331 

orally  to  a  ring  canal  around  the  mouth.  The  ring  canal  bears 
usually  several  (up  to  five  pairs)  Polian  vesicles,  which,  with 
Tiedemann's  vesicles  of  the  starfishes,  are  now  regarded  as  appen- 
dages which,  like  lymph  glands,  produce  the  leucocytes.  From 
the  ring  canal  radiate  five  radial  canals  which  give  off  right  and 
left  in  pairs  the  ambulacral  canals.  These  in  turn  connect  with 
the  ambulacra  and  ampullae,  the  highly  characteristic  locomotor 
organs  of  the  echinoderms.  An  ambulacrum  is  a  muscular  sac 
which  can  be  distended  and  lengthened  by  forcing  in  fluid  from 
the  ambulacral  vessels,  on  the  other  hand  can  be  retracted  and 
shortened  by  its  muscles.  The  ampulla  is  a  sac  connected  with 
the  ambulacrum  and  projecting  into  the  body  cavity.  In  locomo- 
tion the  animal  extends  its  ambulacra,  anchors  them  by  the  suck- 
ing disc  at  the  tips,  and  then  pulls  the  body  along  by  contraction  of 
the  ambulacral  muscle.  In  the  sessile  crinoids  and  the  ophiuroids 
(which  move  by  their  snake-like  arms)  the  ambulacra  are  not 
locomotor  but  tactile  in  function,  lacking  suckers  and  ampullae. 
So  among  the  holothurians  and  sea  urchins  the  ambulacra  are  in 
many  places  replaced  by  tentacles.  Frequently  each  radial  canal 
ends  in  an  unpaired  tentacle  with  olfactory  functions. 

The  arrangement  of  the  ambulacral  system  conditions  the 
arrangement  of  other  organs.  Alongside  the  stone  canal  is  a 
saccular  organ  formerly  called  the  ( heart/  but  now  regarded  as  a 
lymphoid  gland  (ovoid  gland,  paraxon  gland).  Ring  and  radial 
canals  are  accompanied  by  corresponding  blood  canals,  with  which 
are  often  associated  two  vessels  to  the  alimentary  canal.  There  is 
a  similar  nerve  ring  and  radial  nerve,  frequently  in  the  ectoderm, 
to  which  may  be  added  an  enteroccelic  or  apical  nervous  system, 
possibly  of  peritoneal  origin. 

The  courses  of  the  radial  vessels  and  nerves  mark  out  five  chief 
lines  in  the  animal,  the  radii;  between  them  come  the  secondary 
radii  or  interradii.  The  stone  canal,  madreporite,  and  lymphoid 
gland  are  interradial  in  position,  as  are  the  gonads,  usually  five 
single  or  five  pairs  of  racemose  glands;  in  some  cases  but  one  is 
present.  The  gonads  are  supported  in  the  spacious  ccelom  by 
special  bands,  while  mesenteries  support  the  alimentary  tract  and 
its  derivatives. 

Respiratory  organs  are  represented  by  very  various  structures:  branchiae, 
or  thin-walled  outpushings  of  the  coelom,  either  around  the  mouth,  as  in 
Echinoidea,  or  on  the  aboral  surface,  as  in  the  Asteroidea,  the  bursae  of  the 
Ophiuroidea,  the  branchial  trees  of  the  Holothuroidea  and  the  various 
parts  of  the  ambulacral  system. 


332 


ECIIINODEUMA. 


The  Echinoderma  are  exclusively  marine,  occurring  in  large 
numbers  even  in  the  deepest  seas.  Many  groups,  like  the  Crinoids, 
are  largely  bathybial,  others  frequent  rocky  coasts.  At  the  period 
of  reproduction  the  urchins,  starfish,  and  holothurians  frequent 
the  shallow  waters,  passing  their  sexual  cells  into  the  sea,  where 
fertilization  occurs.  In  some,  however,  the  young  are  carried 
about  in  brood  cases  until  the  earlier  developmental  stages  are  past. 


m       y 


-\---ct 


FIG.  306.—  Echinoderm  larvae.  (After  J.  Miiller.)  a,  anus;  m,  mouth;  the  black  linei 
the  course  of  the  ciliated  bands.  /,  form  common  to  all ;  //,  ///,  developmental 
stages  of  auricularia  (Holothurian) ;  7F,  V,  stages  of  the  Asteroid  bipmnaria; 
FI,  pluteus  of  a  spatangoid;  F//,  larva  (Brachiolaria)  of  Asterias  (orig.).  m» 
mouth ;  v,  vent. 

Where  there  is  no  brood  pouch  the  young  escape  from  the  egg 
as  larvas  which  swim  at  the  surface,  and  are  distinguishable  from 
the  adults  (fig.  306,  /)  by  their  soft  consistency,  transparency,  and 
bilateral  symmetry.  By  the  development  of  lobe-like  processes 
and  slender  arms  supported  by  calcareous  rods  the  larvae  assume 
the  most  different  and  bizarre  shapes  (plutei  of  echinoids  and 
ophiuroids,  brachiolaria  and  bipinnaria  of  asteroids,  auricularia  of 
holothurians),  all  of  which  can  be  referred  back  to  a  common  type 
with  tri-regional  alimentary  tract  and  a  ciliated  band  around  the 
mouth,  strikingly  resembling  tornaria,  the  larva  of  Balanoglossus* 
The  different  appearances  of  the  larvae  are  due  to  the  drawing  out 
of  the  ciliated  band  into  lobes  and  arms,  and  also  to  its  becoming 
broken  into  parts  which  unite  themselves  into  complete  rings 
(fig.  306,  F). 

The  metamorphosis  of  the  bilateral  larva  into  the  radial  adult  is  very 
complicated.  It  begins  early  with  the  formation  of  outgrowths  from  the 
archenteron  (fig.  307),  which  become  separated  and  form  the  anlagen  of 


I.  ASTEROIDEA. 


333 


the  ccelom  and  ambulacral  system.  This  becomes  divided,  and  one  por- 
tion develops  itself  as  a  ring  around  the  oesophagus,  the  future  ring 
canal,  and  from  this  five  outgrowths,  the  radial  canals,  arise.  Since  these 
canals,  as  they  grow  out,  carry  the  body  walls  before  them,  the  arms  in  the 
starfishes,  which  show  the  process  most  clearly,  arise  as  outgrowths  which 
recall  buds  (fig.  308).  This  has  given  rise  to  one  view  which  regards  the 
arms  as  individuals,  the  whole  body  (and  hence  that  of  all  echinoderms)  as 
a  colony  of  five  individuals.  According  to  this  the  development  would  be 
a  kind  of  alternation  of  generations,  the  larva  being  the  asexual  genera- 


FIG.  307. 


FIG.  308. 


FIG.  307.— Formation  of  the  coelom  in  Echinus.  (From  Korschelt  and  Heider.)  A, 
first  anlage  of  coelom;  B,  later  stage;  C,  complete  constriction  of  coelom  (vaso- 
peritoneal  vesicle)  from  archenteron 

FIG.  308.— Formation  of  Ophiuran  from  the  pluteus  larva.  (After  Miiller,  from  Kor- 
shelt-Heider.) 

tion  which  by  budding  produces  the  colony.  Yet  this  view  does  not  agree 
with  the  actual  relations,  since  it  draws  an  untenable  contrast  between  the 
larva  and  the  perfect  echinoderm.  The  most  important  organs  of  the 
former  are  carried  over  into  the  latter,  and  the  echinoderm  brings  the  anla- 
gen  to  further  development.  In  the  insects  many  features  which  are  lack- 
ing or  incompletely  developed  in  the  larva  are  developed  in  the  course  of 
the  metamorphosis.  There  is  a  metamorphosis  in  the  echinoderms  as  in 
insects.  It  is  a  question  as  to  which  group  of  Echinoderma  is  the  most 
primitive,  but  ease  of  treatment  makes  it  best  to  begin  with  the  Asteroidea. 

Class  I.  Asteroidea  (Starfish). 

Two  parts  can'  be  recognized  in  the  body  of  a  starfish,  a 
central  disc  and  the  arms,  usually  five  in  number,  which  radiate 
from  it  (fig.  316).  The  relations  in  which  these  parts  stand  to 
each  other  vary  between  two  extremes.  In  many  starfish  the 
arms  play  the  chief  role  and  the  disc  appears  as  only  their  united 
proximal  ends  (figs.  309,  310).  On  the  other  hand  the  disc  may 


334 


ECHINODERMA. 


increase  at  the  expense  of  the  arms,  absorbing  these  in  its  growth  so 
that  they  form  merely  the  angles  of  a  pentagonal  disc  (fig.  311). 
In  both  arms  and  disc  two  surfaces  are  recognized,  oral  and 
aboral,  which  pass  into  each  other,  usually  without  a  sharp  margin. 
In  the  normal  position  the  oral  side  is  downwards  and  has  in  the 


FIG.  309.— Comet  form  of  Linckia  multiflora.    (From  Korschelt-Heider.)    One  of  the 
arms  is  producing  a  new  animal  by  budding. 


Fia.  310. 


FIG.  311. 


FlG.  310.— Ophidiaster  ehrenbergi.  (After  Haeckel).  Comet  form:  one  of  the  original 
arms  shown  only  in  part. 

FIG.  311. — Culcita  pentangularis^  aboral  view.  (From  Ludwig.)  a,  madreporite;  &,  re- 
flexed  end  of  ambulacral  grooves. 

centre  the  mouth  and  radiating  from  it  to  the  tips  of  the  arms  the 
five  ambulacral  grooves.  On  the  aboral  surface  is  the  anus  (when 
not  degenerate)  near  the  centre,  and  excentric  from  it  in  an  inter- 
radius  is  the  madreporite  (in  many  armed  species  two  to  sixteen 
radii  may  have  madreporites). 

A  line  passing  through  the  madreporite  and  the  opposite  arm  divides  the 
body  into  symmetrical  halves.  This  ray  is  frequently  spoken  of  as  anterior, 
since  in  the  irregular  sea  urchins  (Spatangoids)  the  homologous  arm  is 
clearly  anterior,  while  the  madreporic  interradius  is  posterior.  This  plane 
of  symmetry  does  not  correspond  with  that  of  the  larva.  The  two  rays  on 
either  side  of  the  madreporite  form  the  bivium,  the  three  others  the 
trivium. 

The  skin  is  everywhere  protected  by  large  and  small  plates 
jointed  together.  These  make  a  dry  starfish  hard  and  stiff,  but 
in  life  it  is  extremely  flexible,  the  arms  can  be  bent  in  any  direc- 


/.    A8TEROIDEA. 


335 


tion,  and  the  animal  can  work  its  way  through  narrow  openings. 
Of  the  skeletal  pieces  the  ambulacral  plates  need  special  mention, 
A  B 


FIG.  312.— -4,  cross-section  of  starfish  arm  (orig.).  a,  adambulacral  plates;  am,  ambu. 
lacra;  op,  ambulacral  plates;  6,  branchiae;  c,  coelom:  /i,  hepatic  caeca;  i,  inter- 
ambulacral  plates;  n,,  radial  nerve;  p,  ampulla;  r,  radial  canal;  r,  radial  blood- 
vessel. B,  ambulacral  plates,  ventral  view,  showing  the  ambulacral  pores 
between. 

These  form  the  roofs  of  the  ambulacral  grooves,  and  between  them 
are  openings,  the  ambulacral  pores,  through  which  connexion  is 
made  between  the  ambulacra  and  ampullae.  In  each  arm  the  pairs 
of  ambulacral  plates  meet  above  the  groove  like  the  rafters  of  a 


FIG. 


313.— Asteriscus  verruculatus,  aboral  surface  removed.    (After  Gegenbaur.) 
gonads ;  /i,  hepatic  caeca ;  /,  stomach  with  anus. 


roof.  Laterally  each  ambulacral  plate  abuts  against  a  small  inter- 
ambulacral  plate,  bearing  usually  movable  spines.  Beyond  these 
comes  the  adambulacral  plates,  and  then  those  of  the  aboral  sur- 
face. Each  ambulacral  area  terminates  at  the  tip  of  the  arm  with, 
an  unpaired  (ocular)  plate. 


336 


ECIIINODERMA. 


The  organs  lie  in  part  in  the  coelom,  in  part  in  the  ambulacra! 
grooves.  The  alimentary  tract  is  in  the  ccelom  and  extends 
straight  upward  from  the  mouth  to  the  aboral  surface,  where  it 
ends  with  an  anus  or  is  entirely  closed  (figs.  313,  314).  By  a 


FIG.  314.— Section  through  ray  and  opposite  interradius  of  a  starfish  (qrig.).  B, 
branchiae;  (7,  cardiac  pouch  of  stomach;  .E1,  eye  spot;  G,  gonad;  //,  'liver';  M% 
mouth;  N,  radial  nerve;  P,  pyloric  part  of  stomach;  RC,  ring  canal;  RD,  -radial 
canal  of  water-vascular  system ;  tf,  stone  canal. 

constriction  it  is  divided  into  a  larger,  lower  cardiac  portion  and 
a  smaller,  upper  pyloric  division.  From  the  latter  arise  five  hepatic 
ducts  which  connect  with  five  pairs  of  hepatic  glands  lying  in  the 
arms.  The  cardiac  division  gives  origin  to  five  gastric  pouches 
which  can  be  protruded  from  the  mouth  or  retracted  by  appro- 
priate muscles.  The  gonads  are  five  pairs  of  racemose  glands  lying 
in  the  basis  of  the  arms  and  opening  interradially  between  the 
arms.  Lastly,  the  stone  canal,  extending  from  the  aboral  madre- 
porite  to  the  ring  canal,  and  the  lymphoid  gland  lie  in  the  coelom. 
The  radial  nerve,  canal,  and  blood-vessel  lie  in  the  roof  of  the 
ambulacra!  groove  between  the  ambulacra.  The  nerve  ends  at  the 


FIG.  315.— Longitudinal  section  of  eye  of  Asterias.    (Orig.) 

tip  of  the  arm  in  a  compound  eye  spot  colored  with  red  or  orange 
pigment  which  experiment  shows  is  sensitive  to  light.  A  second 
nerve  has  been  described  lying  in  the  ccelom  of  the  arm.  The 
ambulacral  system  corresponds  with  the  foregoing  description 


//.    OPHIUROIDEA, 


337 


(p.    330),   the  ampullae  as  well  as  the  five  or  more  Polian   and 
Tiedemann's  (racemose)  vesicles  projecting  into  the  co3lom. 

Since  the  arms  contain  nearly  all  important  organs,  the  physiological 
independence  of  these  is  easily  understood.  Arms  broken  off  not  only 
live,  but  regenerate  first  the  disc  and  then 
new  arms  which  appear  at  first  like  small 
buds  (comet  form,  figs.  309,  310).  This 
separation  of  arms  may  occur  through 
accident,  or  it  may  be,  and  not  infre- 
quently is,  produced  by  the  animal  itself. 

Examples  of  species  with  well-devel- 
oped arms  and  ambulacra  in  four  rows 
are  furnished  by  the  ASTERID^E,  repre- 
sented on  our  shores  by  the  five-finger 
Aster  ias  *  and  Leptasterias,*  and  Heli- 
aster*  with  numerous  arms.  In  the 
SOLASTERID^E  the  ambulacra  are  two- 
rowed;  arms  sometimes  numerous.  Py- 
tlionaster  (fig.  316).  In  the  ASTERINID^E 
the  arms  are  short  or  the  body  is  pentag- 
onal, no  large  plates  on  the  margins  of 
the  arms.  Asteriscus  (fig.  313).  In  other 
forms  (Culcita*  fig.  317,  Hippasteria* 

CtenodiSGUS*)    the  body    is   more   Or   less    FIG. 

pentangular,  the  margin   being  covere.d 
with  large  plates. 

Class  II.  Ophiuroidea  (Brittle  Stars). 

In  these  the  animal  consists,  as  in  the  Asteroidea,  of  disc  and 
arms,  the  latter  sometimes  branched,  but  the  internal  anatomy  is 

different.  The  ambulacral  plates 
have  been  drawn  inside  the  arm 
and  each  pair  fused  to  a  large 
'vertebra'  (fig.  317).  As  a  result 
the  co3lom  of  the  arms  is  greatly 
reduced,  the  hepatic  caeca  are  lack- 
ing, and  the  alimentary  canal,  which 
lacks  an  anus,  is  confined  to  the 
disc.  By  the  ingrowth  of  ven- 
FIG.  SIT.  —  Section  of  Ophiuroid  arm  tral  plates  the  ambulacral  grooves 

(ong.).    a,  ambulacrum;  Z),  blood  ves-  _     .  G 

sei;  c,  coeiom;  w,  muscles  of  arm;  n,  are  converted  into  tubes,  and  the 

nerve;  ?•,  radial  water  tube;  v/verte-         ,     ,  ,  .   ,        ,      _  .  . 

bra  '  (coalesced  ambulacral  plates).      ambulacra,      which      lack      SUCking 

discs,  are  tactile  organs,  locomotion  being  effected  by  the  snake- 
like  motion  of  the  arms.     The  madreporite  is  on  the  ventral  sur- 


316  —  Pythonaster       murrayf. 


338 


ECHINODERMA. 


face.  Also  on  the  ventral  surface  are  five  slits  which  connect 
with  as  many  bursae,  thin-walled  respiratory  sacs  into  which  the 
sexual  organs  open. 

In  many  brittle  stars  (Ophiocnida,  Opliiothelia,  Ophiocoma),  especially 
in  young  specimens,  there  is  a  kind  of  asexual  generation  (schizogony),  the 
animal  dividing  through  the  disc,  the  halves  regenerating  the  missing  parts. 
The  classification  is  based  largely  on  small  details.  In  the  majority  the 
arms  are  unbranched  (Ophiopholis  *  (fig.  318),  Opliwglypha*  Amphiura  *), 
but  in  the  EURYALID^E,  or  basket  fish,  the  arms  are  branched  (Astrophy.ton,* 
fig.  319),  but  not,  as  usually  stated,  dichotomously. 


FIG.  318. 
FIG.  318.— Ophiopholis  aculeata* 


FIG.  319. 
(From  Morse.) 
FIG.  319.— Astrophyton  arborescent,  basket  fish.    (From  Ludwig.) 


Class  III.  Crinoidea  (Pelmatozoa). 

The  crinoids  or  sea  lilies  are  on  the  road  to  extinction.  In 
early  times,  especially  in  the  paleozoic,  they  were  very  abundant, 
but  to-day  there  are  but  few  genera  and  species,  these  mostly 
restricted  to  the  greater  depths  of  the  ocean,  only  the  Comatulidae 
occurring  near  the  shore.  The  crinoids  are  attached  to  the  sea 
bottom  by  a  long  stalk  which  contains  a  central  canal  (fig.  320). 
This  stalk  is  composed  of  cylindrical  discs  and  often  bears  five  rows 
of  outgrowths,  the  cirri.  In  the  Comatulidse  (fig.  321)  the  adult 
is  not  thus  attached,  swimming  about  in  the  water  with  the  arms 
or  moving  about  on  the  tang.  In  their  earlier  stages  these  animals 
have  a  stalk  (fig.  322),  passing  through  a  Pentacrinus  stage,  a 
proof  that  the  fixed  condition  was  the  primitive  one.  In  these 
forms,  when  the  separation  takes  place,  one  joint  of  the  stalk  with 
its  cirri  remains  attached  to  the  animal,  as  the  centrodorsal  united 
with  the  lowest  cup  plates,  the  infrabasals  (fig.  321). 

On  the  upper  joint  of  the  stalk  is  a  cup-shaped  body  (theca) 
the  edges  of  which  bear  five  or  ten  (usually  branched)  arms.  The 


III.    CRINOIDEA. 


339 


Fio.  320.— Pentacrinus  macleayanus.    (After  Wyville  Thompson.) 


FIG.  321.  FIG.  322. 

FIG.  321.— Adult  of  Antedon  macronema.    (After  Carpenter.) 

FIG.  322.— Different  Pentacrinus  stages  (a,  b,  c)  of  Antedon  rosacea.    I,  arms;  2,  cim; 
3,  stalk. 


340 


ECHINODERMA. 


walls  of  the  theca  are  covered  with  polygonal  calcareous  plates. 
Usually  the  stalk  bears  five  plates,  the  basalia,  and  then  come  five 
radialia,  alternating  in  order  with  the  basalia  (fig.  323).  In  some 


FIG.  323.— Hyocrinus  bethleyanus.  A,  tipper  end  of  stalk  with  cup,  and  the  bases  of 
the  arms;  b,  basalia;  ftr,  brachialia;  r,  radialia.  J?,  oral  surface  of  cup  with 
mouth,  five  oralia,  and  the  bases  of  the  arms. 

there  is  a  circle  of  infrabasalia  in  a  line  with  the  radialia.  Fre- 
quently the  elements  of  the  arm,  the  brachialia,  are  directly  attached 
to  the  radials  (fig.  323).  But  often  the  arm  branches  once  or 
several  times  dichotomously,  and  the  first  branching  takes  place  at 
the  base,  so  that  the  arms  seem  to  spring  from  the  theca.  In 
these  cases  the  first  brachialia  are  considered  as  part  of  the  theca 
and  are  called  radialia  distichalia  (figs.  320,  321).  From  the  arms 
arise,  right  and  left,  a  row  of  pinnulae,  lancet-shaped  processes 
supported  by  calcareous  bodies  in  which  the  sexual  products  ripen 
until  freed  by  dehiscence  (fig.  325). 

The  mouth  opening,  in  the  middle  of  the  oral  disc  which  closes 
the  theca,  is  frequently  surrounded  by  five  radial  plates,  the  oralia. 
The  mouth,  which  in  contrast  to  other  echinoderms  is  directed 
upwards,  connects  with  a  spacious  digestive  tract  in  which  oesopha- 
gus, stomach,  and  intestine  can  be  distinguished.  The  anus  is 
interradial  and  near  the  mouth  (fig.  324).  Five  ambulacral 
grooves  begin  at  the  mouth  and  extend  out  on  the  arms,  branching 
with  them  and  extending  to  the  tips  of  the  pinnulae.  These  are 


///.    CRINOIDEA. 


341 


ciliated  and  serve  as  conduits  to  bring  food  to  the  mouth.  Nervous, 
ambulacral,  and  blood  systems  begin  with  a  circumoral  ring.  They 
follow  the  ambulacral  grooves  as  in  the  asteroids,  but  the  ambulacra 


FIG.  334. 


t  IG.  325. 


FIG.  324.— Oral  area  of  criuoi&(Antedon\  showing  by  dotted  lines  the  course  of  the  in- 

testine  from  the  mouth  (m)  to  the  anus  (a) ;  y,  ciliated  grooves  leading  from  the 

arms  to  the  mouth  (orig.). 
FIG.  325  —Cross-section  of    pinnula  of  Antedon.    (After  Ludwig.)    a,  axial  nerve 

cord-  c,  ciliated  cups;  c,  c,  coeliac  canal;  g,  gonad;  s,  sacculi;   sc,  subtentacular 

canal ;  t,  tentacles. 

here  have  no  suckers  nor  ampullae  and  are  merely  tactile  tentacles. 
A  typical  stone  canal  is  also  lacking;  in  its  place  are  five  or  several 
hundred  tubules  leading  from  the  ring  canal  to  the  ccelom.  Oppo- 
site their  ccelomic  mouths  are  fine  pores  in  the  oral  disc  through 
which  water  enters  to  pass  through  the  tubules  into  the  ambulacral 
system.  The  ambulacral  nervous  system  is  weakly  developed. 
The  enterocoale  system,  on  the  other  hand,  is  well  developed  and 
forms  the  axial  cord  running  through  the  brachialia  and  radialia 
to  unite  in  a  ring  in  the  centrodorsal.  A  problematical  organ, 
the  so-called  dorsal  organ,  also  begins  in  the  centrodorsal  and 
extends  up  through  the  axis  of  the  theca  to  the  oral  disc.  It  is 
possibly  a  lymphoid  gland,  possibly  a  structure  for  the  transfer  of 
nutriment,  and  is  apparently  homologous  with  the  '  heart '  of  the 
starfish. 


342  ECIHNODERMA. 

Sub  Class  I.  Eucrinoidea. 

The  foregoing  account  applies  entirely  to  the  Eucrinoidea,  which  may 
be  divided  into  two  groups  : 

Order  I.  TESSELLATA  (Palseocrinoidea).  Theca  with  its  side  walls 
composed  of  immovably  united  thin  plates  ;  the  ambulacral  grooves  usu- 
ally completely  covered  by  calcareous  plates.  Exclusively  paleozoic. 

Order  II.  ARTICULATA  (Neocrinoidea).  Ambulacral  grooves  open, 
tneca  with  compact,  in  part  movably  articulated,  plates.  This  order  left 
the  other  in  the  mesozoic  age,  and  some  families  have  persisted  until  now. 
Rhizocrinus*  (fig.  323)  and  Pentacrinus  (fig.  320),  deep  seas  ;  the  COMA- 
TULID^E  are  fixed  in  the  young,  free  in  the  adult.  Antedon*  (fig.  321). 

Sub  Class  II.  Edrioasteroidea  (Agelacrinoidea). 

Theca  of  irregular  plates  ;  arms  unbranched  and  lying  on  the  theca. 
Possibly  the  ancestors  of  the  noncrinoid  echinoderms.  Paleozoic.  Agela- 
crinus. 

Sub  Class  III.    Cystidea. 

Exclusively  paleozoic  ;  body  spherical,  composed  of  polygonal  plates. 
Stalk  and  arm  structures  rudimentary,  sometimes  lacking.  The  AMPHO 
RIDA  are  by  some  regarded  as  ancestral  of  all  echinoderms.  Holocystites, 
Echinosphcerites  (fig.  326). 


FlO.    326.  Fio.  327. 

FlO.  328.—Echinosphceritesaurantium.    (From  Zittel  ) 

FIG.  3SR.—Pentremitesflorealis.    (From  Zittel).     Lateral,  oral,  and  aboral  views. 

Sub  Class  IV.  Blastoidea. 

Arms  lacking ;  the  mouth  surrounded  by  five  petal-like  ambulacral 
areas.  The  group  appears  at  end  of  Silurian  and  dies  out  with  the  carbon- 
iferous. Pentremites  (fig.  327). 


IV.    ECHINOIDEA. 


343 


Class  IV.  Echinoidea  (Sea  Urchins). 

The  structure  of  the  sea  urchins  is  best  understood  in  the 
spherical  forms  (figs.  328,  330). 
Mouth  and  anus  lie  at  opposite 
poles  of  the  main  axis,  each  open- 
ing immediately  surrounded  by 
areas  covered  by  calcareous  plates, 
the  arrangement  of  which  varies 
with  the  family^  Around  the  anus 
is  the  periproct,  around  the  mouth 
the  peristome,  the  latter  bearing 
sphasridia  and  in  the  Echinoids  five 
pairs  of  interambulacral  gills.  Be- 
tween peristome  and  periproct  the  _ 

FIG.  328.— Ccelopleuriisflnridanus. 

body  wall  is  composed  ot  calcareous 

plates,  which,  except  in  the  Echino- 

thuridse,    are    immovably    united. 

Aside  from  the  extinct  Palaachei- 

noidea  the  plates  are  arranged  in  twenty  meridional  rows,  or,  more 

accurately,  in  ten  double  rows,  two  rows  being  always  intimately 

associated  together.      Five  of  these  double  rows  are  ambulacral, 


(After 

Agassiz.)  Aboral  "view,  the  spines 
removed  to  show  the  ambulacral  (a) 
and  (ib)  interambulacral  areas,  end- 
ing respectively  in  the  ocular  and 
genital  plates ;  in  the  centre  the  four 
plates  of  the  periproct. 


FIG.  329. 


FIG.  330. 


FIG.  329.— Clypeaster  suhdepres&us.    (After  Agassiz.)    Aboral  view,  showing  the  peta- 

loid  ends  of  the  ambulacral  areas. 
FIG.  330.— Diagrammatic  longitudinal  section  through  a  sea  urchin. 

the  alternating  five  interambulacral.  Both  bear  small  hemispheri- 
cal articular  surfaces  on  which  are  situated  the  spines,  either  long 
and  pointed  or  swollen  to  spherical  plates.  These  spines  are  ex- 
tremely mobile  and  are  moved  by  muscles  so  that  they  serve  both  as 
protecting  and  locomotor  structures.  The  ambulacral  plates  are 


34:4:  ECHINODERMA. 

distinguished  from  the  interambulacral  by  the  ambulacral  pores 
by  which  the  ambulacra  on  the  surface  are  connected  with  the 
internal  ampullae.  In  most  sea  urchins  the  paired  grouping  of 
the  pores  results  from  the  fact  that  a  double  canal  extends  from 
ampulla  to  ambulacrum. 

In  the  arrangement  of  the  ambulacra  two  modifications,  the  band  form 
and  the  petaloid,  occur.  In  the  first  the  ambulacra  are  equally  developed 
from  peristome  to  periproct  (fig.  328).  In.  the  second  oral  and  aboral 
regions  may  be  distinguished  (fig.  329).  In  the  oral  region  alone  are  loco- 
motor  feet  always  present,  but  these  are  so  irregularly  arranged  that  no 
striking  figure  results.  In  the  aboral  area  the  ambulacra  are  tentacular 
in  character  and  are  regularly  arranged,  their  pores  bounding  five  petal- 
like  figures  around  the  periproct,  very  distinct  after  removal  of  the  spines. 
In  the  Echinoids,  the  Cidarids  excepted,  the  interambulacral  plates  around 
the  peristome  show  five  pairs  of  notches  for  the  gills,  five  pairs  of  thin- 
walled  branching  extensions  of  the  body  cavity. 

Ambulacral  and  interambulacral  fields  both  end  at  the  periproct 
with  an  unpaired  plate,  the  five  ambulacral  plates  (radialia  of  mor- 
phology) being  called  ocular  plates,  since  they  often  bear  pigment 
spots  formerly  regarded  as  eyes.  They  are  perforated  by  the  end 
of  the  radial  canal  and  nerve,  the  latter  here  uniting  with  the  epi- 
thelium of  the  skin.  The  five  interambulacral  plates  are  called 
genital  plates,  since  they  usually  contain  the  openings  of  the  genital 
ducts.  One  is  often  madreporite  as  well. 

The  interior  of  the  body  is  occupied  by  a  spacious  coelom,  to 


nd    d 


FIG.  331. — Sea  urchin  opened  around  the  equator.  A,  ambulacral  area;  J,  interam- 
bulacral area;  L,  lantern;  d,  intestine;  ed,  anal  end  of  intestine;  g,  gonads;  nd, 
siphon;  oe,  oasophagus;  p,  p',  ring  canal  and  Polian  vesicles;  st,  stone  canal. 

the  walls  of  which  the  thin-walled  alimentary  tract  is  fastened  by 
a  mesentery.    In  the  Clypeastroids  this  tract  forms  a  simple  spiral, 


IV.   ECHINOIDEA. 


345 


but  elsewhere  it  is  a  double  spiral.  It  ascends  from  the  mouth* 
turning  once,  and  then,  bending  on  itself,  coils 
in  the  reverse  direction  to  the  anus  (fig.  331). 
Usually  the  first  portion  of  the  canal  is  accom- 
panied by  a  siphon,  an  accessory  tube  opening 
into  the  main  tube  at  either  end.  Except  in 
the  Spatangoids  the  mouth  is  surrounded  by 
five  sharp-pointed  calcareous  plates,  the  teeth,  FTG.  332.  —  Aristotle's 
which  in  the  Echinoids  are  supported  by  a  l^rofusiimd^°l(^t- 
complicated  system  of  levers,  fulcra,  and  mus-  d?i«?h™a^iveoU;  r«i 
cles,  the  'lantern  of  Aristotle'  (fig.  332). 

The  ring  canal  and  the  ring  of  the  blood  system  lie  on  the 
lantern,  the  stone  canal  and  ovoid  gland  (' heart')  extending 
upwards  from  them  (fig.  330).  The  blood-vascular  ring  gives  oft 
two  blood-vessels  which  run  along  the  alimentary  canal,  while  from 
the  ring  canal  arise  five  ambulacral  or  radial  canals  which  run  on 


A 

iurl 
a,  anus ;  g,  genital  pores ;  i,  ambulacral  areas  ;  m,  madreporite ;  o,  mout 


FIG.  333  — Oral  (A)  and  aboral  (B)  surfaces  of  the  sand  dollar,  Echinarachnius  par 

ith. 


the  inner  side  of  the  test  accompanied  by  nerves  which  radiate 
from  a  nerve  ring.  The  gonads  are  five  (rarely  four  or  two) 
unpaired  organs  in  the  aboral  half  of  the  test,  opening  through  the 
genital  plates,  that  is,  interradially  as  in  the  starfish. 

Order  I.  Palaeechinoidea. 

Paleozoic  forms  with  five  ambulacral  areas,  the  interambulacral  areas 
containing  more  than  two  rows  of  plates.  Melonites. 

Order  II.  Cidaridea  (Regulares). 

Ambulacral  areas  band-like,  body  more  or  less  spherical,  mouth  and 
anus  polar.  Here  belong  the  common  urchins,  represented  on  our  coasts, 
by  Toxopneustes*  Strongylocentrotus,*  Arbacia*  C&lopleurus*  (fig.  328). 


346 


ECHINODERMA. 


Order  III.  Clypeastroidea. 

Irregular  flattened  echinoids  with  central  mouth  and  teeth  ;  anus  out- 
side the  periproct  in  the  posterior  interradius,  sometimes  marginal ;  five 
petaloid  ambulacral  areas.  Clypeaster  (tropical),  Echinarachnius*  (sand 
dollar,  fig.  333),  Mellita*  with  holes  through  the  test. 

Order  IV.  Spatangoidea. 

Bilateral  flattened  forms  more  or  less 
heart-shaped  ;  mouth  and  anus  excentric, 
no  teeth;  usually  five  petaloid  ambulacral 
areas  and  four  genital  plates.  From  the 
forward  position  of  the  mouth  it  follows 
that  only  two  ambulacral  areas  (bivium, 
p.  334)  are  upon  the  lower  surface.  Warmer 
seas.  Spatangus*  Echinocardium,  Brissus. 

Class  V.  Holothuroidea. 

The   sea   cucumbers   are    most  re- 

moved  of  any  sroup from  the  to™1 

echinoderm  appearance.       At  the  first 
anus.  The  bivium  without  tu-  glance  the  skin  appears  naked  and  the 

characteristic  plates  absent.    Yet  these 

are  imbedded  in  the  skin  in  the  shape  of  plates,  wheels,  and  anchors 
(fig.  335).     The  integment  is  tough,  leathery,  and  muscular, with 


FIG.  335.— Dermal  plates  of  Holothurians.    A,  Myriotrochus  rinkii.    (After  Daniels- 
sen.)    _B,  Thyone  briareus ;  C,  Synapta  girardii  (orig.). 

longitudinal  and  circular  fibres.  The  saccular  body  gives  these 
forms  a  worm-like  appearance,  strengthened  by  its  elongation  in 
the  main  axis,  and  with  the  mouth  and  anus  at  the  poles.  Unlike 
other  echinoderms  these  move  with  the  main  axis  parallel  to  the 
ground,  a  condition  which,  to  a  greater  or  less  extent,  leads  to  a 
replacement  of  radial  by  bilateral  symmetry.  One  surface  (trivium) 
becomes  ventral,  the  bivium  dorsal,  and  in  many  the  trivial  ambu- 


HOLOTHUROIDEA. 


347 


<Fio.  336.— Anatomy  of  Caudina  arenata.  (After  Kingsley.)  a,  anastomoses  of  dorsal 
blood-vessel;  b,  branchial  tree;  d,  dorsal  blood-vessel;  /,  mesenterial  filaments; 
g,  genital  opening;  ?,  alimentary  canal;  I,  longitudinal  muscles;  m,  mouth:  <>, 
genital  duct;  p,  pharyngeal  ring  ;  ?•,  gonads,  cut  away  on  right  side;  f,  ampullae 
of  tentacles  ;  v,  ventral  blood-vessel. 


348  ECUINODERMA. 

lacra  alone  are  locomotor,  those  of  the  bivium  being  tactile  or 
wholly  absent. 

In  the  body  cavity  (fig.  336)  lies  the  alimentary  canal,  which 
(except  in  Synapta)  is  coiled  in  a  uniform  manner,  although  many 
minor  convolutions  may  obscure  this.  It  passes  backwards  in  the 
median  dorsal  interradius,forwardin  the  left  ventral  interradius,  and 
then  back  in  the  right  dorsal  interradius  to  the  anus.  It  is  held  in 

position  by  mesenteries  (fig.  337),  and 
near  the  anus  by  numerous  muscular 
filaments.  Into  the  terminal  portion 
one  or  two  branchial  trees  may  empty. 
These  are  tubular  sacs  with  small 
branched  outgrowths  which  are  filled 
with  water.  The  similarity  of  these  to 
the  excretory  organs  of  some  Gephy- 
raea  (p.  317)  was  one  ground  for  re- 
FIG.  337.  —  Transverse  section  of  garding  those  forms  as  intermediate 

Hnlothuria  tubulosa.    (After  Lud-  f 

wig.)  d,  digestive  tract ;  db,  dor- between     worms     ana      ecnmoderms. 

sal  blood-vessel;  0,  gonad  duct;  „,,  n     n 

n,  skin;  tm,  longitudinal  muscles;  They  are  to  be  regarded  as  respiratory, 

liv,  left  branchial  tree ;  wi,  mesen-     •  i-i  •    j*      n       .en    j       -j.i 

teries ;  r',  r3,  ambuiacrai  complex  since  they  are  periodically  mled  with 


f  resh  water.     In  many  species  <  Cuvie- 

™,  right  branchial  tree.  rian  organg  ,  OC(Jur  .  thege  are  morpho. 

logically  specially  modified  portions  of  the  branchial  tree  and  are 
either  connected  with  them  or  separately  with  the  cloaca.  Many 
zoologists  regard  them  as  defensive  structures  because  of  their 
sticky  nature  and  because  they  can  be  cast  out  through  the  anus. 
The  oesophagus  is  usually  surrounded  by  a  ring  of  five  radial 
and  five  interradial  plates  which  serve  as  points  of  attachment  for 
the  longitudinal  muscles.  Just  behind  it  lie  the  ring  canal,  ring 
nerve,  and  the  ring  of  the  blood  system,  each  giving  off  a  radial 
branch  which  here  runs  inside  the  muscular  sac  of  the  body.  From 
the  beginning  of  the  radial  canals  (rarely,  as  in  Synapta,  from  the 
ring  canal)  tubes  extend  outward  to  form  the  extremely  sensitive 
retractile  tentacles  which  surround  the  mouth,  and  which  either 
branch  (Dendrochirotae)  or  bear  frilled  shield-shaped  extremities 
(Aspidochirotae).  A  single  Polian  vesicle  is  usually  present,  and 
the  stone  canal  (except  in  the  Elasipoda)  connects  with  the 
coelom.  Blood-vessels  going  from  the  vascular  ring  form  rich 
anastomoses  on  the  alimentary  canal.  Only  a  single  gonad  (or  a 
pair  of  united  gonads)  occurs.  This  consists  of  numerous  tubules 
which  open  usually  interradially  near  the  mouth. 


F.   HOLOTHUROIDEA. 


349 


The  regenerative  powers  of  these  animals  are  of  interest.  In  unfavor- 
able conditions  (hence  in  preserving  the  animals  in  alcohol  without  nar- 
cotization with  chloral)  they  void  the  whole  viscera  and  yet  may  live  and 
reproduce  the  lost  parts.  In  certain  species  are  found  a  few  parasites. 
One  or  two  harbor  a  small  fish  (Fierasfer)  in  their  cloaca  and  branchial 
trees.  A  parasitic  snail,  Entoconcha  mirabilis,  lives  in  one  species  of 
Synapta,  and  a  mussel,  Entovalva  mirabilis,  in  another. 

Order  I.  Actinopoda. 

Radial  canals  present,  sending  branches  to  the  tentacles  and  am- 
bulacra when  present.  Divided  into  Pedata,  with  ambulacra,  and  Apoda. 
without.  The  PEDATA  include  the  HolothuridsB  with  peltate  tentacles. 


FIG.  338.— Cucumaria  frondosa,  sea  cucumber.  (From  Emerton.) 
Holothuria  *  in  warmer  waters,  one  species  furnishing  the  trepang  of 
Chinese  markets.  The  CUCUMARIID^E  represented  in  our  waters  by  Cucu- 
maria *  (Pentacta)  with  regular  rows  of  ambulacra,  Thyone  *  with  them 
scattered,  and  Psolus*  scaly  with  a  creeping  disc.  The  deep-sea  ELA- 
SIPODA  belong  to  the  Pedata.  The  APODA  are  represented  by  Gaudina  * 
(fig.  336)  and  Molpadia* 

Order  II.  Paractinopoda. 

No  radial  canals  nor  ambulacra.     Tentacular  canals  arising  from  ring 
canal.     Myriotrochus*  Synapla*  Oligotrochus*  (fig.  339). 


350  ECHINODERMA. 

Summary  of  Important  Facts. 

1.  The  ECHINODERMA  share  the  radiate  structure  with  the 
Coelenterata,  but  differ  from  them  (a)  in  the  numerical  basis  of 
the  symmetry  (five) ;  (b)  in  that,  as  embryology  shows,  they  have 
descended  from  bilateral  forms. 

2.  Farther    characters    are   the   existence    of   a   coelom,    the 
ambulacral  system,  and  the  mesodermal  spiny  skeleton,  which  has 
given  the  name  to  the  phylum. 

3.  The  ambulacral  S3^stem  is  locomotor  and  occurs  nowhere  else. 
It  consists  of  a  sieve-like  plate,  the  madreporite  (not  always  pres- 
ent), which  passes  water  to  the  stone  canal,  and  from  this  to  the 


FIG  339,—Oligotrochus  vitreus.*   (After  Danielssen  and  Koren.) 

ring  canal  and  the  radial  canals  to  fill  the  ampullae  and  ambulacra. 
Lateral  branches  supply  the  tentacles  and  cause  their  extension. 

4.  Blood-vessels  and  nerve  cords  run  in  the  same  radii  as  the 
radial  canals  of  the  ambulacral  system;  stone  canal,  madreporite, 
ovoid  gland,  and  genital  ducts  are  interradial. 

5.  The  Echinoderma  are  divided  into  five  classes:  (1)  Aster- 
oidea,   (2)  Ophiuroidea,   (3)  Crinoidea,   (4)    Echinoidea,   and   (5) 
Holothuroidea. 

6.  The  ASTEROIDEA  have  a  disc  and  (usually)  five  arms  into 
which  the  gastric  pouches  and  hepatic  caeca  extend.     The  ambu- 
lacral groove  open. 

7.  The  OPHIUROIDEA  also  have  disc  and  arms,  but  the  ambu- 
lacral groove  is  closed  and  the  hepatic  caeca  absent. 

8.  The    CRINOIDEA   have   a   cup-shaped  body  bearing  arms, 
usually  branching,  with  pinnulae,  and  a  stalk,  usually  with  cirri. 
They   are   either   temporarily   or    permanently   attached.       The 
Crinoidea    are    subdivided    into    Eucrinoidea,    Edrioasteroidea, 
Cystidea,  and  Blastoidea. 

9.  The  ECHINOIDEA  are  usually  spherical  or  oval,  armored  with 
calcareous  plates  which  extend  as  meridional  bands  from  peristome 
to  periproct,  five  pairs  of  ambulacral  and  five  of  interambulacral. 

10.  The  ambulacral  plates  end  at  the  periproct  with  a  single 
ocular  plate;    the  interambulacral  with  a  similar  genital  plate. 
The  madreporite  is  fused  with  one  of  the  genital  plates. 


MOLLUSC  A.  351 

11.  The  regular  sea  urchins  have  the  anus  in  the  centre  of  the 
periproct,  the  mouth  in  the  peristome;  the  ambulacral  areas  are 
band-like. 

12.  The  Clypeastroidea  have  a  central  mouth,  the  anus  outside 
the  periproct  in  the  posterior  interradius;  the  ambulacral  areas 
petaloid. 

13.  The  Spatangoidea  are  markedly  bilateral,  the  mouth  an- 
terior, the  anus  posterior;  ambulacral  areas  petaloid. 

14.  The  HOLOTHUROIDEA   are   elongate   and   worm-like;    the 
skeletal  system  greatly  reduced;  they  are  more  or  less  bilaterally 
symmetrical  and  have  usually  a  single  gonad  and  two  branchial 
trees.     They  are  divided  into  Actinopoda,  with  radial  canals,  and 
Paractinopoda,  without. 

PHYLUM  VI.     MOLLUSCA. 

At  the  first  glance  the  molluscs,  like  the  flatworms  and  leeches, 
give  the  impression  of  parenchymatous  animals.  A  spacious  coelom 
is  absent;  what  was  formerly  regarded  as  a  body  cavity  is  a  system 
of  sinuses  surrounding  the  viscera  and  connected  with  the  blood 
system,  and  is  especially  developed  in  the  Acephala.  More  recently 
the  view  has  gained  ground  that  the  molluscs  have  descended 
from  ccelomate  animals,  and  from  forms  in  which,  by  encroach- 
ments of  a  connective  tissue  and  muscular  parenchyma,  the  coelom 
has  been  reduced  to  the  inconspicuous  remnants  of  the  pericardium 
and  the  lumen  of  the  gonads. 

Where  the  molluscan  organization  is  well  developed,  as  in  the 
snails,  four  parts  may  be  recognized  in  the  body  (fig.  340),  The 
visceral  sac  forms  the  chief  mass  of  the  body;  it  is  less  rich  in 
muscles  than  the  rest  because  it  is  reduced  to  a  thin  peripheral 
layer  by  the  alimentary  canal,  liver,  nephridia,  and  gonads.  In 
front  it  is  continuous  with  the  head,  which,  according  to  the  group, 
is  more  or  less  marked  off  by  a  neck,  and  bears,  besides  the  mouth, 
the  tentacles  and  eyes,  the  most  important  sense  organs.  Below, 
the  visceral  sac  passes  into  a  muscular  mass,  usually  used  for  loco- 
motion, the  foot.  From  the  back  extends  the  pallium  or  mantle, 
a  dermal  fold  which  envelops  a  goodly  part  of  the  body.  The 
Acephala  (fig.  340,  C)  have  a  double  mantle,  right  and  left,  both 
halves  springing  from  the  dorsal  line  and  extending  down  over  the 
visceral  sac  and  foot.  The  cephalopods  (fig.  340,  A)  and  the  snails 
(fig.  340,  B),  on  the  other  hand,  have  an  unpaired  mantle  which 
arises  from  about  the  central  part  of  the  back  and  either  extends 


352 


MOLL  U8C A. 


down  on  all  sides  or,  like  a  cowl,  covers  either  the  anterior  or 
posterior  parts  of  the  body.  The  mantle  is  of  importance  in  two 
ways :  its  outer  surface  is  covered  with  epithelium  which,  like  that 
of  the  adjacent  surface,  has  the  power  of  secreting  shell,  a  thick 
cuticular  layer  of  organic  matter  (conchiolin)  largely  impregnated 
with  calcic  carbonate.  The  inner  surface  of  the  mantle,  together 

SOU 


FIG.  340.— Diagrams  of  three  molluscan  classes.  A.  a  cephalopod  (Sepia) ;  B,  a  gas- 
teropod  (Helix);  C,  an  acephal  (Anodontd).  a,  anus;  c,  cerebral  ganglion;  /it,  foot; 
m,  mantle  chamber;  .sc/i,  shell;  £,  siphon;  v,  visceral  ganglion.  Visceral  sac 
dotted;  mantle  lined,  shell  black. 

with  the  outer  surface  of  the  body,  bounds  a  space,  the  mantle  cavity, 
which,  from  its  most  important  function,  is  also  called  the  bran- 
chial chamber.  Since  most  molluscs  are  aquatic,  special  vascular 
processes  of  the  body,  the  gills  or  branchiae,  lie  in  this  space;  in 
the  terrestrial  forms  its  walls  serve  as  lungs  and  thus  are  respiratory. 
From  the  foregoing  it  will  be  seen  that  the  character  of  the 
mantle  must  exert  an  influence  on  the  shape  of  the  shell  and  on 
the  respiratory  organs.  Paired  mantle  folds  necessitate  two  valves, 
right  and  left,  to  the  shell;  a  right  and  left  branchial  chamber, 
and  right  and  left  gills.  With  an  unpaired  mantle  the  shell  is 


MOLLUSCA.  353 

always  unpaired,  while  the  gills  may  retain  their  primitive  paired 
condition. 

The  gills  in  the  mantle  cavity  are  called  ctenidia,  from  their  resem- 
blance to  combs  with  two  rows  of  teeth.  Each  consists  of  an  axial  portion 
(back  of  the  comb),  containing  the  chief  blood-vessels  and  two  rows  of 
branchial  leaves.  The  whole  is  united  to  the  wall  of  the  branchial  cavity 
by  the  axis  (fig.  385).  In  many  aquatic  forms  the  ctenidia  are  lacking, 
and  then  the  respiration  is  either  diffuse  by  the  skin  or  by  accessory  gills 
which  by  structure  (usually  outside  the  mantle  cavity)  are  distinguished 
from  the  ctenidia. 

Those  parts  of  the  surface  of  the  mollusc  which  are  not  covered 
by  the  shell  have  a  columnar  epithelium  which  is  frequently  ciliated 
and  which  contains  unicellular  mucus  glands,  especially  abundant 
on  the  edge  of  the  mantle.  These  give  these  animals  the  soft  slip- 
pery skin  which  is  implied  in  the  name  Mollusca  (mollis,  soft). 
Many-celled  glands,  like  the  byssus  gland  of  the  Acephala,  the 
pedal  gland  of  many  snails,  occur. 

Although  the  existence  of  head,  foot,  and  mantle  is  very  char- 
acteristic of  the  molluscs,  they  are  not  always  present.  In  the 
Acephala  there  is  no  distinct  head  region;  many  gasteropods  lack 
the  mantle  and  hence  the  shell;  in  the  Cephalopoda  the  foot  is 
converted  into  other  appendages,  the  siphon  and  arms.  These 
modifications  are  to  be  explained  by  degeneration  and  evolution. 
In  the  nervous  system  are  also  some  highly  characteristic  features. 
As  a  rule  it  consists  of  three  pairs  of  ganglia  associated  with 
important  sense  organs  and  connected  by  nerve  cords.  One  pair 
lies  dorsal  to  the  oesophagus  and  corresponds  to  the  supraoesophageal 
ganglion  of  the  worms;  it  is  the  brain  (cerebrum)  and  supplies 


V^- 


Fia.  341.— Nervous  systems  of  Molluscs.  A,  most  gasteropods;  J9,  acephals;  C,  cepha- 
lopods  and  pulmonates.  c,  cerebral;  pa,  parietal,  pe,  pedal,  pi,  pleural,  and  v, 
visceral  ganglia. 

the  tentacles  and  eyes.  A  second  pair  lies  ventral  to  the  alimentary 
tract  on  the  front  part  of  the  muscle  mass  of  the  foot :  these  are 
the  pedal  ganglia  which  are  connected  with  the  otocysts.  The 
third  pair,  the  visceral  ganglia,  are  also  ventral,  and  near  them 
are  the  third  sense  organs,  which  are  widely  distributeed  through 
the  Mollusca,  and  which  from  position  and  structure  are  regarded 


354  MOLLUSC  A. 

as  organs  of  smell  (osphradia).  They  are  thickened  patches  of 
ciliated  epithelia  extending  into  the  mantle  cavity.  Pedal  and 
visceral  ganglia  are  united  to  the  cerebrum  by  nerve  cords,  the 
cerebropedal  and  cerebrovisceral  connectives  respectively.  Accord- 
ingly as  these  connectives  are  long  or  short  the  ganglia  are  wide 
apart  or  united  into  a  nerve  mass  around  the  oesophagus. 

Primitive  Mollusca  (Amphineura)  have  a  simpler  condition.  The 
cerebral  ganglia  lie  dorsal  to  the  oesophagus  and  are  united  by  a  cord 
around  the  oesophagus  (fig.  344).  From  it  are  given  off  two  pairs  of  lat- 
eral nerve  tracts,  the  ventral  or  pedal  cords,  and  lateral  or  pleural  cords, 
the  latter  united  by  a  loop  dorsal  to  the  anus.  By  a  concentration  of 
ganglion  cells  the  pedal  cords  give  rise  to  the  pedal  ganglia,  and  similarly 
the  pleural  cords  form  three  pairs  of  ganglia,  the  pleural  and  the  parietal, 
as  well  as  the  visceral  already  mentioned,  of  the  cerebrovisceral  cord  (fig. 
341,  A).  The  pleural  ganglia  are  connected  with  the  pedal  by  nerve  cords; 
the  parietal  innervates  the  osphradium.  When  farther  concentration  takes 
place  the  pleural  may  unite  with  the  cerebral,  and  the  parietal  with  the 
visceral  (fig.  341,  B),  or  both  may  fuse  with  the  visceral  (C).  In  the  latter 
case  the  visceral  ganglion  (in  the  wider  sense)  is  associated  with  the  pedal 
by  the  pleuropedal  connective  ;  while  in  the  other  the  connective  is  appa- 
rently absent  because  fused  with  the  cerebropedal.  Although  the  otocyst 
receives  its  nerve  from  the  pedal  ganglion,  the  centre  of  innervation  lies  in 
the  cerebrum. 

The  heart,  which  lies  dorsally,  consists  of  auricles  and  ven- 
tricles. The  ventricle  is  always  unpaired,  but  there  are  two  auricles 
where  two  gills  exist  from  which  the  blood  flows  to  the  heart,  but 
with  the  loss  of  one  gill  one  auricle  may  disappear.  Distinct  arteries 
and  veins  occur;  capillaries  are  found  only  in  the  Cephalopoda, 
while  in  the  lower  molluscs,  and  especially  in  the  Acephala,  the 
smaller  arteries  open  into  lacunar  spaces  which  were  formerly 
regarded  as  the  body  cavity.  A  completely  closed  vascular  system 
does  not  exist  even  in  the  Cephalopoda. 

The  heart  is  enclosed  in  a  spacious  sac  or  pericardium,  which, 
with  few  exceptions,  is  connected  with  the  nephridia  by  a  ciliated 
canal,  and  in  many  molluscs  (Cephalopoda  and  some  Acephala)  is 
also  related  to  the  gonads.  These  facts  support  the  view,  already 
mentioned,  that  the  pericardium  and  the  lumen  of  the  gonads  are 
the  remnants  of  the  coelom;  for  here,  as  in  the  annelids,  the 
nephridia  open  by  ciliated  nephrostomes  into  the  ccelom,  and  the 
sexual  cells  arise  either  from  the  coelomic  walls  or  from  sacs  cut 
off  from  them.  Even  more  important  for  this  view  would  be 
confirmation  of  the  disputed  statement  that  in  Paludina  vivipara 
the  coelom  (enteroccele)  arises  as  diverticula  from  the  archenteron. 


MOLLUSC  A.  355 

Nephridia  and  sexual  organs  are  primitively  paired,  but  fre- 
quently are  single  by  the  degeneration  of  the  structures  of  one  side. 
The  animals  are  either  hermaphroditic  or  dioecious,  but  the  gonads 
are  always  very  large.  Even  more  room  in  the  visceral  sac  is 
demanded  by  the  digestive  tract  in  which  oesophagus,  stomach,  a 
coiled  intestine,  a  voluminous  liver,  and  frequently  salivary  glands 
may  be  recognized.  The  radula  or  lingual  ribbon  is  also  a  char- 
acteristic organ,  and  its  absence  from  the  Acephala  is  probably  to- 
be  explained  by  degeneration.  It  is  a  plate  or  band  armed  with 
teeth  which  lies  on  the  floor  of  the  pharynx  on  a  ventral  ridge,, 
the  tongue,  and  is  used  for  the  communication  of  food  (figs.  366,. 
367). 

Keproduction  is  exclusively  sexual;  budding,  fission,  or  parthen- 
ogenesis have  not  yet  been  observed.  The  eggs,  united  in  large 
numbers,  are  usually  enveloped  in  jelly  and  are  either  rich  in 
deutoplasm  or  are  enveloped  in  a  nourishing  albumen.  A  few~ 
molluscs  (e.g.,  Paludina  vivipara)  are  viviparous.  A  metamor- 


ties 


FIG.  342.—  Veliger  larva  (trochophore)  of  Teredo  navalte.  (From  Hatschek.)  A,  anus: 
J,  stomach  ;  J^  intestine;  £,  liver;  LM.d,  LM.v,  dorsal  and  ventral  longitudinal 
muscles  ;  Meg,  primitive  mesoderm  cells  ;  JfP,  teloblast  ;  Neph,  protonephros  ;  O, 
mouth  ;  Oe,  oesophagus  ;  R,  rectum  ;  S,  shell  ;  ScM,  hinge  ;  SM.h,  SM.v,  posterior 
and  anterior  adductors;  S»,  apical  plate;  Wkr,  wkr.  pre-  and  postoral  ciliated 
bands  ;  uvs,  cilia  of  apical  plate. 

phosis  is  of  wide  occurrence.  In  such  cases  a  'veliger'  larva 
escapes  from  the  egg  (fig.  342)  ;  in  this  can  be  recognized  head, 
foot,  and  mantle,  even  in  those  cases  where  one  or  the  other  of 
these  is  lacking  in  the  adult.  This  shows  that  the  absence  of 


356 


MOLL  USCA. 


mantle,  shell,  or  head,  which  occur  in  large  groups  of  molluscs, 
is  not  a  primitive  condition,  but  can  only  be  explained  by  degen- 
eration. The  name  veliger  arises  from  the  velum,  a  strong  circle 
of  cilia,  which  surrounds  a  frontal  or  velar  field  in  front  of  the 
mouth,  and  which  serves  as  a  locomotor  organ  for  the  larva.  In 
some  cases  (fig.  343  B}  it  is  lobed  like  the  trochus  of  a  Rotifer. 


Fia.  343.— Veliger  stages.  A,  of  a  snail;   B,   of   a    Pteropod.     (From  Gegenbaur.) 
o,  shell;  op,  operculum  ;  p,  foot ;  t,  tentacle;  v,  velum. 

The  veliger  recalls  the  annelid  trochophore  and  serves  for  the 
distribution  of  the  species;  it  is  therefore  of  great  importance  for 
animals  which,  like  most  molluscs,  are  sedentary  or  slow-moving. 
In  cases  without  metamorphosis  (Cephalopoda,  Pulmonata,  etc.) 
the  veliger  stage  is  frequently  indicated  during  embryonic  devel- 
opment by  a  ridge  of  cells  surrounding  a  preoral  velar  field. 


Class  I.  Amphineura. 

These  forms,  some  of  which  appear  in  the  Silurian,  are  clearly 
the  most  primitive  of  molluscs,  and  are  distinguished  by  a  marked 
bilateral  symmetry.  The  nervous  system  already  described  (p. 
354)  consists  of  pleural  and  pedal  cords  with  scattered  ganglion 
cells  and  no  ganglia,  these  cords  being  connected  by  numerous 
commissures  (fig.  344,  B}. 

Sub  Class  I.  PlacopJiora  (CMtonidce). 

The  chitons  were  formerly  included  among  the  gasteropods 
because  of  the  presence  of  a  creeping  foot  and  a  radula.  They 
are  at  a  glance  distinguished  from  them  by  the  rudimentary  con- 
dition of  the  head  and  the  shell.  This  last  is  unique  among  mol- 
luscs; it  consists  of  eight  transverse  plates  overlapping  like  shingles, 
which  allows  the  animal  to  roll  itself  into  a  ball.  The  edge  of  the 


L    AMPHINEURA. 


357 


mantle  extends  beyond  the  shell  and  is  covered  with  spines,  while 
in  the  mantle  cavity  beneath  are,  right  and  left,  a  series  of  ctenidia. 
Nerves  enter  the  shell  and  end  with  noticeable  sense  organs  (aes- 


FiG.344. — Chiton  squamosus,  dorsal  view.  (After  Haller.)  A,  the  entire  animal;  /?, 
after  removal  of  shell  and  viscera,  a,  anus;  C',  brain;  K,  ctenidia;  o,  mouth; 
P,  pedal  nerve  cord;  pi,  pleurovisceral  nerve  cord. 

thetes  and,  in  some,  eyes,  fig.  345).     The  symmetry  of  the  body  is 
also  expressed  in  the  viscera.     The  anus  is  medial,  and  right  and 


FIG.  345. — Eye  and  aesthetes  of  Acanthopleura  spiniger.  (After  Moseley.)  a,  macrges- 
thete;  b,  micrassthete;  /,  calcareous  cornea;  gr,  lens;  /i,  iris;  fc,  pigmented  cap- 
sule ;  n,  p,  nerves  ;  ?•,  retina. 

left  of  it  are  the  openings  of  the  nephridia  and  sexual  organs.  The 
sexes  are  separate,  the  gonads  unpaired,  while  corresponding  to  the 
paired  arrangement  of  the  gills  there  are  two  auricles  to  the  heart. 

The  Chitons  are  represented  on  our  northeastern  coast  by  several 
small  species  (Trachydermon,*  Auricula*};  farther  south  and  on  the 
Pacific  shores  are  larger  species  (Cryptochiton  *). 


358  MOLLUSC  A. 

Sub  Class  II.   Solenogastres  (Aplacophora). 

Worm-like  forms  without  shell;  the 
foot  rudimentary  and  at  the  bottom  of 
a   ventral   groove.     The  radula  is  also 
reduced;  in  Chcetoderma  it  bears  but  a 
single  tooth.     The  gills  are  either  small 
or  wanting.     The  usually  hermaphrodite 
animals  have  the  gonads  emptying  into 
an  unpaired  chamber  (pericardium?)  and 
FIG.  m.-Neomenia  corinata,  thence   to   the   exterior    by  the   paired 
BSS^IS^^SS  nephridia.      Clmtoderma  in   New   Eng- 
terior;c,  ventral  groove.          land;  ^eomenia,  Dondersia. 

Class  II.  Acephala  (Lamellibranchiata,  Pelecypoda). 

These  have,  among  the  molluscs,  the  least  powers  of  locomo- 
tion. Some  are  fixed,  the  majority  burrow  slowly  through  sand 
or  mud ;  only  a  few  spring  by  means  of  the  foot  or  swim  by  open- 
ing or  closing  the  shells.  Hence  it  is  that  they  need  more  pro- 
tection than  other  species,  and  this  is  afforded  by  the  strong  shells 
in  which  the  body  can  usually  be  completely  enclosed.  This  shell 
recalls  that  of  the  brachiopod  in  that  it  consists  of  two  halves  or 
valves,  but  these  valves  are  right  and  left  rather  than  dorsal  and 
ventral,  and  hence  are  usually  symmetrical  in  shape.  Only  when 
the  animal  rests  permanently  on  the  right  or  left  side  is  this  sym- 
metry lost,  and  then  the  symmetry  of  the  soft  parts  is  affected. 

The  two  lobes  of  the  mantle  which  secrete  the  shell  on  their 
outer  surface  arise  from  the  back  of  the  animal  and  grow  down- 
wards, forwards,  and  backwards,  so  that  they  envelop  the  whole 
-(fig.  352).  Hence  the  oldest  and  the  most  thickened  part  of  the 
.shell,  the  umbo,  occurs  near  the  back  (fig,  347).  Around  this 
the  lines  of  growth  are  arranged  concentrically,  lines  which  show 
how,  by  gradual  growth  of  the  mantle,  the  shell  has  increased  in 
.size.  On  the  back  the  valves  approach  each  other,  and  in  the 
majority  are  movably  connected  by  a  hinge,  which  consists  of 
projections  ('  teeth ')  in  one  valve  fitting  into  depressions  in  the 
•other.  In  the  Brachiopoda  the  valves  are  opened  by  appropriate 
muscles;  in  the  Acephala  by  an  elastic  hinge  ligament  usually  placed 
dorsal  to  and  behind  the  hinge.  The  shell  is  closed  by  adductor 
muscles  which  extend  through  the  body  from  shell  to  shell,  leav- 
ing their  impressions  or  scars  on  the  inner  surface  (fig.  347). 


//.   ACEPHALA. 


359 


Usually  there  occur  an  anterior  and  a  posterior  adductor  equally 
well  developed  (Dimyaria);  less  frequently  the  anterior  is  rudi- 
mentary (Heteromyaria)  or  entirely  disappears  (Monomyaria). 


FIG.  348. 
FIG.  347.— Left  valve  of  Crassatella  plumhea,  inner  and  outer  surfaces.    (From  Zittel.) 

The  outer  surface  showing  lines  of  growth  ;  no  pallial  sinus. 
FIG.  348.  —Right  valve  of  Mactra  gtuitorwm,  with  pallial  sinus.  (From  Lud  wig-Leunis.) 

Letters  for  both  figures:  a',  anterior;   a",  posterior  adductor  scar;  e,  hinge;   I, 

internal  ligamental  groove ;  m,  pallial  line ;  s,  pallial  sinus. 

When  the  muscles  are  relaxed  (as  always  occurs  at  death)  the  elastic 
ligament  opens  the  valves. 

The  heterodont  hinge  is  the  typical  form  (fig.  348);  each  valve  bears  a 
group  of  teeth  near  the  umbo,  those  of  the  left  alternating  with  those  of 
the  right.  Besides  these  '  cardinal  teeth  '  there  are  in  front  and  behind 
'lateral  teeth?  often  produced  into  ridges.  The  ligament  lies  behind  the 
hinge  and  is  usually  visible  from  the  outside  (external  ligament),  but  is 
occasionally  transferred  to  the  interior  (internal  ligament,  fig.  347).  The  so- 
called  schizodont  and  desmodont  hinges  are  modifications  of  the  hetero- 
dont. Then  there  are  Acephala  of  apparently  primitive  character  which 
either  lack  the  hinge  (dysodont),  or  have  one  composed  of  numerous  teeth 
in  a  series  symmetrical  to  the  umbo  (taxodont),  or  of  two  strong  teeth  like- 
wise symmetrical  to  the  umbo  (isodont).  In  these  cases  the  ligament  is 
developed  in  front  of  as  well  as  behind  the  umbo,  and  may  be  either 
external  or  internal. 

Since  the  secretion  of  shell  takes  place  most  rapidly  at  the  edge 
of  the  mantle,  both  are  closely  united,  the  union  being  strength- 
ened by  small  muscles.  So  the  edge  of  the  shell  has  a  different 
appearance  from  the  rest,  this  part  being  marked  off  by  a  pallial 
line  parallel  to  the  margin  (fig.  347).  In  many  species,  the  Sinu- 


360 


MOLLUSC  A. 


palliata,  the  line  at  the  hinder  end  makes  a  large  bay  (pallial 
sinus)  (fig.  348,  s).  Since  the  mantle  folds  are  membranes  with 
free  margins,  it  follows  that  when  the  shell  is  closed  these  edges  are 
pressed  together,  which  would  prevent  the  free  entrance  and  exit 
of  water.  To  accommodate  this  each  mantle  has  its  margin  exca- 
vated at  the  posterior  end,  so  that  when  brought  together  two 
openings,  an  upper  and  a  lower,  result  (fig.  349,  0).  The  lower 


FlG.  349.— Ventral  views  of  siphonate  and  asiphonate  acephals.  A,  Anodonta  cygnea  ,' 
B,  Isocardia  cor  ;  (7,  Lutraria  elliptica.  a,  anal  siphon  ;  />,  branchial  siphon  ;/,  foot; 
A;',  outer,  A.",  inner  gill  lamella;  wi,  mantle;  s,  shell. 

of  these  is  the  branchial  opening  by  which  fresh  water  passes  into 
the  mantle  (branchial)  chamber;  it  flows  out  after  passing  over 
the  gills,  along  with  the  faeces,  through  the  upper  or  cloacal  open- 
ing. In  many  bivalves  the  free  edges  of  the  mantle  grow  together, 


FIG.  350.— Section  of  shell  of  Aru 


;.    c,  cuticula;  p,  prismatic  layer;  I,  nacreous 
layer. 


leaving  three  openings,  one  for  the  protrusion  of  the  foot,  the 
others  the  two  just  described,  which  are  now  called  the  incurrent 
(branchial)  and  excurrent  (cloacal)  siphons  (fig.  349,  B).  By 
further  development  the  margins  of  these  openings  are  drawn  out 


//.   ACEPHALA. 


361 


into  two  long  conjoined  tubes  (fig.  349,  A},  which  for  their  retrac- 
tion need  special  muscles,  which  are  attached  to  the  valves  and 
thus  cause  the  pallial  sinus  referred  to  above  (fig.  348). 

In  the  shell  three  layers  may  be  distinguished  (fig.  350) :  on  the  outside 
a  thin  organic  cuticula  and  below  two  layers  largely  of  calcic  carbonate. 
In  many  these  two  layers  are  distinguished  as  the  prismatic  layer  and  the 
nacreous  layer,  the  first  consisting  of  closely  packed  prisms;  the  nacreous 
layer  of  thin  lamellae  generally  parallel  to  the  surface.  These  by  their  free 
edges  produce  diffraction  spectra  and  so  the  iridescent  appearance  of  the 
shell;  the  finer  the  lines  thus  formed  the  more  beautiful  the  play  of  colors. 
This  is  especially  noticeable  in  the  mother-of-pearl  shells  Meleagrina  and 
Margaritina  margaritifera.  When  foreign  substances  get  between  mantle 
and  shell  they  stimulate  a  greater  secretion  of  nacreous  substance  and 
become  surrounded  by  layers  of  it.  In  this  way  pearls  are  formed. 


K?    K3 


FIG.  351.— Anatomy  of  Anodonta,  the  mantle,  gill,  and  liver  of  the  right  side 
removed,  the  pericardium  opened.  1,  2,  anterior  and  posterior  adductors;  I,  II, 
III,  cerebral,  pedal,  and  visceral  ganglia;  a,  anus;  7.1,  ib2,  upper  and  lower  limbs 
of  organs  of  Bojanus ;  frr,  branchial  siphon;  d,  intestine;  e,  nephridial  opening; 
fu,  foot ;  0,  gonad ;  Ti1,  ft2,  ventricle  and  auricle  of  heart ;  7c',  insertion  of  both 
lamellae  of  right  gill ;  fc3, 7c4,  inner  and  outer  lamellae  of  left  gill;  Z,  left  liver  ;  I', 
its  opening  in  ?n,  stomach;  rn7,  pallial  line;  r1,  anterior,  ra,  posterior  retractor 
muscle:  sp,  nephrostome :  v,  labial  palpus.  The  arrows  show  the  planes  of  sec- 
tion of  fig.  352. 

The  gills  lie  between  the  mantle  and  the  body  and  from  their 
lamellar  character  have  given  rise  to  the  name  Lamellibranchiata. 
(figs.  351,  352).  Two  gill-leaves  occur  on  either  side,  although 
occasionally  the  outer  or  both  may  degenerate.  Frequently  the 
gills  of  the  two  sides  unite  behind  the  body  and  produce  a  parti- 
tion which  separates  the  mantle  cavity  into  a  small  dorsal  cloacal 


MOLLUSC  A. 

chamber  and  the  larger  lower  respiratory  cavity.  Into  the  cloaca 
empty  the  anus  and  the  water  which  has  passed  over  the  gills;  it 
opens  to  the  exterior  through  the  excurrent  siphon.  The  incurrent 
siphon  leads  into  the  branchial  chamber.  In  front  of  the  gills 
are  two  more  pairs  of  leaf -like  lobes,  the  labial  palpi,  between 
which  is  the  mouth. 

The  gills  are  variously  developed.  The  Nuculidse — the  most  primitive 
of  living  Acephala — have  true  ctenidia  consisting  of  an  axis  grown  to 
the  body  and  an  inner  and  an  outer  row  of  gill  leaves  (fig.  355).  From 
this  the  filibranch  type  is  easily  derived.  The  gill  leaves  grow  out  into 


...*' 


FIQ.  352.— Projection  of  sections  shown  by  the  arrows  in  fig.  351.  61,  b2,  upper  and 
lower  limbs  of  nephridium  (organ  of  Bojanus) ;  rf,  intestine ;  e,  nephridiopore ; 
/M,  foot ;  g,  gonad ;  h1.  ventricle  surrounding  the  intestine  ;  h2,  auricle :  fc1,  fc2,  inner 
and  outer  gill  lamellae ;  J,  hinge  ligament ;  m,  mantle ;  n,  cerebro-visceral  com- 
missure ;  sp,  nephrostome ;  v,  venous  sinus. 

long  filaments,  each  bent  on  itself  so  that  it  presents  two  limbs,  a  descend- 
ing and  an  ascending.  These  branchial  threads  are  so  matted  together 
that  they  give  the  impression  of  a  continuous  leaf.  In  the  true  lamellar 
gill  the  threads  of  the  filibranch  grow  together  at  intervals,  leaving  open- 
ings, the  gill  slits.  Since  there  is  an  ascending  and  a  descending  limb,  it 
follows  that  each  gill  consists  of  an  inner  and  an  outer  leaf  (fig.  352),  leav- 
ing a  space  between  into  which  the  gill  slits  open.  This  internal  space  in 
some  serves  to  contain  the  young. 

The  complete  enclosure  of  the  body  in  the  mantle  folds  has 
led  to  a   degeneration   of  the  head  and  its  normal  appendages 


//.   ACEPHALA. 


363 


{Aeephala).  Hence  there  are  only  two  divisions  in  the  body, 
dorsally  the  visceral  sac  and  ventrally  the  foot.  The  foot,  degener- 
ate in  many,  has  a  broad  sole  only  in  Pectunculus  and  the  Nuculi- 
dre;  usually  it  is  hatchet-shaped  (Pelecypoda),  that  is,  compressed 
with  a  rounded  ventral  margin.  It  may  be  enormously  expanded 
and  contracted  again.  This  expansion  is  often  explained  by  the 
taking  of  water  into  the  blood,  but  now  it  is  generally  accepted 
that  it  is  accomplished  by  forcing  blood  from  other  regions  into 
it.  While  the  foot  by  this  extensibility  can  serve  as  a  locomotor 
organ,  it  also  functions  in  many  as  an  organ  of  attachment. 
Inside  is  a  large  byssus  gland  which  can  secrete  silky  threads,  the 
fcyssus  (fig.  353),  one  end  of  which  is  fastened  to  foreign  objects  by 


FIG.  353.— Mytilus  edulis*.  (After  Blanchard.)  a,  edge  of  mantle  ;  b,  spinning  finger  of 
foot ;  c,  byssus ;  d,  e,  retractors  of  foot ;  /,  mouth  ;  0,  labial  palpi ;  7t,  mantle  ;  i,  j, 
inner  and  outer  gills. 

means  of  a  finger-like  process  of  the  foot,  while  the  other  end 
remains  in  connection  with  the  foot.  Molluscs  which  have  a  byssal 
gland  are  found  anchored  by  a  thick  bunch  of  byssal  threads  to 
stones,  etc. 

The  heart,  surrounded  by  a  pericardium,  usually  occupies  the 
most  dorsal  part  of  the  visceral  sac.  It  consists  of  a  ventricle  and 
a  pair  of  auricles  (figs.  351,  352,  7*1,  7*2).  The  auricles  receive  the 
blood  direct  from  the  gills;  the  ventricle  forces  it  out  through 
anterior  and  posterior  aortae  (fig.  351),  the  latter  lacking  in  many 
species. 

The  excretory  organs  (organs  of  Bojanus)  lie  immediately 
below  the  pericardium.  The  organs  of  the  two  sides  touch  in  the 


364  MOLLUSCA. 

middle  line.  Each  consists  of  a  dorsal  smooth-walled  chamber 
and  a  lower  portion  traversed  by  threads,  both  connected  behind 
but  separated  elsewhere  by  a  thin  partition.  The  lower  chamber 
is  connected  in  front  with  the  pericardium  by  a  ciliated  canal,  the 
nephrostome,  while  the  upper  opens  to  the  outside  by  a  short 
canal,  the  ureter,  the  external  opening  being  in  the  region  of  the 
inner  cavity  of  the  inner  gill.  In  this  way  a  connexion  is  estab- 
lished from  the  pericardium  to  the  exterior,  the  apparatus  being 
apparently  a  true  nephridium.  In  many  it  serves  also  as  genital 
duct,  but  usually  the  genital  and  reproductive  ducts  are  separate. 
The  animals  are  usually  dioecious,  the  gonads  being  acinose 
glands. 

The  digestive  tract  (fig.  351)  begins  with  a  short  oesophagus, 
widens  out  to  a  large  stomach  from  which  a  slender  intestine  leads, 
with  many  convolutions,  to  the  anus.  In  the  majority  of  Acephals 
the  terminal  portion  enters  the  pericardium  in  front  and  below, 
passes  through  the  ventricle  and  out  through  the  upper  posterior 
wall  of  the  pericardium.  In  its  course  the  alimentary  tract  i& 
enveloped  by  the  gonads  and  the  voluminous  liver,  the  secretion 
of  the  latter  emptying  by  two  ducts  into  the  stomach.  Usually 
the  stomach  has  a  blind  sac,  in  which  lies  the  'crystalline  style/  a 
rod-like  structure  of  uncertain  significance. 

The  three  typical  molluscan  ganglia  (p.  353)  are  uncommonly 
wide  apart.  The  two  brain  ganglia  (cerebropleural  ganglia)  lie 
either  side  of  the  mouth  at  tho  base  of  the  labial  palpi  and  central 
to  the  anterior  adductor.  They  are  very  small,  since  cephalic 
sense  organs  are  lacking,  and  are  united  by  a  transverse  supra- 
oesophageal  commissure.  The  posterior  ganglia,  composed  of  the 
united  parietal  and  pedal  ganglia,  lie  near  the  anus  ventral  to  the 
posterior  adductor.  The  pedal  ganglia,  rather  far  forward  in 
the  muscles  of  the  foot,  are  closely  approximate.  Of  the  higher 
sense  organs  only  the  otocysts  near  the  foot  are  constant.  The 
labial  palpi  are  also  highly  sensory,  while  two  small  osphradia  occur 
at  the  basis  of  the  gills.  When  eyes  occur  they  are,  as  in  the  scal- 
lops (Pectinidae),  arranged  in  a  row  like  pearls  on  the  margin  of 
the  mantle.  Small  tentacles  with  sensory  powers  may  occur  both 
on  the  margin  of  the  mantle  and  on  the  tip  of  the  siphon. 

Veligers  (fig.  342)  are  very  common  in  development.  When  this  stage 
is  lacking  the  history  may  contain  a  metamorphosis  as  in  the  fresh-water 
Anodonta.  The  young  which  grow  in  the  maternal  gills  are  known  as 
Glochidia,  which  are  distinguished  from  the  adult  by  a  byssus  thread, 
by  only  a  single  adductor,  and  by  a  hook  or  tooth  on  the  free  margin  of 


//.    ACEPHALA:  PRGTOCHONCHI^]. 


305 


the  shell  (fig.  354).  After  escape  from  the  gills  they  swim  about  by 
opening  and  closing  the  shells,  and  by 
means  of  the  hooks  attach  themseJves 
to  passing  fish.  They  produce  an  ulcer 
in  the  skin  of  the  fish  in  which  they 
grow,  and  by  renewal  of  the  shell  and 
the  adductor  muscles  attain  the  de- 
finitive condition.  After  this  metamor- 
phosis they  fall  to  the  bottom,  to  live 
henceforth  half  buried  in  the  mud. 

Structure    of    gills,    hinge,    edge  of 
mantle,  and  adductor  muscles  have  been 
used  as  basis  of  classification,  the  usual 
divisions  being  founded  on  characters  derived   from   only  one  of  these 
organs. 

Order  I.  Protochonchiae. 

The  primitive  character  of  these  forms  is  shown  by  the  struc- 
ture of  the  gills,  which  are  either  ctenidia  (Protobranchiata)  or 


FIG.  354.— Glochidium  of  Anodonta. 
(From  Balfour.)  ait,  adductor  ;  by, 
byssus  ;  s,  sense  hairs  ;  sli,  shell. 


FIG.  355.— Anatomy  of  Nucula.  (After  Drew.)  oa,  anterior  adductor;  ft,  byssal  gland; 
c,  cerebral  ganglion;  ct,  ctenidium;  /,  foot;  7?,  heart;  7,  labial  palpus;  o,  otocyst; 
Pi  pedal  ganglion;  pa,  posterior  adductor;  «,  stomach  ;  t,  appendage  of  palpus; 
r,  visceral  ganglion. 

filamentary  (Filibranchiata),  yet  here  and  there,  as  in  the  scal- 
lops and  oysters  (Pseudolamellibranchiata),  the  fusion  of  gill  fila- 
ments is  already  begun.  Hinge  and  ligament  are  symmetrical 
with  regard  to  the  umbo,  or  vary  little  from  symmetry.  The  hinge 
may  be  lacking,  and  the  ligament  is  wholly  or  in  part  internal. 
The  mantle  edges  are  free,  and  rarely  is  there  the  first  trace  of  fusion. 


366 


MOLLUSC  A. 


FIG.  356.—  Yoldia  limatula*    (From  Binney-Gould.) 


FlO.  357.— A,  Modiolaplicatula*;    B.   Pecten  irradians*;   C,  Mytilua  edults.*     (From 

Binney-Gould.) 


//.   AGEPHALA:  HETEROCONCHI^E.  367 

Sub  Order  I.  DIMYARIA.  Two  equally  developed  adductors.  The 
taxodont  NUCULID^E  have  ctenidia,  a  broad  foot,  pleural  and  cerebral  gan- 
glia separate,  and  gonads  emptying  through  the  nephridia,  all  points  which 
show  them  extremely  primitive.  Niicula*  Leda*  Yoldia.*  The  ARCID^E. 
are  also  taxadont,  but  filibranch.  Scapharca*  Argina*  SOLEMYID^E. 

Sub  Order  II.  ANISOMYARIA.  Anterior  adductor  rudimentary 
(Heteromyaria)  or  wanting  (Monomyaria).  With  the  exception  of  the 
isodont  SPONDYLID^E,  all  the  families  lack  a  hinge  (dysodont).  To  the 
Heteromyaria  belong  the  MYLILIDJE,  or  mussels,  with  strong  byssus  and 
shells  pointed  anteriorly.  Modiola*  Pinna*  Mytilus  edulis,  abundant  on 
our  mud  flats ;  eaten  in  Europe,  but  occasionally  poisonous.  Dreissenia 
polymorpha,  a  brackish  and  fresh-water  species,  has  spread  from  the 
Caspian  through  central  Europe.  Lithodomus  *  bores  into  stone.  The 
AVICULID^E  of  warm  seas  have  wing-like  projections  either  side  of  the  umbo. 
The  pearl  oysters  of  the  East  and  West  Indies  (Meleagrina)  belong  here. 
The  OSTR^EID^E  and  the  PECTINID.E  are  monomyarian.  The  Ostrasidas,  or 
oysters,  usually  become  attached  by  the  right  valve.  Our  American 
Ostrcea  mrginiana  differs  from  the  European  species  in  having  the  sexes 
separate.  The  Pectin  idas,  or  scallops,  are  free-swimming  and  are  well 
known  for  their  highly  developed  green  eyes  on  the  edge  of  the  mantle. 

Order  II.  Heteroconchiae. 

Gills  always  lamellar,  their  outer  surface  frequently  plaited. 
Hinge — in  rare  cases  (Anodonta)  lost  by  degeneration — is  hetero- 
dont  or  modified  from  a  heterodont  condition.  The  mantle  edges 
but  rarely  free  in  their  whole  extent ;  siphons  usually  present,  but 
in  some  so  small  (Integripalliata)  as  to  cause  no  sinus  in  the  pallial 
line;  in  others  (Sinupalliata)  large,  the  pallial  line  having  a  marked 
sinus.  Anterior  and  posterior  adductors  equally  developed. 

Sub  Order  I.  INTEGRIPALLIATA.  The  UNIONID.E  (Naiadee)  include 
the  fresh- water  mussels,  of  which  hundreds  of  species  occur  in  the  Missis- 
sippi basin,  some  of  which  are  markedly  iridescent  and  afford  material 
for  pearl  buttons.  In  some  pearls  of  value  are  occasionally  found.  Unio,* 
Anodonta*  The  tropical  TRIDACNID.E,  with  small  siphons,  includes  the 


FIG.  358.— .A,  Saxicava  arctica;  B,  Astarte  sulcata ;  C.  Siliqua  costata.    (From  Binney- 

Gould.) 

largest  Acephala,  Tridacna  gigas,  the  shell  of  which  may  be  four  feet 
long  and  weigh  three  hundred  pounds.     The  heart  shells 


368 


MOLL  USC A. 


Cardium*,  Serripes*)  and  ASTARTID^E,  marine,  and  the  fresh-water  CYCLA- 
DID.E  (Cyclas,  Pisidium  *),  about  the  size  of  peas,  belong  here,  as  probably 
do  the  extinct  RUDISTID.E  of  the  cretaceous. 

Sub  Order  II.  SINUPALLIATA.     The  VENERID.E  with  swollen  shells, 
represented  by  the  quahog,  Venus  mercenaria  on  our  east  coast  and  by 
brightly  colored  species  in  the  tropics;  the  MACTRID^E  or 
hen  clams,  and  the  flattened  delicate  TELLINID^E  (Tellina*, 
Macoma*),  have  short  siphons.    In  others  the  siphons  are 
so  large  that  they  cannot  be  entirely  retracted  within  the 
shell.     This  is  the  case  in  the  MYID^E,  represented  in  all 
northern  seas  by  the  long  clam,  Mya  arenaria,  and  in  the 
razor  clams  (SOLENID^E;  SolenEnsatella*).    The  allied  SAXI- 
CAVID^   have   burrowing  species.      These   forms   connect 
with  others  in  which  the  united  siphons  far  exceed  the  rest 
of  the  body  in  length,  giving  the  animal  a  worm-like  ap- 
pearance (fig.  359).     Since  the  valves  do  not 
cover  the  whole  shell,  they  are  supplemented 
by  accessory  shells,  or  the  worm-like  body 
secretes  a  tube   in   which   the  rudimentary 
valves  are   imbedded  (fig.   360).      The  PHO- 
LADID^E,  some  of  which  are  phosphorescent, 
burrow  in  wood,  clay,  or  stone.     The  shell  is 
well  developed.     In  the  ship  worms  (TERE- 
DHXE)  the  shells,  on  the  other  hand,  are  small, 
while  in  some  species  the  burrows  made  by 
these  animals  in  wood  are  lined  by  calcareous 
deposits.     The  several  species  of  Teredo*  by 
their  boring  habits  do  much  damage  to  wood 
in   the   sea,  especially  in   the  tropics.     The 
GASTROCH^ENID^E  also  form  tubular  shells,  the 
valves  being  imbedded  in  the  tube  (fig.  360); 
at  the  smaller  anterior  end  the  tube  is  open, 
but  the  other  end  is  closed  by  a  perforated 
plate,   giving    these    animals  the    name   of 
*  sprinkling-pot  '  shells. 

Lastly,  there  should  be   mentioned   the 
Septibranchiata,  in   which  the 

have  the  shaPe  of  a  sePtum  perforated 
LudWig-Leu-  by  gill  slits  separating  the  branchial  and  clo- 
chambers>     siienia^  Cuspidaria. 


B 

FlG.  359.  —  Te- 
redo navalis, 
ship  worm  in 
its  tube,  the 
siphons  (a, 
anal;  b,  bran- 
chial) drawn 
out  of  the 
tube  (r);  k, 
shell.  B,teeth 
of  the  shell 
enlarged. 


Ispe5$?  little-known 


nis.)  a,  shell. 


III.   SCAPHOPODA.    IV.    GASTEROPODA. 


Class  III.  Scaphopoda  (Solenoconchae). 

The  tooth  shells  are  primitive  forms  which  have  some  resem- 
blances to  the  Acephala  in  the  paired 
liver  and  iiephridia  and  in  structure  of  the 
nervous  system  (with  the  exception  that  a 
buccal  ganglion  is  present  and  the  pleural 
ganglia  are  distinct  from  the  cerebral). 
In  some  points  they  are  primitive  (persis- 
tence of  jaws  and  radula),  but  in  others 
they  are  considerably  modified.  They  lack 
gills,  have  unpaired  dioecious  gonads, 
rudimentary  heart  (no  auricle),  and  have 
two  bunches  of  thread-like  tentacles  either 
side  of  the  mouth.  The  mantle  lobes, 
which  are  paired  in  the  larva,  unite  below, 
forming  a  sac  open  at  either  end,  and  this 
secretes  a  shell  shaped  like  the  tusk  of  an 
elephant,  from  the  larger  end  of  which 
protrudes  the  long  three-lobed  foot  used 
for  boring  in  the  sand.  Dentalium  (fig.  361),  Entalis* 


FIG.  361.—  Dentalium  elephan- 
tinum,  tooth  shell ;  left  the 
animal,  right  the  shell 

£foot ;   ?,  liver  region;  o, 
inder  opening  of  mantle. 


Class  IV.  Gasteropoda. 

Although  more  highly  organized  than  the  Acephala,  the  snails 
are  in  some  respects  more  primitive.  The  regions  of  the  body- 
foot,  visceral  sac,  head,  and  mantle — occur  in  all  orders,  although 
in  each  one  or  more  forms  may  occur  in  which  one  or  another  part 
is  lost. 

As  a  rule  the  foot  is  flattened  ventrally  to  a  creeping  sole.  In 
it  may  be  distinguished  anterior  and  posterior  processes,  the  pro- 
podium  and  metapodium,  a  sharp  lateral  margin,  the  parapodium, 
and,  above  these,  appendages  or  ridges,  the  epipodia.  Inside  the 
foot  is  usually  a  pedal  gland. 

The  head  bears  (1)  the  tentacles,  a  pair  of  muscular  lobes  or 
hollow  retractile  processes;  (2)  a  pair  of  primitive  vesicular  eyes, 
which  usually  lie  at  the  basis  of  the  tentacles,  but  may  rise  even 
to  their  tips.  In  many  snails  the  eyes  are  on  special  stalks  which, 
as  in  the  stylommatophorous  Pulmonata,  form  a  second  pair  of 
tentacles.  The  protrusion  of  the  tentacles  is  caused  by  an  inflow 
of  blood,  their  retraction  by  muscles  attached  to  the  tip  which 
draw  them  in  like  a  finger  of  a  glove. 


370  MOLLUSC  A. 

The  mantle  begins  on  the  back  and  extends  thence  forward 
over  the  body  to  near  the  beginning  of  the  head.  It  covers  the 
mantle  cavity,  a  spacious  chamber,  which  in  the  water-breathing 
Prosobranchiata,  etc.,  contains  the  gills  (ctenidia)  and  opens 
outward  by  a  large  aperture  under  the  margin  of  the  mantle. 
The  edge  of  the  mantle  may  be  produced  into  a  long  groove-like 
siphon,  conveying  water  to  and  from  the  branchial  chamber, 
which  is  of  importance  in  determining  the  shape  of  the  shell. 
When,  by  degeneration  of  the  gill,  the  animals  become  air-breath- 
ing, the  mantle  cavity  becomes  a  lung,  and  the  opening,  by  growth 
of  the  mantle  edges  to  the  body,  becomes  a  small  spiraculum, 
closed  by  muscles. 

The  visceral  sac,  by  the  great  development  of  the  gonads  and 
liver,  becomes  very  large.  Since  growth  downwards  is  prevented 
by  the  muscular  foot,  the  organs  press  towards  the  back,  carrying 
before  them  the  dorsal  wall  at  the  origin  of  the  mantle  folds,  the 
line  of  least  resistance.  Some  organs,  like  nephridia  and  heart, 
may  be  pressed  into  the  mantle  cavity.  When  the  visceral  sac,  as 
often  occurs,  becomes  enormous,  it  does  not  stand  directly  upwards, 
but  coils  from  left  to  right  in  a  spiral.  The  older  the  animal 
the  more  the  spiral  coils  and  the  larger  the  last  or  body  whorl. 
The  visceral  spiral  therefore  begins  at  the  tip  with  narrow  whorls 
which  increase  in  size  with  approach  to  the  rest  of  the  body. 

From  the  foregoing  the  shape  of  the  shell  is  easily  understood. 
As  a  secretion  of  the  mantle  it  takes  the  form  which  the  mantle 
assumes  under  the  influence  of  the  visceral  sac.  With  slight  devel- 
opment of  the  visceral  sac  it  forms  a  flattened  cone  (fig.  362,  A), 
or  is  slightly  coiled  at  the  apex,  as  in  the  abalone  (B).  When  the 
visceral  sac  is  greatly  elongate  the  shell  is  correspondingly  an 
elongate  cone.  It  is  rarely  irregularly  coiled  (Vermetidae,  fig. 
362,  0).  It  is  usually  coiled  like  a  watch  spring  in  one  plane,  or 
like  a  spiral  staircase ;  in  the  latter  case  the  shell  is  more  or  less 
conical  (fig.  362,  D,  E)  and  one  can  speak  of  its  apex  and  base. 
In  the  middle  of  the  base  is  usually  a  depression,  the  umbilicus. 
Sometimes  the  coils  are  loose  and  do  not  touch  in  the  axis  con- 
necting umbilicus  and  apex,  so  that  one  can  look  into  the  space, 
but  usually  the  coils  fuse  together  into  a  calcareous  pillar,  the 
columella,  around  which  the  whorls  pass  (fig.  362,  E,  c). 

The  shell  increases  to  a  certain  size  by  additions  from  the  mantle 
edge;  and  since  this  determines  the  aperture,  the  shell  is  marked 
with  parallel  lines  of  growth.  The  pigment  is  elaborated  on  the 
edge  of  the  mantle,  and  in  the  formation  of  the  shell  passes  into 


IV.  GASTEROPODA. 


371 


it,  causing  its  color  pattern.  When  the  siphon  is  present  the 
shell  shows  a  corresponding  process.  Thus  are  distinguished 
holostomate  shells  with  smooth  mouths  (fig.  362,  D)  and  siphono- 


FIG.  362.— Various  forms  of  shells.  (After  Schmarda,  Bronn,  and  Clessin.)  A,  Patella 
costata:  B,  Haliotis  tuberculata;  C.  Vermetus  dentiferm ;  D,  Lithoglyphus  naticoides; 
E,  shell  of  Murex  opened  to  show  c,  columella  ;  8,  siphon. 

stome   shells,   in   which  the  anterior  margin  is  drawn  out  in  a 
groove  (fig.  362,  E). 

A  simple  conical  shell  without  further  evidence  is  not  proof  of  primi- 
tive structure.  It  may  arise  from  the  spiral  form  by  degeneration,  if  the 
visceral  sac  be  reduced.  Thus  the  shells  of  Fissurella  and  Patella  are  to  be 
explained,  for  the  viscera  here  show  the  results  of  an  earlier  spiral  twist. 

In  most  places  the  union  between  shell  and  soft  parts  is  not  very  firm, 
but  the  connexion  at  the  aperture  is  more  intimate,  while  a  muscle  is  at- 
tached to  the  columella  (musculus  columellaris)  at  about  the  middle  point 
of  its  height,  the  other  end  being  inserted  in  the  foot.  It  is  for  the  retrac- 
tion of  the  animal  within  the  shell,  first  the  anterior  part  with  the  head 
and  then  the  rest  with  the  metapodiuin.  In  this  the  metapodium  is  folded 
so  that  its  dorsal  surface  lies  towards  the  aperture.  Hence  in  many  species- 
this  surface  secretes  a  door,  or  operculum,  which  closes  the  aperture  when 
the  body  retracts.  Since  the  aperture  increases  in  size  with  growth,  the- 
operculum  must  also  enlarge,  which  is  accomplished  in  a  spiral  manner 
(fig.  362,  D),  the  process  sometimes  showing  in  a  spiral  line  on  the  out- 
side. So-called  eye  stones  are  the  opercula  of  small  Trochidee  and  Tur- 
binida3.  Land  snails  are  usually  without  opercula,  but  at  certain  times, 


372 


MOLLUSC  A. 


as  in  hibernation,  they  can  close  the  shell  by  a  calcareous  plate,  the  epi- 
pJiragm.  In  the  spring  this  separates  from  the  shell  and  is  lost. 

In  most  gasteropods  the  shell  is  coiled  to  the  right,  but  in  some  species 
(fig.  363)  the  whorls  are  constantly  turned  to  the  left, 
while  reversed  specimens  occasionally  occur  in  many 
species  which  are  normally  dextral. 

In  the  shell  there  are  at  most  two  layers,  an  inner 
lamellar  layer  (not  always  present),  which  sometimes 
is  highly  iridescent,  and  an  outer  porcellanous  layer, 
which  is  °Pa(lue  and  contains  the  pigment.  In  rare 
of  Lanistes  carinatus.  cases  the  mantle  and  consequently  the  shell  are  lack- 
£sTn  u'ing,  or  the  mantle  is  present  but  the  shell  is  rudi- 

mentary and  not  visible  externally  because  the  mantle  folds  have  grown 
over  it.  In  these  cases  the  visceral  sac  is  not  prominent.  Since  the  shell- 
less  forms  possess  a  mantle  and  shell  in  the  young,  the  adult  conditions 
are  explained  by  degeneration. 

Only  a  few  gasteropods  are  like  the  Amphineura  and  Acephala 
in  being  bilaterally  symmetrical.  Usually  the  spiral  twist  of  the 
visceral  sac  has  resulted  in  a  torsion  of  other  parts  from  left  to 
right,  in  which  alimentary  tract,  nephridia,  gills,  heart,  and  nerv- 
ous system  take  part.  The  intestine  is  bent  in  this  way,  the  anus 
opening  into  the  mantle  chamber  on  the  right  side,  or  the  twisting 
may  be  continued  so  far  as  to  double  the  intestine  on  itself,  the 
anus  being  in  the  middle  line  in  front,  near  the  head.  Nephridia, 


TTlG.  364.— Three  diagrams  illustrating  the  torsion  of  the  body  and  the  twisting  of  the 
nervous  system  in  gasteropods.  (After  Lang.)  A,  bilateral,  B,  asymmetrical, 
C,  streptoneurous  condition.  The  reference  letters  are  placed  upon  the  organs 
of  the  primitive  left  side,  a,  anus:  c,  cerebral  ganglion:  0,  ctenidium;  7,  auri- 
cle; w,  mouth;  n,  nephridial  opening;  o,  osphradium;  pa,  parietal  ganglion; 
pe,  pedal  ganglion;  pi,  pleural  ganglion;  v,  ventricle. 

gills  (with  them  the  osphradia),  and  heart  wander  in  company,  so 
that  the  organs  primitively  belonging  on  the  left  side  may  be  trans- 


IV.  GASTEROPODA. 


373 


f erred  to  the  right  and  vice  versa.  With  this  there  is  a  tendency 
to  asymmetry  and  the  loss  of  the  organs  (usually  of  the  primitively 
left  side).  When  the  nervous  system  takes  part  in  the  twisting 
a  notable  crossing  of  the  cerebrovisceral  commissures  takes  place, 
known  as  streptoneury  or  chiastoneury  (fig.  364,  c). 

The  alimentary  canal  begins  with  a  muscular  region  which  in 
some  groups  is  developed  into  a  large  protrusible  proboscis  (fig. 
365).  The  pharynx,  which  follows,  contains  the  tongue,  a  ventral 
ridge  supported  by  one  or  more  cartilages  and  covered  by  a  cutic- 
ular  layer,  the  radula  or  lingual  ribbon  (odondophore).  The  upper 

surface  of  the  radula  is  armed 
with  sharp,  backwardly  di- 
rected teeth  (fig.  366)  which, 
are  usually  arranged  in  trans- 
verse and  longitudinal  rows,, 
but  which  vary  so  in  num- 
ber, form,  size,  and  arrange- 
ment that  they  are  of  value  in 
classification.  Although  the 
radula  covers  the  tongue,  it  is 


x     r 


FIG.  365. 


FIG.  366. 


FIG.  365.— Pyrula  tuba,  male.  (After  Souleyet.)  The  mantle  has  been  cut  on  the 
right  side  and  turned  to  the  left,  reversing  the  pallial  organs,  a,  anus ;  c,  ctenid- 
ium  ;  cm,  columellar  muscle ;  /,  foot ;  7i,  heart  in  pericardium ;  i,  intestine ;  I, 
liver;  777,  mantle;  m/,  floor  of  mantle  cavity;  ?7,  nephridium;  ns,  opening  of 
nephridium;  o,  osph radium ;  p,  proboscis;  pe,  penis;  t,  testes;  v,  vas  deferens 
cut  in  two. 

FIG.  366.— Pharyngeal  region  of  Helix  pomatia.  A,  side  view ;  B,  section,  m,  muscle; 
oe,  oesophagus  ;  r,  radula  ;  rs,  radula  sac  ;  sp,  salivary  duct ;  z,  lingual  cartilage. 

formed  in  the  radula  sac,  which  lies  behind  the  tongue.  From 
this  it  grows  forward  like  a  nail  over  its  bed  as  fast  as  it  is  worn 
out  in  front.  It  is  opposed  in  eating  by  a  single  median  or  a 
pair  of  lateral  jaws  (lacking  in  carnivorous  forms). 

The  rest  of  the  alimentary  canal  is  convoluted,  the  anus  being 


374  MOLLUSC  A. 

usually  on  the  right  side  in  front,  in  or  beside  the  mantle  chamber 

(figs.  365,  370,  371).     Rarely  it  empties  in  the  middle  line  behind. 

(Esophagus,   stomach,  and  intestine  are  slightly  marked   off 

from  each  other.     The  convolutions  of  the  intestine  are  enveloped 


FIG.  367.— Row  of  teeth  from  the  radula  of  Trochus  cinerartus.    (After  Schmarda.) 

by  the  liver,  which  by  its  large  size  forms  the  chief  part  of  the 
visceral  sac.  A  pair  of  salivary  glands  empty  into  the  pharynx, 
these  in  the  Doliidae  secreting  free  sulphuric  acid. 

The  nervous  system  usually  differs  from  that  of  other  molluscs 
in  that  the  pleural  and  parietal  ganglia  are  free  (p.  353).  If  the 
commissures  be  short,  the  ganglia  are  collected  near  the  pharynx 
and,  thus  freed  from  the  body  torsion,  are  symmetrical  (orthoneu- 
rous,  fig.  368,  //).  If  the  cerebrovisceral  commissure  be  longer, 
the  result  is  almost  always  streptoneury  (chiastoneury).  Pleural 
and  visceral  ganglia  hold  their  place,  but  the  right  parietal  ganglion 
crosses  above  the  intestine  to  the  left  side  (hence  called  supra- 
intestinal),  while  the  left  passes  under  the  intestine  to  the  right 
side  (subintestinal),  the  cerebrovisceral  commissure  being  twisted 
like  the  figure  8.  The  strong  development  of  the  pharynx  is  ac- 
companied by  buccal  ganglia.  The  existence  of  streptoneurous 
forms  among  the  orthoneurous  Opisthobranchs  (Actoson)  and  Pul- 
monata  (Chilina)  shows  that  orthoneury  in  these  groups  has  arisen 
from  streptoneury. 

Gills,  heart,  and  nephridia  are  best  treated  together.  Certain 
genera  (Haliotis,  Fissurella)  recall  the  Acephala  in  having  these 
organs  in  pairs,  while  the  intestine  passes  through  the  heart.  As 
a  rule  the  asymmetry  induced  by  the  torsion  of  the  body  has 
resulted  in  the  loss  of  the  ctenidium,  osphradium,  nephridium, 
and  auricle  of  one  (the  primitively  left)  side.  Prosobranchs  and 
Opisthobranchs  are  recognized  accordingly  as  the  gills  are  on  the 
anterior  or  posterior  part  of  the  body.  In  the  Opisthobranchs 
(fig.  369)  the  ctenidia  have  been  lost  and  are  replaced  by  secondary 
gills  on  the  back.  Here  the  heart  is  in  front  of  the  gills;  it  receives 
blood  from  behind  and  forces  it  forward  to  the  head  by  an  aorta. 
In  the  Prosobranchs  the  heart  has  been  twisted  about  ninety 


IV.  GASTEROPODA. 


375 


degrees,  so  that  the  auricle  is  in  front  and  the  ctenidium  in  front 
of  this  (fig.  370),  while  the  aorta  leads  backwards.  The  nephrid- 
ium,  which  communicates  with  the  pericardium  by  a  nephro- 
stome,  is  rarely  a  racemose  gland;  usually  it  is  saccular,  the  lumen 


/z. 


FIG.  368. 


FIG.  369. 


FIG.  368.— I,  streptoneurous  nervous  system  of  Paludina.  (After  Hering,  from 
Gegenbaur.)  II,  orthoneurous  system  of  Linmcen.  (After  Lacaze-Duthiers.) 
A,  visceral;  B,  buccal;  C,  cerebral;  p,  pedal;  PI,  pleural;  s6,  sp,  s\ib-  and  supra- 
intestinal  ganglia;  n,  olfactory  nerve;  p,  otocyst. 

FIG.  369. — Diagram  of  circulation  in  Doris.  (After  Leuckart.)  a,  auricle;  c,  gills 
around  anus;  t,  tentacle;  v,  ventricle;  ar,  vessels  returning  venous  blood  from  the 
body. 

bearing  gland  cells  and  concretions;  its  duct  either  empties  into 
the  mantle  cavity  or  beside  the  anus. 

The  sexual  organs  in  some  forms  (Cyclobranchs  and  many 
Zygobranchs)  empty  into  the  nephridia.  They  show  two  extremes. 
On  the  one  hand  are  completely  dioecious  species,  on  the  other 
there  may  be  complete  hermaphroditism  (many  Tectibranchs, 
Pteropoda),  in  which  the  male  and  female  organs  are  united 
throughout  their  extent.  Intermediate  stages  occur;  those  of 
the  pulmonates  are  described  below. 


376  MOLLUSC  A. 

In  the  Helicidae  there  is  a  hermaphrodite  gonad  which  lies  together 
with  the  liver  in  one  of  the  first  whorls  of  the  shell  (fig.  371,  «).  A  coiled 
genital  duct  follows  which  widens  to  a  thick-walled  'uterus'  (u)  along 


FIG.  370.— Anatomy  of  Cyprcea  tigrfs.  (After  Quoy  et  Gaimard.)  br,  ctenidium;  c, 
heart;  d/,  vas  deferens;  /t,  liver ;  m,  stomach  ;  N,  cerebral  ganglion  ;  oc,  eye  ;  pe> 
penis  ;  ph,  pharynx,  the  radula  drawn  out;  r,  rectum  ;  re,  nephridium;  £,  testes. 

which  a  second  seminal  canal  appears  to  lie.  Actually  in  the  interior 
there  is  but  a  single  lumen,  the  different  appearances  being  due  to  glands 
in  the  walls.  A  separation  into  vas  deferens  and  vagina  occurs  at  the 
end  of  the  uterus.  The  vas  deferens  (vd)  proceeds  as  a  small  coiled 
canal  to  the  genital  pore.  Here  it  enlarges  to  a  protrusible  penis  (p) 
with  which  is  connected  a  retractor  muscle  and  an  appendage,  the  flagel- 
lum  (fl).  The  vagina  is  broader  and  goes  straight  to  the  genital  pore, 
where  it  meets  the  penis.  Connected  with  the  female  genitalia  are  the 
large  albumen  gland  (ei)  at  the  beginning  of  the  uterus  and  a  receptac- 
ulum  seminis  (r) ;  a  round  vesicle  connects  with  the  vagina  by  a  long 
duct,  and  (not  always  present)  two  '  finger-form  glands.'  Lastly,  the  dart 
sac  (ps)  of  the  vaginal  wall,  which  secretes  a  calcareous  stylet,  the  *  love 
dart,'  which  in  copulation  acts  as  a  stimulus  to  the  male  genitalia.  In 
spite  of  hermaphroditism  a  copulation  lasting  for  days  may  occur,  con- 
nected with  which  is  the  fact  that  in  many  species  the  male  cells  are  first 
matured,  then  the  female  (proterogyny) ;  or  the  reverse  may  occur 
(proterandry). 

The  sexual  opening  is  almost  always  on  the  right  side,  beside 
the  anus  or  in  front  of  it  on  the  head.  Its  position  may  be  rec- 
ognized in  hermaphroditic  species  and  in  dioecious  males  by  the 


IV.  GASTEROPODA. 


377 


grooved  dermal  fold,  the  penis  (fig.  370,  pe).  Occasionally  this  is 
separated  from  the  genital  pore,  but  is  connected  with  it  by  a  cili- 
ated groove. 

The  terrestrial  snails  lay  their  large  tough-shelled  eggs  in  damp 
earth;  in  the  aquatic  forms  the  eggs  are  laid  in  masses,  usually 


FIG.  371.— Anatomy  of  Helix  pomatia,  the  roof  of  the  pulmonary  sac  cut  at  the  left 
side  and  turned  to  the  right;  the  pericardium  and  visceral  sac  opened  and  the 
viscera  separated,  a,  anus;  c,  columellar  muscle;  d,  intestine;  ei,  albumen 
gland ;  /,  finger-form  gland;  /Z,  flagellum:  fu,  foot ;  0,  cerebral  ganglion ;  h,  heart ; 
Z,  liver;  ZM,  lung  ;  m,  stomach;  w,  nephriaium;  n',  its  opening;  p,  penis;  ps,  dart 
sac;  r,  receptaculum  seminis;  s,  pharynx;  sp,  salivary  gland;  M,  uterus;  t>, 
vagina  ;  vd,  vas  def  erens  ;  2,  hermaphrodite  gonad. 

gelatinous,  each  egg  with  a  layer  of  albumen  and  a  firm  shell. 
Occasionally  there  is  a  kind  of  nest,  as  is  the  case  with  lanthina 
which  carry  the  mass  of  eggs,  attached  to  the  foot,  about  with 
them.  A  few  gasteropods  are  viviparous. 

In  the  development  the  great  constancy  with  which  the  veliger 
stage  (figs.  342,  343)  appears  is  noticeable.  Most  marine  larvae 
swim  by  their  velum  (often  divided)  at  the  surface  before  creeping 
at  the  bottom.  But  in  those  cases  where  the  snail  leaves  the  egg 


378  MOLLUSC  A. 

in  the  definitive  condition  the  velum  is  usually  developed  in 
embryonic  life,  sometimes  so  strongly  that  the  embryo  rotates  in 
the  surrounding  fluid. 

Order  I.  Prosobranchia. 

The  Prosobranchs,  like  most  gasteropods,  have  the  twisting  of 
the  visceral  complex  from  left  posterior  to  right  anterior,  so  that 
the  anus  lies  on  the  right  side  near  the  head,  the  nervous  com- 
missures are  twisted  into  an  8,  and  the  nephridia  of  the  right 
side  have  been  carried  to  the  left,  where  they  lie  far  forward. 
This  has  twisted  the  heart  so  that  it  receives  branchial  blood  from 
in  front  and  sends  it  backwards  through  the  aorta.  The  sexes 
are  separate  and  the  shell  and  mantle  are  usually  well  developed. 
Accordingly  as  the  mantle  is  drawn  out  in  a  siphon  or  not,  the 
shells  are  siphonostomate  or  holostomate  (p.  371).  Certain 
Prosobranchs  are  near  the  primitive  Amphineura  in  the  reten- 
tion of  both  ctenidia,  both  auricles,  and  both  nephridia,  but  in  the 
great  majority  only  one  gill  (the  primitive  right)  is  present  and  the 
corresponding  auricle  alone  is  well  developed,  although  the  other 
may  exist  in  a  rudimentary  condition. 

Sub  Order  I.  ASPIDOBRANCHIA  (Diotocardia,  Scutibranchia), 
Ctenidium  bipectinate  (fig.  372)  or  absent.  There  are  usually  two 
auricles  and  two  nephridia.  DOCOGLOSSA  (limpets),  auricle  single ; 


FIG.  372.  FIG.  373. 

FIG.  372.— Fissurella  patagonica,  ventral  view.    (From  Bronn.)    br,  the  paired  gills; 

p,  foot. 
Fio.  373.— Acmcea  testudinalis*  limpet.   (From  Binney-Gould.) 

one  or  no  ctenidinm ;  intestine  not  passing  through  heart,  shell  conical. 
ACM.EIDJE  with  ctenidium.  Acmcza*  (fig.  373).  PATELLID.E,  ctenidia 
lacking,  replaced  by  a  ring-like  mantle  gill.  Patella  (fig.  362,  A).  ZYGO- 


IV.    GASTEROPODA:  PROSOBRANCHIA. 


379 


BRANCHIA.  Two  ctenidia  (fig.  372),  shell  with  marginal  slit  or  with 
holes  corresponding  to  an  anal  notch  in  the  mantle;  auricles  and  ne- 
phridia  paired;  heart  traversed  by  intestine.  FISSURELLID^,  keyhole  lim- 
pets; shell  conical,  with  apical  opening.  HALIOTID^E,  abalones;  shell 
weakly  spiral,  flat,  with  a  series  of  holes.  Haliotis*  (fig.  362,  B).  AZYGO- 


Fio.  374.— American  Pectinibranch  gasteropoda.  (From  Binney-Gould.)  A,  Crepidula 
fornicata;  B,  Lacuna  vincta;  C,  llyanassa  obsolete, ;  D,  Littorina  palliata;  E,  L. 
litorea ;  F,  Urosalpinx  cinerea ;  G,  Purpura  lapillus  ;  H.  Buccinum  undatum ;  I, 
Lunatia  heros. 

BRANCHIA.  One  ctenidium,  but  two  auricles.  TROCHID^E,  operculum 
liorny  ;  Trochus,  Margarita*  TURBINID^E,  top  shells;  operculum  calca- 
reous. Turbo,  Phasianella. 

Sub  Order  II.     PECTINIBRANCHIA  (Monotocardia,  Ctenobranchia). 
^Ctenidium  unipectinate,  osphradium  well  differentiated  (fig   365),  intestine 


380  MOLLUSC  A. 

not  passing  through  the  heart.  Many  groups  are  recognized,  based  upon 
the  structure  of  the  lingual  ribbon.  Of  the  thousands  of  species  only  a 
few  groups  can  be  included  here.  KHACIIIGLOSSA  ;  siphonostornate, 
predatory.  MURICID.E  (Murex,  Purpura*  Urosalpinx*)  have  an  anal 
gland  secreting  a  substance  first  colorless,  turning  to  purple  by  exposure 
to  air.  The  Tynan  purple  was  produced  by  Murex  trunculus.  Urosalpinx 
cinereus*  drills  into  oysters.  BUCCINID.E,  whelks,  VOLUTID^E,  and  OLIVID^E 
belong  here.  TOXIGLOSSA ;  CONID^E,  with  large  oesophageal  poison 
gland,  some  species  producing  severe  wounds.  Conus,  tropical;  Bele.* 
T^ENIOGLOSSA ;  NATICID.E,  Neverita*  and  Lunatia,*  common  snail  of 
Atlantic  coast,  their  egg-masses  being  the  familiar  sand  saucers.  LIT- 
TORINID.E  ;  periwinkles.  CYPR^EID^B,  cowries;  Cyprcea  moneta  of  India  is 
used  as  money  in  Africa.  AMPULLA  RID^E;  amphibious,  part  of  branchial 
cavity  acting  as  lung,  part  containing  ctenidium.  PALUDINID^E,  fresh 
water.  CYCLOSTOMID.E,  tropical  terrestrial  forms,  the  mantle  cavity  a 
lung. 

HETEROPODA.  In  all  details  of  gills,  genitalia,  heart,  and  nervous- 
system  these  are  true  Pectinibranchs,  but  from  an  exclusively  pelagic  life 
have  acquired  peculiar  modifications.  As  in  most  pelagic  animals  the 
body  is  gelatinous  and  transparent.  The  head  is  elongate,  and  the  body  is- 
enlarged  so  that  usually  it  cannot  be  retracted  into  the  shell.  Most  char- 
acteristic is  the  division  of  the  foot  into  pro- and  metapodium  (fig.  375),  the 


FIG.  375.— Carinaria  mediterranea  (after  Gegenbaur),  shell  removed.  A,  metapodium  ; 
a,  anus  ;  ar,  aorta ;  #,  visceral  sac  ;  ftr,  branchiae,  the  heart  above ;  'if,  vas  def- 
erens  ;  o,  mouth  ;  oc,  eye  with  tentacle ;  05,  oasophagus :  p.  propodium  :  ns.  penis  ; 
I,  II,  III,  cerebral,  pedal,  and  visceral  ganglia. 

latter  forming  a  tail-like  elongation  of  the  body.  The  propodium  is  verti- 
cally flattened  and  by  its  undulations  serves  as  a  swimming  organ.  The 
Heteropoda  are  predaceous  and  extremely  voracious  ;  they  swim  back 
downwards.  The  ATLANTID^  can  completely  withdraw  into  the  shell  and 
close  it  with  an  operculum  ;  the  CARINARIHLE  (fig.  375)  have  a  shell  which 
scarcely  covers  the  visceral  complex  ;  the  PTEROTRACHEID.E  have  no  shells. 


IV.    GASTEROPODA:    OPISTHOBRANCHIA. 


381 


Order  II.  Opisthobranchia. 

The  Opisthobranchia  have  not  varied  from  the  primitive  sym- 
metry to  such  an  extent  as  have  Prosobranchs  and  Pulmonates. 
The  anus  is  in  the  plane  of  symmetry  or  only  slightly  removed 
from  it,  although  it  may  be  placed  far  forwards.  The  nervous 
system  is  orthoneurous,  the  twist  being  straightened  (except  in 
Actseonidae).  The  heart  also  retains  its  primitive  position,  receiv- 
ing blood  from  behind  and  forcing  it  forward  to  the  body  through 
the  aorta  (fig.  369).  In  rare  cases  a  (right)  ctenidium,  a  poorly 
developed  mantle,  and  a  thin  shell  enveloped  in  the  latter  occur. 
Usually  these  have  been  lost  and  the  place  of  the  ctenidium  is 
taken  by  accessory  gills  of  various  forms  or  a  dermal  respiration 


FIG.  ZlG.—Hyalcea  complmata  from  above.  (After  Gegenbaur.)  a,  arms ;  6r,  gill ;  c, 
heart  ;  </,  gonad ;  7i,  liver ;  m,  mantle ;  oe,  oesophagus ;  re,  nepliridium ;  v, 
stomach ;  7J,  pedal  ganglion  and  otocyst. 

occurs.  It  is  interesting  to  note  that  the  larvae  have  well-developed 
mantle  and  shell.  Also  important  from  the  systematic  standpoint 
is  the  existence  of  hermaphroditism,  the  genital  duct  opening 
on  the  right  side.  Many  of  the  Opisthobranchs  afford  fine 
examples,  in  form  and  coloration,  of  protective  resemblance.  All 
are  marine. 

Sub  Order  I.  TECTIBRANCHIA.  Mantle  and  usually  a  shell  and 
ctenidium  present,  parapodial  processes  often  present.  /Scaphander,* 
Bulla*  Philine*  Aplysia. 


382 


MOLLUSCA. 


Sub  Order  II.  PTERpPODA.  Pelagic  forms  which  in  most  points  of 
structure  agree  with  the  Tectibranchs.  The  head  and  usually  eyes  and 
tentacles  are  lacking,  while  the  fins  (in  reality  greatly  developed  para- 
podia)  are  highly  characteristic,  giving  the  name  '  wing-footed '  to  these 
forms.  Like  the  Tectibranchs  they  are  hermaphroditic,  orthoneurous, 
have  a  single  ctenidium  and  a  posterior  auricle.  The  THECASOMATA 


FIG.  377. — -4,  Clione  papilionacea ;  B,  Hyalea  tridentata.    (After  Verrill.) 

have  shells,  those  of  LIMACINHWE  (spiral)  and  HYALEID^E  (pyramidal)  being 
calcareous.  The  CYMBULID^B  have  transparent  gelatinous  pseudo-shells 
formed  by  the  subepithelial  connective  tissue.  The  long  nearly  cylindrical 
shells  of  the  CAVOLINIDTE  make  up  much  of  the  *  pterpod  ooze '  of  the  deep 
seas.  GYMNOSOMATA  ;  shell  lacking.  Pneumodermon,  with  suckers  like 
those  of  cephalopods  on  the  proboscis.  Clione,*  arctic. 

Sub  Order  III.     NUDIBRANCHIA.     Shell,    ctenidia,  and    osphradia 
lacking ;  most  possessing  accessory  gills  (or  cerata)  of  varying  form  and 


FIG.  378.  FIG.  379. 

FIG.  378.— Dor  is  bilamellata.* 
FIG.  37$.—jEolidia  papillosa.    (From  Ludwig-Leunis.) 

distribution.  In  the  DORIDIID^E  they  form  a  cluster  of  retractile  bushes 
around  the  anus  (fig.  378).  In  the  TRITONIID^:  they  are  in  two  rows,  right 
and  left  (often  branched)  upon  the  back.  The  J&OLIVJE  have  several  rows 
(Dendronotus*),  while  in  the  ELYSIID^E  cerata  are  lacking.  The  ^Eolidae 
are  noteworthy  in  having  nematocysts  like  those  of  the  coelenterates 
(p.  229). 


IV.    GASTEROPODA:  PULMONATA. 


383 


Order  III.  Pulmonata 

In  several  respects  the  Pulmonata  are  intermediate  between  the 
Prosobranchs  and  Opisthobranchs.  Like  the  latter  they  are 
orthoneurons  and  hermaphroditic  (p.  376).  On  the  other  hand 
the  respiratory  organ  is  far  forward  near  the  head,  with  the  result 
here,  as  in  the  Prosobranchs,  that  the  auricle  is  forward,  the  aorta 
behind.  The  opisthopneumous  Testacellidae  have  the  lungs  at 
the  posterior  end  of  the  body.  Here  and  there  streptoneurous 
conditions  occur  (Cliilina). 

The  lung,  the  most  characteristic  feature  of  the  order,  is  a 
spacious  sac  arising  from  the  mantle  cavity  along  with  the  degen- 
eration of  the  ctenidium.  It  begins  on  the  right  side  and  like  a 
half-moon  stretches  some  distance  on  the  left.  On  the  right  side 
is  a  small  opening,  the  spiracle,  with  a  sphincter  muscle,  and  in  its 
margin  the  anus  and  sometimes  the  ureter.  The  roof  of  the  lung 
is  occupied  by  a  rich  network  of  blood  vessels  (fig.  371,  lu)  which 
draw  the  blood  from  a  marginal  vein,  collect 
it  in  a  main  trunk  and  carry  it  to  the  heart. 

Many  pulmonates  are  aquatic,  but  since  they  have 
no  gills  they  must  occasionally  come  to  the  surface  to 
fill  the  lung  sac  with  air.  This  is  the  case  with  many 
pond  snails  of  the  Limnseidse,  but  some,  which  live 
at  great  depths,  as  in  the  lakes  of  Geneva  and  Con- 
stance, and  consequently  cannot  reach  the  surface, 
use  the  skin  and  to  some  extent  the  lung  for 
water-breathing.  Several  genera  (Planorbis,  Pulmo- 
branchia,  Siphonaria)  have  given  rise  to  secondary 
gills. 

Sub  Order  I.  STYLOMMATOPHORA.  Four  re- 
tractile tentacles,  the  eyes  being  borne  at  the  tips  of 
the  second  and  longer  pair.  The  HELICIDJS  have  a 
well-developed  shell,  closed  by  an  epiphragm  (p.  372) 
during  hibernation.  Helix,*  many  hundred  species 
distributed  among  many  sub  genera.  Pupa,*  Acha- 
tina,  Bulimus,  many  tropical  species.  LIMACID^E. 
Shell  reduced,  completely  concealed  in  the  mantle. 
Limax*  Avion*  Ariolimax.* 

Sub  Order  II.  BASSOMATOPHORA.  Only  one  pair  of  non-retractile- 
tentacles,  the  eyes  at  their  base.  LIMN.EID.E,  pond  snails,  living  in  shallow 
ponds  and  brooks.  Limncea*  Planorbis.* 


FIG.  380.— Limax  cine- 
reus.  (After  Ludwig- 
Leunis.)  s.  spiracle. 


384: 


MOLLUSC  A. 


Class  V.  Cephalopoda. 

The  Cephalopoda  are  distinguished  among  the  molluscs  by  their 
size  and  their  high  organization.  The  majority  measure,  includ- 
ing the  arms*  from  eight  inches  to  three  feet  in  length,  a  few  are 
smaller  (two  to  seven  inches),  while  especially  rare  are  the  huge 
giants,  some  of  which  may  be  over  forty  feet  in  length.  These 
large  species  for  a  long  time  were  only  known  from  the  tales  of 
sailors,  who  said  that  the  animals  had  grasped  vessels  with  their 
large  muscular  arms  and  had  drawn  them  into  the  sea.  In  the  last 
half-century  some  of  these  forms,  belonging  to  the  genus  Arclii- 


FIG.  381.  FIG.  382. 

TIG.  381.— Octopus  tonganus  from  the  side.    (After  Hoyle.)    Funnel  and  mantle  fold  to 

the  right ;  back  and  eyes  on  the  left. 
FIG.  382.— Loligo  kobiensis,  ventral  view.    (After  Hoyle.) 

teuthis,  have  been  stranded  by  storms  on  the  coasts  of  Newfound- 
land and  Japan.  One  of  these  Newfoundland  specimens  had  a 
body  twenty  feet  long  from  head  to  tail,  and  one  of  the  arms  was 
thirty-five  feet  in  length.  Since  these  arms  are  composed  entirely 
•of  muscle,  it  is  easily  conceivable  that  they  might  swamp  a  small 
vessel. 

The  body  of  a  cephalopod  is  divided  by  a  constriction  into 
head  and  trunk.  At  the  extremity  of  the  head  is  the  mouth,  and 
around  this  a  circle  of  arms  or  tentacles.  Each  tentacle  is  taper- 
ing and  bears  on  its  oral  surface  rows  of  suckers  (in  some  species 
altered  to  hooks).  The  Octopoda  have  eight  of  these  arms,  all 


V.  CEPHALOPODA. 


385 


equal  in  size  (fig.  381),  four  on  the  right  side,  four  on  the  left. 
The  Decapoda  (fig.  382)  have  in  addition  two  longer  arms  which 
bear  suckers  only  on  the  enlarged  tips  and  can  be  retracted  into 
special  pouches.  This  additional  pair  come  between  the  third 
and  fourth  of  the  Octopoda,  counting  from  the  dorsal  side. 

Behind  the  crown  of  tentacles  are,  right  and  left,  the  pair  of 
large  eyes  which  superficially  closely  resemble  those  of  the  verte- 
brates, since  they  have  a  transparent  cornea  and  a  large  pupil  sur- 
rounded by  an  iris.  Internally  the  resemblance  is  not  less  pro- 
nounced (fig.  383).  Behind  the  iris  is  a  lens  and  a  vitreous  body, 


A     Int 


FIG.  383. 


FIG.  384. 


FIG.  383.— Diagrammatic  secticm  of  Cephalopod  eye.  (After  Gegenbaur.)   ae,  argentea 

(choroid) ;  C,  cornea ;  ci,  ciliary  process ;  go,  optic  ganglion ;  ik,  iris  ;  fc,  cartilages; 

_L,  lens ;  p,  pigment  layer ;  Re,  cellular  layer  of  retina ;  JJr,  rod  layer  of  retina; 

w,  white  body. 
FIG.  384.— Schematic  section   of  eye  of  Nautilus.    (From  Balfour.)    A,  aperture  of 

optic  cup  ;  Int,  iris-like  fold  of  integument ;  JV.op,  optic  nerve;  .R,  retina. 

the  latter  being  bounded  by  the  retina  and  this  in  turn  by  a  pig- 
mented  silvery  layer,  the  argentea  or  choroid,  which  contains 
cartilages  recalling  the  sclerotic  coat.  Two  striking  peculiarities 
separate  these  eyes  from  those  of  the  vertebrates  and  show  that 
they  have  arisen  independently  and  have  an  entirely  different  de- 
velopmental history.  (1)  The  cornea  in  many  species  has  an 
opening  by  which  water  enters  the  anterior  chamber;  (2)  the  layer 
of  rods  in  the  retina  abuts  against  the  vitreous  body  and  the  gan- 
gl ionic  layer  lies  behind,  while  in  the  vertebrates  the  reverse  is 
the  case. 


386  MOLLUSCA. 

The  foregoing  description  applies  to  but  part  of  the  Cephalop- 
oda. The  highly  different  Naufcilidse  have  a  large  number  of 
lobe-like  processes  on  the  head,  these  without  suckers.  The  eyes 
are  deep  pits,  opening  to  the  exterior  by  a  small  aperture,  the  base 
of  the  pit  being  occupied  by  the  retina,  while  lens,  vitreous  body, 
iris,  and  cornea  are  lacking  (fig.  384).  It  is  to  be  noticed  that  the 
other  cephalopod  eyes  pass  through  a  Nautilus  stage. 

In  the  trunk  anterior  and  posterior  sides  are  distinguishable, 
the  two  passing  into  each  other  on  the  sides.  The  anterior  side 
(which  corresponds  only  in  part  to  the  ventral  side  of  other  mol- 
luscs) is  wholly  covered  by  the  mantle,  a  strong  muscular  fold, 
which  takes  its  origin  from  the  periphery  of  the  body,  often 
encroaching  upon  the  back  and  always  terminating  with  free  mar- 
gins at  the  head.  On  opening  the  mantle  by  a  ventral  incision  (fig. 
385)  the  two  ctenidia  (four  in  Nautilus)  are  seen  on  either  side. 


FIG.  385.— Sepia  officiiialis,  the  mantle  and  left  nephridial  sac  opened  to  show  the 
vena  cava  leading  to  the  branchial  heart.  «,  anus;  6,  d,  lock  of  siphon  and  mantle ; 
(/,  genital  opening;  K,  head;  fc,  ctenidium  ;  n,  nephridial  sac  ;  n',  nephridial  open- 
ing ;  up,  nephrostome  ;  t,  ink  sac  ;  !fr,  siphon. 

Between  them  in  the  middle  line  is  the  anus,  and  right  and  left 
of  this  and  a  little  behind  are  the  nephridial  openings  (four  in  Nau- 
tilus, which  also  has  osphradia).  More  laterally  are  the  sexual 
openings,  of  which  one  (usually  the  right)  is  commonly  absent. 


F.    CEPHALOPODA.  387 

At  the  head  the  mantle  opens  by  a  slit  to  the  exterior,  but  it  can 
be  closed  and  fastened  by  various  locking  contrivances  (in  Sepia, 
Loligo,  etc.,  by  button-like  projections  (fig.  385,  d]  which  fit  into 
corresponding  sockets  (b)  on  the  trunk).  When  thus  closed  the 
communication  with  the  exterior  is  by  a  special  conical  muscular 
tube,  the  funnel  or  siphon,  which  is  fastened  to  the  body  and  has 
a  wide  mantle  aperture.  Since  the  cephalopods,  by  contraction 
of  the  mantle  wall,  can  drive  the  water  from  the  mantle  cavity 
through  the  siphon  with  great  force,  they  can  swim  very  rapidly  by 
the  reaction.  Here,  too,  Nautilus  is  peculiar  in  that  the  siphon  is 
throughout  life  composed  of  two  overlapped  folds,  which  is  sig- 
nificant since  in  the  embryos  of  other  forms  the  siphon  (fig.  396) 
arises  as  two  separate  folds  which  later  unite  to  produce  the  defini- 
tive condition.  A  typical  foot  is  lacking,  but  comparative  mor- 
phology shows  that  the  siphon  is  composed  of  a  pair  of  epipodia, 
while  the  arms  are  differentiations  of  fused  foot  and  head. 

Head  and  trunk  are  covered  with  a  thin  mucous  skin,  which  shows  in 
a  marked  degree  the  power  of  changing  color.  Loligo  will  pass  from  a 
dark  red  to  a  translucent  white  ;  Octopus  has  an  even  greater  gamut  of 
color.  These  color  changes  are  possible  since  in  the  cutis  there  is  a  silvery 


FIG.  386.— Female  Nautilus,  the  shell  bisected.  (From  Ludwig-Leunis.)  1,  mantle;  2, 
dorsal  lobes ;  3,  tentacles :  i,  head  fold ;  5,  eye ;  6,  siphon ;  7,  position  of  nid- 
amental  gland ;  8,  shell  muscle ;  9,  living  chamber ;  10,  partitions  between 
chambers ;  11,  siphuncle. 

layer  over  which  are  numerous  different-colored  pigment  cells  or  chroma- 
tophores,  in  which  radial  muscle  fibres  are  inserted.  On  contraction  of 
these  the  chromatophores  are  flattened  and  thus  influence  the  color  ;  when 
the  fibres  relax  the  pigment  cells  contract  to  small  spots.  In  deep-sea 
cephalopods  phosphorescent  organs  have  been  observed. 

Notwithstanding  the  soft  bodies  a  well-developed  shell  occurs 
in  living  cephalopods  only  in  Nautilus  and  Argonauta  (figs.  386, 


388 


MOLL  USC A. 


398).     Externally   the   shell    of   the  former,  coiled  in   a  plane, 
resembles  that  of  certain  snails  like  Planorbis;  but  on  section  it 


Fio.  387.— Spirula,  with  internal  shell.    (After  Owen.) 

is  seen  to  be  divided  by  partitions  into  numerous  chambers  which 
increase  in  size  towards  the  aperture.  Only  in  the  last  is  the 
animal  situated:  the  others  are  filled  with  air.  Each  partition 
has  a  small  opening,  and  through  these  runs  a  strand  of  tissue,  the 
siphuncle.  Among  the  fossil  cephalopods  many  forms — the  Nau- 


FIQ.  388.— Diagram  of  shells,  etc.  of  various  cephalopods.  (After  Lang.)  A,  Sepia; 
B,  Belosepia;  C,  Belemnites ;  D,  Ostracoteuthis ;  E,  Ommaiftrephe*.  a,  anterior;  p, 
posterior  ;  ph,  phragmocone  ;  pr,  proostracum  ;  ?•,  rostrum  ;  s,  siphon. 

tiloids  and  Ammonites — have  a  similar  chambered  shell;  but  in 
other  recent  forms  and  in  many  extinct  species  the  shell  has 
undergone  a  more  or  less  complete  degeneration.  In  Spirula 
peronii  (the  animals  of  which  are  extremely  rare,  the  dead  shells 
common)  there  is  a  similar  chambered  shell,  buried  in  the  mantle 
(fig.  387).  In  the  Decapoda  the  equivalent  of  the  shell  is  com- 
pletely concealed  in  the  back  of  the  animal.  In  the  Sepias  it  is 
a  lamellar  calcareous  structure,  the  well-known  cuttle  bone;  in  the 


V.    CEPHALOPODA.  389 

Loliginidae  it  forms  a  <  pen  '  of  purely  organic  nature  (fig.  340,  A). 
Like  true  shell  these  dorsal  structures  are  products  of  the  external 
epithelium,  only  the  epithelium  which  forms  them,  the  shell 
gland,  has  become  folded  in  and  the  walls  have  united  over  it. 

The  shell  of  Argonauta  (fig.  398)  is  different.  It  occurs  only  in  the 
female,  is  thin  as  paper,  spirally  coiled  at  the  tip,  and  is  only  in  part  a 
secretion  of  the  body,  for  a  part  of  it  is  formed  by  two  tentacles  which  are 
expanded  for  this  purpose.  Internal  partitions  are  lacking,  and  this  shell 
serves  as  a  nest  for  the  eggs.  A  word  or  two  may  be  added  to  correlate 
the  recent  and  fossil  shells  of  the  Dibranchiata,  which  are  always  internal 
and  more  or  less  rudimentary.  The  fossil  Belemnites  (fig.  388,  c)  had  a 
chambered  shell  ('  phragmocone ')  perforated  for  the  siphuncle.  In  front 
this  is  prolonged  ventrally  into  a  thin  broad  plate,  the  proostracum,  while 
behind  it  is  inserted  in  a  calcareous  sheath,  the  guard  or  rostrum.  From 
this,  by  comparison  with  the  fossil  Belosepia  (5),  it  is  seen  that  the  cuttle 
bone  as  it  appears  in  commerce  (A)  is  the  anterior  part  of  the  chambered 
shell,  its  Iamina3  being  the  partitions,  while  in  the  animal  the  rostrum  and 
siphuncle  are  in  part  retained.  On  the  other  hand,  comparison  with 
the  fossil  Ostracoteuthis  (D)  shows  that  in  Ommastrephes  (E)  we  have  but 
a  remnant  of  the  phragmocone,  while  the  bulk  of  the  pen  is  proostracum. 
In  Loligo  the  phragmocone  is  entirely  lacking. 

The  mouth,  situated  in  an  oval  buccal  mass,  lies  between  two 
horny  jaws,  like  the  beak  of  a  parrot 
(fig.  389);  then  follows  a  pharynx 
with  a  radula,  and  in  turn  a  long 
oesophagus,  often  with  a  crop-like  dila- 
tation. The  oesophagus  opens  into  a 
wider  pouch,  the  stomach,  with  which 
is  connected  a  blind  sac,  frequently  Fl°- 389  ~ Jaws  of  Se 
coiled.  Here  the  tract  doubles  on  itself  and  goes  straight  to  the 
anus,  or  makes  one  or  two  convolutions  in  its  course  (fig.  390). 
One  or  two  salivary  glands  (upper  and  lower,  the  latter  poisonous 
in  Octopus)  open  into  the  oesophagus,  and  a  pair  of  liver  sacs 
(frequently  fused)  open  by  two  bile  ducts  into  the  gastric  blind 
sac.  These  ducts  may  bear  racemose  glands  called  the  pancreas. 
Lastly,  the  ink  sac  opens  into  the  intestine  near  the  anus.  This 
gland,  which  has  a  duct  of  varying  length,  secretes  in  its  interior 
a  brownish  or  blackish  pigment.  When  alarmed  the  animal  ejects 
this  secretion  and  clouds  the  water  so  that  it  can  escape  unseen. 
This  organ  is  best  developed  in  Sepia  officinalis,  and  its  secre- 
tion forms  the  basis  of  the  well-known  color,  sepia.  Nautilus 
has  no  ink  sac. 

Just  behind  the  buccal  mass  are  the  closely  united  chief  gan- 


390 


MOLLUSC  A. 


mb 


FIG.  390.— Anatomy  of  Octopus  vulgar  is.  a,  amis;  ao,  aorta;  cv,  vena  cava  with  ne- 
phridial  appendage  ;  d,  intestine  ;  go,  optic  ganglion  ;  /<,  systemic  heart ;  e,  crop ; 
K,  head  ;  A-,  ctenidia ;  fc/t,  branchial  heart ;  fcn,  cartilage;  I,  I',  liver  and  gall  duct, 
the  liver  indicated  by  dotted  line;  M,  mantle;  o,  ovary;  od,  oviduct;  p,  pedal 
ganglion  ;  s,  buccal  mass  with  salivary  glands;  s<,  stellate  ganglion  ;  s.y,  stomach 
and  sympathetic  ganglion  ;  7',  basis  of  tentacles  ;  t,  ink  sac  ;  u,  visceral  ganglion ; 
vfc,  auricle  of  systemic  heart ;  *,  spiral  blind  sac. 

glia  of  the  nervous  system  (fig.  391).  A 
single  dorsal  mass  represents  the  cerebral 
ganglia;  connected  with  this  by  broad  com- 
missures, the  pedal  and  visceral  (viscero- 
pleuro-parietal)  ganglia  lie  close  together 
ventrally.  With  these  parts  are  associated 
upper  and  lower  buccal  ganglia.  The  large 
optic  ganglia,  developed  in  the  optic  nerve 
arising  from  the  cerebrum,  are  especially 
characteristic  of  the  Cephalopoda,  as  are  the 
ganglia  stellata,  right  and  left  at  the  anterior 
edge  of  the  mantle  (fig.  390),  which  owe 
their  name  to  the  radiation  of  fibres  to  inner- 
vate the  mantle.  An  unpaired  sympathetic 
ganglion  lies  at  the  junction  of  stomach  and 
blind  sac.  Cerebral,  pedal,  visceral  and  optic 
ganglia  are  enclosed  in  the  cephalic  cartilage, 


FIG.  391.— Nervous  system 
of  Sepia  officinalis  from 
the  side,  obi,  inferior 
buccal  ganglion ;  gbs,  su- 
perior buccal  ganglion ; 
0c,  cerebral  ganglion  ;  grp, 
pedal  ganglion ;  0u,  vis- 
ceral ganglion  ;  rnib,  buc- 
cal mass;  ce,  oesophagus ; 
op,  optic  ganglion. 


F.    CEPHALOPODA.  391 

which  has  the  shape  of  a  ring  with  wing-like  processes.  The 
otocysts  lie  in  the  ventral  arch  of  the  ring.  Two  pits  opening 
"behind  the  eye  are  regarded  as  olfactory,  while  Nautilus  has, 
"besides  osphradia,  two  pairs  of  ciliated  optic  tentacles. 

Most  noticeable  of  the  circulatory  structures  is  the  presence  of 
two  kinds  of  hearts  (fig.  390).  The  systemic  heart  consists  of  two 
(four  in  Nautilus)  auricles  receiving  the  blood  from  the  gills,  and 
a  median  ventricle  from  which  arise  anterior  and  posterior  aortse. 
Then  there  is  a  branchial  heart  at  the  base  of  each  ctenidium 
which  receives  the  blood  from  the  vena  cava  and  pumps  it  into  the 
gill.  Of  venae  cavse  there  are  an  anterior  unpaired  and  two  pos- 
terior paired  trunks,  the  former  dividing  and  sending  a  branch  to 


FIG.  392.— Male  sexual  organs  of  Sepia  offlcinalis.  (After  Grobben.)  ft,  coelomic  sac 
passing  to  the  left  and  above  into  the  pericardium  ;  c,  coalomic  canal  to  the 
vas  deferens  ;  d,  vas  deferens ;  d',  its  opening  to  coelom  ;  /,  portions  of  coelom  ;  ?i, 
Needham's  pocket ;  n',  its  mouth ;  p',  p2,  prostates ;  t,  testis ;  t ',  its  opening  to 
coelom. 

each  branchial  heart.  These  trunks  are  of  importance  in  con- 
nexion with  the  nephridia.  The  nephridial  openings  (p.  386)  lead 
to  two  spacious  sacs  through  which  the  veins  pass  obliquely,  this 
part  of  the  blood  vessels  being  enclosed  by  diverticula  of  the 
lumen,  covered  with  epithelial  excretory  cells.  Near  its  mouth  each 
nephridial  sac  communicates  by  a  nephrostome  with  the  usually 
large  coelom  (pericardium,  gonads,  etc.). 

In  the  Octopoda  the  coelom  is  reduced  to  the  gonads  and  narrow 
canals  leading  from  the  nephrostome  to  the  gonads  and  branchial 
hearts,  but  elsewhere  there  is  a  well-developed  system  of  connected  cavi- 
ties (in  Nautilus  opening  by  two  pores  into  the  mantle  cavity),  consisting 


392 


MOLLUSC  A. 


of  the  pericardium  around  the  systemic  and  branchial  hearts  and  the  thin- 
walled  genital  sac,  one  wall  of  which  bears  the  genital  ducts,  while  on  the 
other  the  sexual  cells  arise  or  the  ducts  of  a  separate  sexual  gland  open 
(fig.  392). 

The  gonads  of  the  always  dioecious  Cephalopoda  are  unpaired 
and  lie  far  back  in  the  visceral  sac.  The  ducts  in  the  female 
Octopoda  (rarely  in  the  males)  and  in  some  Decapoda  (Oigopsida) 
are  paired.  In  Nautilus  only  the  right  duct  is  functional  in 
either  sex,  although  the  left  is  well  developed.  Elsewhere  there 
is  only  the  left  duct.  The  oviducts  are  saccular  with  glandular 
walls;  independently  of  them  two  pairs  of  glands  open  to  the 
exterior,  the  accessory  glands  and  the  large  nidamental  glands. 
The  vas  deferens  (fig.  392)  is  more  complicated.  It  has  swellings 
known  as  seminal  vesicle,  prostate,  and  Needham's  sac,  in  which 
the  sperm atophores  are  stored.  These  have  such  a  complicated 


FIG.  393.— Spermatophore  of  Sepia.    (From  Hatschek,  after  Milne  Edwards.)    a,  dis- 
charging apparatus ;  6,  packet  of  spermatozoa  ;  c,  envelope. 

structure  and  show  such  motions  when  swollen  with  water  that 
they  were  long  regarded  as  parasitic  worms  (fig.  393). 


FIG.  394.— Male  of  Argonnuta  argo.  (After  Miiller,  from  Hatschek.)  J-A,  arms  of  right 
side ;  !.-£.,  arms  of  left  side ;  3,  hectocotylised  arm,  at  the  left  in  its  sac,  at  the 
right  protruded. 

The  spermatophores  are  conveyed  to  the  female  by  means  of 
more  or  less  modified  (hectocotylised)  arms  of  the  male.  In  a  few 
genera  the  proper  tentacle  becomes  a  '  Hectocotylus '  (fig.  394). 


V.    CEPHALOPODA. 


393 


It  swells  at  its  base  to  a  sac  in  which  the  peripheral  end  is  enclosed. 
This  part  contains  a  canal  for  the  spermatophores,  cuts  loose  from 
the  male,  and  can  creep  about  for  days  in  the  mantle  chamber  of 
the  female.  Since  it  appears  as  if  an  independent  animal,  it  was 
first  described  as  a  parasitic  worm  under  the  name  Hectocotylus. 
Later  it  was  regarded  as  a  rudimentary  male  cephalopod. 

The  eggs  are  either  fastened  singly  to  aquatic  plants  or  are  laid  in  large 
gelatinous  masses.  They  are  rich  in  yolk,  and  in  consequence  undergo  par- 


FIG.  395.— Two  stages  of  the  germinal  area  of  Sepia.  (From  Balfour,  after  Kolliker.) 
cm,  anus ;  br,  ctenidia  ;  /,  siphon  folds  ;  wi,  mouth ;  wit,  mantle  with  shell  gland ; 
oe,  eye  ;  p,  head  lobes  ;  1-5  arms. 


FIG.  396.— Embryos  of  LoUgo  pealei  (prig.),    o,  arms ;  e,  eyes ;  /,  fin ;  g,  ctenidia ;  h,  ot- 
ocyst ;  w,  mantle  ;  s,  siphonal  folds  and  siphon  ;  v,  anus  ;  y,  yolk  sac. 

tial  discoidal  segmentation  (fig.  103).    The  blastoderm,  on  the  end  of  the 
oval  egg,  forms  the  anlagen  of  the  separate  organs  (eyes,  arms,  siphon,  and 


394  MOLLUSC  A. 

shell  gland)  as  flattened  projections  beside  each  other  (fig.  395).  Later 
the  embryonic  body  becomes  distinct  from  the  yolk,  which,  enclosed  in  a 
cellular  envelope,  remains  attached  to  the  rest,  near  the  mouth,  until  its 
substance  is  absorbed  in  the  growth  of  the  young  and  the  animal  is  ready 
for  hatching  (fig.  396). 

The  Cephalopoda  are  exclusively  marine.  Some  inhabit  rocky  shores, 
others  the  high  seas.  All  are  carnivorous  and  in  turn  are  preyed  upon  by 
fishes,  etc.  Classification  is  based  upon  the  number  of  gills  and  number 
and  character  of  the  arms. 

Order  I.  Tetrabranchia. 

With  four  gills,  four  auricles,  and  four  nephridia;  numerous  tentacles 
without  suckers,  a  well-developed  chambered  shell,  siphon  of  two  separate 
parapodia,  and  simple  eyes  (fig.  384).  Only  four  living  species  known,  all 
belonging  to  the  genus  Nautilus.  The  animals,  which  live  in  the  Malay- 
sian regions,  are  rare,  but  their  shells  are  abundantly  cast  up  by  the  sea. 
In  past  time  the  tetrabranchs  were  very  abundant.  The  NAUTILID^E, 
with  straight  (Orthoceras)  or  coiled  shells  (Goniatites,  etc.)  flourished  in 
paleozoic  times.  They  had  simple  septa.  The  AMMONITID^:  with  folded 
septa  were  largely  mesozoic.  Since  no  living  forms  exist,  their  pertinence 
to  the  tetrabranchiates  is  assumed  from  the  character  of  the  shell. 

Order  II.  Dibranchia. 

With  two  nephridia,  two  gills,  and  two  auricles ;  eight  or  ten  arms 
with  suckers  ;  highly  organized  eyes  ;  shell  rudimentary  or  absent. 


FIG.  397.— Octopus  bairdii.*    (From  Verrill.)    A  hectocotylised  arm  on  the  right  side. 

Sub  Order  I.  DECAPOD  A.  Ten  arms,  with  lateral  fins  to  the  body. 
Shell  usually  present.  SPIRULID^E,  with  internal  chambered  loose-coiled 
shell.  Spirula  (fig.  387).  OIGOPSIDA,  pelagic,  with  perforated  cornea  (p.  385) 


V.    CEPHALOPODA  :  SUMMARY  OF  IMPORTANT  FACTS.    395 

and  two  oviducts.  Ommastrephes  common  in  New  England ;  Archi- 
teuthis*tt\e  giant  squid  (p.  384).  MYOPSIDA.  Oviduct  single  (left) ;  cornea 
unperforated.  Loligo*  common  squid;  Rossia*;  Sepia,  cuttle  fish,  fur- 
nishing the  '  cuttle  bone '  once  used  in  medicine,  now  fed  to  cage  birds, 
and  the  pigment  sepia. 

Sub  Order  II.   OCTOPODA.    Eight  arms  webbed  at  their  base;  shell 
very  rudimentary,  sometimes  fragmentary  or  wanting ;   oviducts  paired, 


FIG.  398.— Argonauta  argo,  paper  sailor,  female.    (After  Rymer  Jones.) 

OCTOPODID^E,  Octopus  *  (fig.  397),  Alloposus.*  ARGONAUTID^E,  female  with 
boat-like  shell  (fig.  398),  males  much  smaller  and  without  shell.  Argonauta 
argo,  paper  nautilus.  In  the  Argonautidse  and  PHILONEXID.E  the  hecto- 
cotylus  separates  of  itself. 

Summary  of  Important  Facts. 

1.  The   MOLLUSC  A   are  parenchymatous  animals  with  re- 
duced coelom.     They  consist  of  head,  visceral  sac,  mantle,  and  foot. 

2.  The  head  bears  eyes  and  tentacles. 

3.  The  foot  is  an  unpaired  muscular  mass  used  in  locomotion. 

4.  The  mantle  bounds  the  mantle  cavity  which  is  connected 
with  respiration;  it  either  functions  as  a  lung  or  covers  the  gills 
(ctenidia).     It  secretes  the  shell  from  its  outer  surface. 

5.  Foot,  head,  mantle,  and  with  the  latter  the  shell,  may  be 
lost  in  many  groups. 

6.  The  molluscs,   without  exception,    agree    in  the    nervous 
system. 


396  MOLLUSC  A. 

7.  Three  pairs  of    ganglia  with  which  three  pairs  of  sense- 
organs  are   connected  almost  always  occur:    a,  cerebral   ganglia 
and  eyes;  b,   pedal  ganglia  and  otocysts;  c,   visceral  ganglia  and 
osphradia  (olfactory). 

8.  The  heart  is  dorsal  and  arterial;  it  is  enclosed  in  a  peri- 
cardium (reduced  ccelom)  which  connects  with  the  nephridia  by 
nephrostomes. 

9.  There  is    always   a  single   ventricle  arid,  according  to  the 
number  of  respiratory  organs,  one,  two,  or  four  auricles. 

10.  The  alimentary  canal  is  well  developed;  the  liver  large; 
salivary  glands  usually  present.      In  most  there  is  a  pharynx  or 
buccal  mass  with  radula  and  jaws. 

11.  A  veliger  stage  is  common  in  development. 

12.  The  Mollusca  are  divided  according  to  the  respiratory  organs 
and  appendages  of  the  body  into  five  classes:  (1)  Amphineura;   (2) 
Acephala;  (3)  Scaphopoda;  (4)  Gasteropoda;  (5)  Cephalopoda. 

13.  The  AMPHINEURA  have  an  extremely  simple  nervous  sys- 
tem in  which  the  three  pairs  of  typical  ganglia  are  replaced  by 
nerve  tracts. 

14.  The  ACEPHALA,  or  Lamellibranchia,  lack  head  and  ceph- 
alic appendages. 

15.  They  are  bilaterally  symmetrical  and  have  paired  organs: 
mantle  folds,  bivalve  shell,  nephridia,  and  gonads. 

16.  In  many  Acephala  (Asiphonia)  the  mantle  folds  are  com- 
pletely separated  ventrally. 

17.  In  the  Siphonata  the  lower  edges  of  the  mantle  are  united, 
leaving  three  openings:  (1)  in  front  for  the  foot;  (2)  behind  and 
below,  the  branchial  siphon  for  the  ingress  of  wrater  and  nourish- 
ment; (3)  behind  and  above,  the  anal  or  excurrent  siphon  for  the 
water  used  by  the  gills  and  the  faeces. 

18.  There  are  two  pairs  of  gills,  which  may  be  comb-like  (true 
ctenidia),  filiform,  or  most  commonly  lamellar. 

19.  Correspondingly  the  heart  has  two  auricles;  the  unpaired 
ventricle  is  usually  traversed  by  the  rectum. 

20.  The  foot  is  a  compressed  muscular  mass  frequently  pro- 
vided with  a  byssus  gland. 

21.  The  shell  consists  of  cuticular,  prismatic  layer  and  nacreous 
layer.     It  is  closed  by  two  adductors  and  opened  by  an  elastic 
ligament. 

22.  Some  Acephals  (Protoconcha)  are  very  primitive  in  their 
gill  and  hinge  structure;  others  (Heteroconcha)  are  more  highly 
developed. 


V.    CEPHALOPODA:  SUMMARY  OF  IMPORTANT  FACTS.    397 

23.  The  SCAPHOPODA  are  primitive  forms  with  tubular  shells. 

24.  The  GASTEROPODA  (Cephalophora,  or  snails)  have  a  distinct 
head  bearing  eyes  and  tentacles;  a  creeping   foot,   an  unpaired 
mantle  (occasionally  absent),  and  a  univalve  shell. 

25.  The    mantle   cavity  contains  one   or  less  frequently  two 
ctenidia,  or  these  may  be  degenerate  and  a  lung  may  occur. 

26.  Nephridia  and  auricles  are  rarely  paired  (with  paired  gills) ; 
the  gonads,  always  unpaired,  are  hermaphroditic  or  dioecious. 

27.  The  shell  is  always  unpaired;  it  is  usually  coiled  in  a  (right- 
hand)  spiral,  and  is  frequently  closed  with  an  operculum. 

28.  According  to  characters   derived   from    nervous    system, 
sexual  organs,  heart,  and  respiratory  organs  the  Gasteropods  are 
divided    into    (1)  Prosobranchia;    (2)  Opisthobranchia:  and  (3) 
Pulmonata. 

29.  The   Opisthobranchia  are  hermaphroditic;    orthoneurous; 
have  gills  of  various  kinds  (or  none),  and  have  the  auricle  always 
behind  the  ventricle;  shell  and  mantle  reduced  or  absent. 

30.  The   Pteropoda   are   pelagic    Opisthobranchs  with  wing- 
like  processes  of  the  foot  and  frequently  reduced  shell  or  none. 

31.  The  Prosobranchia  have  the  gills  (ctenidia — occasionally 
paired)  far  in  front,  and  in  consequence  the  auricle  in  front  of 
the  ventricle;  they  are  streptoneurous  and  dioecious;  the  mantle 
and  shell  well  developed. 

32.  The    Heteropoda    are    pelagic    Prosobranchia   with    foot 
divided  into  fin  and  tail,  shell  rudimentary,  or  naked. 

33.  The  Pulmonata  are  in  some  respects   (orthoneurous  and 
hermaphroditic)  Opisthobranch-like;  in  other  respects — as  in  posi- 
tion of  heart,  development  of  shell  and  mantle — like  the  Proso- 
T^ranchs;  the  mantle  cavity  functions  as  a  lung. 

34.  The  CEPHALOPODA  have  no  true  foot;  but  its  homologues 
are  to  found  in  the  siphon  and  in  the  tentacles,  usually  provided 
with  suckers,  on  the  head;  they  have  an  unpaired  mantle  and  a 
single  shell  or  none. 

35.  The  unpaired  mantle  cavity  contains  one  or  two  pairs  of 
'ctenidia.     The  water  is  forced    from  the  mantle  cavity  through 
the  siphon. 

36.  The  number  of  auricles  corresponds  with  the  number  of 
ctenidia;  besides  the  systemic  heart  there  are  one  or  two  pairs  of 
branchial  hearts,  elsewhere  unknown  in  molluscs. 

37.  The  sexes  are  separate. 

38.  The  ink  sac  is  peculiar  to  Cephalopoda. 

39.  The  eye  is  (usually)  highly  developed  (with  retina,  choroid, 


398  ARTHROPODA. 

iris,  cornea,  vitreous  body,  and  lens),  as  is  the  nervous  system, 
which  has,  in  addition  to  the  usual  centres,  optic,  sympathetic, 
and  stellate  ganglia. 

40.  The  eggs  have  a  discoidal  segmentation. 

41.  The    Cephalopoda   are   divided   into   Tetrabranchia    and 
Dibranchia. 

42.  The  Tetrabranchia  (extinct  save  for  Nautilus)  have  four 
gills,  a  chambered  shell,  primitive  eyes,  and  finger-like  cephalic 
lobes  in  place  of  tentacles. 

43.  The  Dibranchia  have  two  gills,  eight  or  ten  tentacles  with 
suckers,  and  the  shell  is  reduced  or  absent. 

PHYLUM  VII.  ARTHROPODA. 

Under  the  term  Arthropoda  are  included  the  spiders,  crabs, 
insects,  and  myriapods,  which,  together  with  the  annelids,  were 
united  by  Cuvier  to  form  his  sub-kingdom  Articulata.  Annelids 
and  arthropods  agree  in  many  features.  They  are,  as  the  term 
articulates  implies,  segmented  animals,  and  they  differ  from  the 
vertebrates,  which  are  also  segmented,  in  the  extension  of  the  seg- 
mentation, the  ringing  of  the  body,  to  the  external  surface.  The 
boundaries  between  the  successive  segments,  which  cannot  be  rec- 
ognized in  the  skin  of  the  fish  or  other  vertebrate,  are  marked  in 
the  articulates  by  a  constriction  of  the  body  wall,  whence  the  old 
names  evro^cx,  Insecta,  applied  to  these  forms.  The  articulates 
are  further  characterized  by  a  ladder-like  nervous  system  in  which 
the  brain,  present  in  most  invertebrates,  is  supplemented  by  a 
ventral  chain  composed  of  ganglia  metamerically  arranged.  The 
most  evident  distinctions  between  the  annelids  and  the  arthropods 
are  (1)  the  character  of  the  segmentation  and  (2)  the  presence  of 
jointed  appendages. 

In  superficial  appearance  the  lines  between  the  segments  are 
constricted  more  deeply  in  the  arthropods  than  in  the  annelids. 
The  cause  of  this  lies  in  the  character  of  the  integument  (fig.  25,  /), 
which  is  developed  as  a  hard  armor,  in  which  two  layers  are  rec- 
ognizable, the  epidermis  (often  called  hypodermis)  and  the  chitin- 
ous  layer.  The  epidermis  is  a  thin  cubical  or  pavement  epithelium, 
while  the  chitinous  layer  is  of  greater  thickness  and,  since  it  is 
secreted  by  the  epidermis,  is  stratified  parallel  to  the  surface.  Its 
firmness  is  due  to  the  chitin,  which  is  unlike  most  organic  substances 
in  its  resistance  to  acids  and  alkalis;  only  under  the  action  of 
sulphuric  acid  and  heat  is  it  broken  up  into  sugar  and  ammonia. 


ARTHROPODA.  399 

A  firm  chitinous  armor  would  render  the  animal  incapable  of 
motion  were  there  not  joints  between  the  parts.  While  the  seg- 
ments themselves  are  heavily  armored,  the 
cuticle  between  them  is  reduced  to  a  delicate 
articular  skin,  and  this  is  so  protected  by  a 
kind  of  telescoping  of  the  segments  that 
injury  in  these  softer  regions  is  nearly  im- 
possible (fig.  399). 

Since  the  ringing  of  the  body  is  connected  with 
this  armoring,  it  disappears  with  the  need  for 
such  protection.  The  hermit  crabs  (fig.  480)  are 
instructive  illustrations  of  this.  These  animals  live  FIG.  399.— Diagram  of  Ar- 

.,     ,     „       rr,,  thropod  jointing ;  A,  m 

with  the  abdomen  inserted  in  a  snail  shell.     1  hat     expanded,  B,    in  con- 
part  of  the  body  which  projects  from  the  shell  is     S^tiT^nectin'g 

armored,  while  the  abdomen   is  soft-skinned  and     membranes,  the   mus- 
.     .      .  cles  indicated  by  dotted 

without  traces  of  external  ringing.  lines.   (After  Graber.) 

The  hardened  cuticula  causes  the  periodic  molting  (ecdysis  or  exuvia- 
tion). When  once  hardened  it  is  incapable  of  distension  and  so  would 
prevent  farther  growth.  Hence  when  the  body  has  completely  filled  the 
armor,  the  latter  splits  in  definite  places  and  the  animal  crawls  out  of  the 
old  '  skin '  (exuvia)  and  rapidly  increases  in  size  while  the  new  cuticula  is 
yet  soft  and  extensible. 

Another  result  of  the  cuticula  is  seen  in  the  peculiar  relations  of  both 
ordinary  and  sense  hairs.  These  are  cuticular  structures,  each  usually 
secreted  by  a  single  epidermal  cell  and  renewed  after  each  molt.  Each 
hair  has  a  ball-like  head  situate  in  a  socket  in  the  surrounding  chitin,  and 
hence  is  movable  ;  it  is  traversed  by  a  canal  in  which  is  a  process  of  the 
underlying  matrix  cell.  In  the  case  of  sensory  hairs  these  structures  are 
connected  with  a  nerve  (fig.  77).  The  sense  cell,  like  a  bipolar  ganglion 
cell,  has  two  processes ;  one  peripheral,  which  enters  the  axis  of  the  hair, 
the  other  central,  which  runs  as  a  nerve  fibre  to  the  central  nervous  sys- 
tem. The  cell  itself  may  be  in  the  epithelium  or  situated  deeper  and 
interpolated  as  a  ganglion  cell  in  the  sensory  nerve. 

Another  important  character  is  the  heteronomous  segmentation, 
which,  in  the  lowest  forms  (Peripatus  and  Myriapods),  is  little 
pronounced,  but  elsewhere  leads  to  a  marked  inequality  of  the 
divisions  of  the  body  and  to  a  greater  centralization  of  structure. 
Different  body  regions  may  be  distinguished.  A  few  segments  at 
the  anterior  end  always  fuse  and  form  a  head  (fig.  400,  0) ;  behind 
this  there  is  usually  a  second  segment  complex,  the  thorax  (T),  and 
then  a  third,  the  abdomen  (^4).  An  apparent  reduction  of  regions 
can  occur  when  the  head  and  thorax  unite  (fig.  401,  Ct)  to  form 
a  cephalothorax ;  or  again  the  number  of  regions  may  be  increased 
(fig.  402)  by  a  division  of  the  abdomen  into  abdomen  proper  (A) 


400 


ARTHROPODA. 


FIG.  400.  FIG.  401. 

FIG.  4CO.— Campodea  staphylinus.    (From  Huxley.)    A,  abdomen  :  C,  head  ;  T,  thorax. 
FIG.  401.— Palcemon  serratus.    (From  Ludwig-Leunis.)     A,  abdomen ;    CJ,   cephalo- 
thorax. 


FIG.  403.  FIG.  403. 

Fin.  402.— And'-octonus  australis.    (From  Blanchard.)    A,  abdomen  ;  Ct,  cephalothorax; 

P<  post- abdomen  ;  1,  chelicerae;  3,  pedipalpi ;  3-6,  legs. 
FIG.  403.— Gamasus     coleoptratorum.    (From  Taschenberg.) 


ARTIIROPODA.  401 

and  post-abdomen  (P).  Finally,  in  many  arthropods  (e.g.,  the 
mites  or  acarina,  fig.  403)  it  is  impossible  to  recognize  regions  or 
somites  because  internal  fusion  of  parts  has  obliterated  the  exter- 
nal evidences  of  segmentation. 

In  order  clearly  to  understand  what  is  meant  by  head,  thorax, 
etc.,  requires  a  consideration  of  the  second  character  distinguish- 
ing the  arthropods  from  the  annelids,  the  jointed  appendages, 
which  give  the  name  to  the  former  group.  The  arthropodan 
appendages  are  highly  developed  parapodia,  differing  in  being 
jointed  to  the  body,  in  consisting  of  a  series  of  joints  themselves, 
and  in  having  their  intrinsic  musculature.  As  was  first  pointed 
out  by  Savigny,  there  is  but  a  pair  of  appendages  to  a  somite,  and 
this  belongs  to  the  ventral  surface.  Hence  it  follows  (Savigny^ 
law)  that  if  any  region  shows  no  external  signs  of  segmentation,  but 
bears  more  than  one  pair  of  appendages,  we  conclude  that  the 
region  is  a  complex  of  at  least  as  many  somites  as  there  are  pairs 
of  appendages.  Thus  the  unsegmented  head  of  an  insect  consists 
of  four  somites,  the  cephalothorax  of  a  lobster  of  thirteen,  for  the 
one  bears  four,  the  other  thirteen,  pairs  of  appendages.  Ontogeny 
supports  this,  for  in  the  embryo  the  somites  are  clearly  visible.* 
It  is  not  necessary  that  each  somite  in  the  adult  should  bear  ap- 
pendages, since  these  may  disappear  in  growth  without  leaving 
a  trace. 

The  appendages  subserve  many  functions  (fig.  404).  Their 
primary  purpose  is  locomotion.  Locomotor  appendages  (pereio- 
poda,  feet  or  legs)  are  long  and  consist  of  a  number  of  well-de- 
veloped joints  which  may  form  flattened  oars  or  may  be  provided 
with  claws  for  creeping  (8).  Besides  locomotor  appendages  there 
are  tactile  appendages  or  antennae  (1),  chewing  appendages  (jaws, 
mandibles,  maxillae,  2-4) 9  ^se  fee*  or  pleopoda  (9)  of  varying 
functions,  and  forms— maxillipeds  (5-7} — transitional  between  jaws 
and  legs. 

Aside  from  their  tactile  function,  antennae  are  characterized 
by  position  and  innervation.  They  are  always  placed  in  front 
of  the  mouth  and  receive  their  nerve  supply  from  the  supra- 
03sophageal  ganglion,  while  all  other  appendages  are  innervated 
from  the  ventral  chain. 

The  form  of  the  jaws  is  strikingly  modified.  One  or  two 
basal  joints  serve  for  the  comminution  of  food,  and  these  parts 

*  This  statement  is  not  exactly  correct,  for  in  certain  insects  and  in  the 
lobster  there  is  one  somite  which  is  entirely  lost  in  the  adult. 


402 


ARTHROPOD  A. 


are  strong  and  are  covered  especially  on  the  medial  side  with  a  hard, 
toothed  chitin  (figs.  404,  2;  410,  //,  F; 
507).  The  other  joints  may  entirely 
disappear.  When  they  persist  they  form 
a  more  or  less  leg-like  appendage,  the 
palpus.  Since  several  appendages  may 
be  modified  into  jaws,  the  first  are  called 
mandibles,  the  next  maxillae,  and  second 
maxillae  may  follow.  The  maxillipeds 
may  have  more  the  appearance  of  jaws, 
at  other  times  are  more  leg-like  (fig.  404, 
5-7). 

The  false  feet  (pleopoda)  are  small 
and  inconspicuous  appendages  which  may 
have  various  functions:  they  may  serve 
as  gills  or  supports  for  the  gills,  as  places 
for  the  attachment  of  eggs,  as  organs  for 
the  transfer  of  sperm,  or  as  swimming  or 
creeping  organs. 

These  appendages  have  constant  po- 

Fio.  ^.-Appendages   of   the  sitions  in   the   body-       First    °n   the   head 

maSdfbie^'f^^fl^^nd  come  the  antennae  and  then,  in  the  region 
ffpedsd-mJ"waii'fng  ^leg^"  of  ^e  mouth,  the  jaws  and,  so  far  as 
pieopod.  '  they  are  present,  the  maxillipeds.  Third 

come  the  true  feet,  and  lastly,  when  they  exist,  the  false  feet. 
Those  somites  which  bear  antennae  or  jaws  belong  to  the  head,  those 
bearing  walking  feet  to  the  thorax,  while  the  somites  of  the  abdo- 
men bear  either  false  feet  or  lack  appendages.  As  a  sequence  the 
cephalothorax  is  that  region  of  the  body  which  bears,  besides  an- 
tennas and  jaws,  legs  as  well. 

The  extremities  of  Arthropoda  have  given  rise  to  various  disputes. 
Many  zoologists  speak  of  a  pre-antennal  somite  and  a  pre-antennal  ap- 
pendage, referring  to  the  eye  stalk  of  a  part  of  the  Crustacea,  which,  how- 
ever, differs  markedly  in  its  development  from  the  true  appendages. 
Those  who  accept  an  ocular  somite  must  add  one  to  the  number  of  somites 
as  stated  in  this  volume.  A  second  theory  regards  the  antennae  as  ventral 
appendages  innervated  from  the  ventral  chain  which  secondarily  become 
dorsal  and  receive  their  nerves  from  the  brain.  This  view  is  firmly 
grounded  for  the  second  antennae  of  the  Crustacea.  Other  questions  are 
as  to  the  possible  loss  of  segments  and  appendages. 

The  concentration  or  fusion  of  somites  to  body  regions  has  had 
an  influence  upon  the  internal  structure  and  especially  upon  the 
nervous  system  (fig.  405).  A  ladder-like  nervous  system  consists, 


ARTIIROPODA. 


403 


as  was  pointed  out  (p.  124),  of  a  dorsal  brain  (supraoesophageal 
ganglia)  and  a  ventral  chain  of  ganglia,  all  connected  by  longi- 
tudinal nerve  cords,  the  brain  being  connected  with  the  rest  by 
cords  or  commissures  passing  on  either  side  of  the  oesophagus. 
The  ventral  chain  should  contain  as  many  pairs  of  ganglia  as  there 
are  somites,  but  this  is  not  the  case  except  in  the  embryo.  The 
tendency  is  rather  towards  a  fusion  of  ganglia,  especially  of  those 
somites  which  unite  or  fuse.  This  fusion  of  ganglia  occurs  to  a 
ABC  D 


FIG.  405.— Different  degrees  of  concentration  of  the  ventral  cord  of  Arthropods. 
(From  Gegenbaur.)  A,  Termite  (after  Lespes) ;  B,  water  beetle  (after  Blanchard); 
C,  fly  (after  Blanchard);  Z>,  Thelyphonid  (after  Blanchard).  o,  abdomen  ;  </'•*,  <?3, 
ganglia  of  ventral  cord ;  <;z,  infracesophageal  ganglion ;  gs,  supraoesophageal 
ganglion  ;  o,  eye ;  p'-p'**  walking  feet ;  tt\  lung  books ;  I,  chelicerae  ;  2,  pedipalpus. 

varying  extent  in  different  species,  the  extreme  being  reached  in 
the  spiders  and  crabs  (fig.  441),  where  the  whole  ventral  chain 
forms  a  large  ganglionic  mass.  In  all  cases,  however,  the  brain 
remains  distinct  from  the  rest,  its  position  dorsal  to  the  ossophagus 
precluding  its  fusion  with  the  ventral  chain. 

Of  the  sense  organs  the  best  known  are  the  eyes,  of  which  two 
types  are  recognized,  the  simple  (ocellus,  stemma)  and  the  com- 
pound (faceted).  The  ocelli  are  very  small.  In  their  highest 
development,  as  in  spiders  (fig.  406),  they  are  composed  of  lens, 
vitreous  body,  and  retina.  The  lens  is  formed  by  the  cuticula,  the 
rest  from  the  epidermis.  The  lens  differs  from  the  rest  of  the 
cuticle  in  being  transparent,  and  is  usually  thickened  to  a  biconcave 
body  (1)  which  converges  the  light  upon  the  retina.  Behind  the 
lens  comes  a  layer  of  transparent  cells,  the  vitreous  body  (#),  and 


404 


ARTHROPODA. 


behind  this,  in  turn,  the  retina,  consisting  of  cells  which,  at  the 
one  end,  bear  <  rods '  (4  and  7),  at  the  other  pass  into  nerve  fibres. 
The  retina  and  vitreous  body,  surrounded  by  pigment,  form  a 


456  7 

FIG.  406.— Diagrammatic  section  through  anterior  (A)  and  posterior  (B)  eyes  of  Epeira 
diademat*,.  (After  Grenacher.)  The  hinder  eye  shows  the  inverted  retina;  1, 
lens ;  2,  vitreous  body  ;  #,  epidermis,  outside  this,  chitinous  layer ;  A,  rhabdomes ; 
5,  retinal  cells ;  6,  capsule  of  eye  ;  7,  rhabdomes  of  inverted  eye. 

spherical  thickening  sharply  marked  off  from  the  rest  of  the  epi- 
thelium. These  eyes,  like  those  of  vertebrates,  must  form  inverted 
images. 

In  many  spider  eyes  there  is  an  inversion  recalling  that  of  the  verte- 
brates (fig.  406,  B),  the  rhabdorae  lying  behind  the  nuclear  portion  of  the 
cell.  Behind  the  rhabdomes  comes  a  layer  of  strongly  iridescent  cells,  the 
tapetum  lucidum. 


FIG.  407.— Head  of  drone  bee     (After  Swammerdam,  from  Hatschek.)    Showing  the 
large  faceted  eyes  and  between  them  three  ocelli. 

The  compound  eyes  are  much  larger.  They  owe  their  name 
*  faceted  eyes '  to  the  fact  that  the  cuticle  over  them  is  divided 
into  polygonal  (usually  hexagonal)  areas  or  facets  (fig.  407).  Each 


ARTHROPODA. 


405 


facet  corresponds  to  a  small  chitinous  lens  (the  number  of  which 
varies,  in  different  species,  between  a  dozen  and  several  thousand), 


1  23  4 

FIG.  408.— Section  of  compound  eye  of  Forficula.  (After  Carriere,  from  Hatschek.) 
7,  cuticula,  producing  the  cornea  of  many  lenses  over  the  eye;  2,  epidermis, 
which  in  the  eye  forms  the  ommatidia;  3,  basal  membrane;  U,  reentrant  chitin- 
ous fold  ('sclerotic');  5,  rudimentary  larval  eye. 

and  bounds  the  eye  externally,  whence  this  layer  is  called  the 

cornea  (fig.  408).     The  part  of   the  eye  be-        ^ ^^ 

neath  the  cornea  consists  of  radially  arranged        IT  L  r  / 

prismatic  parts  or  ommatidia  which  corre- 
spond in  number  and  position  to  the  facets, 
their  broader  ends  being  placed  beneath  the 
facets,  their  narrower  internal  ends  con- 
necting with  fibres  of  the  optic  nerve  which 
go  to  the  brain.  Each  ommatidium  (fig. 
409)  has  essentially  the  structure  of  an 
ocellus:  (1)  the  lens  (/)  with  its  epi- 
thelium; (2)  the  vitreous  body  (kz)\  (3)  the 
retinula  (rz).  The  vitreous  body  is  usually 
composed  of  four  cells  which  in  the  so-called 
euconous  eyes  surround  a  transparent  body, 
the  crystalline  cone  (&),  secreted  by  these  cells. 
The  retinular  cells  are  almost  always  seven  in 
number,  each  bearing  on  its  inner  surface  a 
rhabdome  (r),  the  seven  rhabdomes  frequently 
fusing  into  a  common  mass.  Each  omma-  FIG.  409.— A  single  om- 

. .  , .  .  ill  •  i       ,  i  matidium    (with    sec- 

tidium  is   surrounded  by  a   pigment  sheath,      tions)  of  a  compound 

.     ,    , .         .,         , .      ii      r»  .,     £  -n  eye.       fc,      crystalline 

isolating  it  optically  irom  its  lellows. 


cone;  fcz,  cone  cells:  /, 
lens  with  hypodermis; 
r.  rhabdomes;  rz,  re- 
tinular cell. 


From  this  it  appears  that  the  compound 
eye  may  be  regarded  as  a  complex  of  ocelli. 
This  anatomical  conception  must  not,  however,  obscure  the  physio- 


406  ARTHROPODA. 

logical.  As  Johannes  Mliller  first  pointed  out,  the  whole  compound 
•eye  forms  but  a  single  erect  picture  composed  of  separate  images 
of  small  area  formed  by  the  separate  ommatidia.  This  '  mosaic 
theory '  has  completely  replaced  the  view  that  each  ommatidium 
formed  a  complete  inverted  picture. 

While  the  number  of  ocelli  varies,  the  compound  eyes  are  almost 
always  two  in  number.  Where,  apparently,  as  in  the  Daphnidse, 
there  is  but  one,  there  is  in  reality  a  fusion.  There  is  also  con- 
stantly present  a  large  optic  ganglion  where  the  optic  nerve  enters, 
but  lying  outside  the  eye  itself. 

The  tactile  organs,  consisting  of  tactile  hairs  (fig.  77),  are  uni- 
form in  structure.  On  the  other  hand  the  senses  of  hearing,  taste, 
and  smell  are  subserved  by  varying  organs.  It  is  to  be  regretted 
that  we  know  but  little  of  these  senses  in  arthropods,  although 
beyond  question  they  are  frequently  well  developed.  The  sense  of 
smell  resides  chiefly  in  the  antennae  and  in  the  palpi  of  the  jaws. 
The  organs  are  olfactory  cones  (modified  hairs)  which  frequently 
lie  in  pits  in  the  skin.  Similar  organs  in  the  mouth  are  probably 
connected  with  taste.  As  organs  of  hearing  (?  equilibration)  besides 
the  otocysts  of  the  Podophthalmata  and  the  tympanal  organs  of 
the  Orthoptera,  the  widely  distributed  '  chordotonal '  nerve  ends 
of  insects  are  to  be  mentioned. 

Concerning  the  alimentary  canal  it  need  only  be  said  that  the 
larger  proportion  of  it  is  formed  of  ectodermal  stomodeum  and 
proctodeum,  while  the  entodermal  portion  (mesenteron)  forms  on 
an  average  but  one  third  of  the  total  length.  At  ecdysis  the  chitin- 
ous  lining  of  these  parts,  including  the  large  chewing  stomach,  is 
cast  with  the  rest  of  the  integument.  The  entire  absence  of  cili- 
ated epithelium  is  noteworthy.  Ciliated  cells  have  never  been 
found  in  arthropods. 

The  most  constant  portion  of  the  circulatory  system  is  the  heart, 
which  usually  lies  immediately  beneath  the  back  and  is  enclosed 
in  a  more  or  less  distinct  sac  which,  although  called  pericardium, 
is  not  a  part  of  the  ccelom.  From  the  pericardium  blood  passes 
into  the  heart  by  openings  right  and  left,  the  ostia.  Since  the 
margins  of  the  ostia  project  far  into  the  lumen  of  the  heart  and  so 
form  folds  functioning  as  valves,  the  heart  itself  may  be  divided 
into  a  series  of  chambers,  especially  distinctly  separated  from  each 
other  by  the  progressive  contraction  of  the  wall  (fig.  66).  The 
chambers  disappear  when,  with  reduction  of  the  body,  the  heart 
shrinks  to  a  sac.  In  small  arthropods  the  heart  together  with 
the  whole  vascular  system  may  be  lost.  Since  the  Annelida  have 


ARTHROPOD  A.  407 

a  well-developed  circulatory  system,  this  loss  in  these  animals 
must  be  regarded  as  secondary  rather  than  as  primitive,  and  is 
explained  by  the  fact  that  with  reduction  in  size  the  organization 
is  simplified. 

The  blood  may  pass  from  the  large  arteries  either  directly  into 
the  large  blood  sinuses  of  the  body,  erroneously  called  the  body 
cavity,  or  by  a  more  complicated  course  through  capillaries  and 
veins  as  well  as  through  the  respiratory  organs.  There  is,  on  this 
account,  the  greatest  difference  in  the  development  of  the  vascular 
system,  but  even  in  the  highest  forms  the  system  is  not  entirely 
closed,  the  blood  passing  to  the  sinuses  of  the  body  (haemocoele, 
p,  110)  and  thence  to  the  pericardium  (probably  arising  from  the 
coalescence  of  veins  and  certainly  not  coelomic),  from  which  it  is 
sucked  through  the  ostia  into  the  heart. 

The  variations  in  the  circulation  depend  upon  the  modifica- 
tions of  the  respiratory  organs,  which  can  be  described  adequately 
only  in  connexion  with  the  various  groups.  In  general  it  can 
only  be  said  that  the  more  respiration  is  localized  in  regions  and 
organs  the  more  nearly  complete  is  the  circulation,  while  with 
respiration  diffused  over  or  through  the  whole  body,  the  vascular 
system,  including  even  the  heart,  may  be  reduced. 

The  various  spaces  in  the  body  are  frequently  encroached  upon 
by  a  fat  body,  a  kind  of  connective  tissue  whose  cells,  richly  laden 
with  fat,  serve  as  a  store  of  nourishment  for  the  animal.  Besides, 
urinary  products,  like  uric  acid,  have  been  found  in  it,  leading  to 
the  conclusion  that  the  fat  body  acts  as  a  reservoir  for  excretory 
substances  before  their  elimination  by  the  excretory  organs.  These 
latter  vary  greatly  in  the  different  groups :  true  nephridia  in  Peri- 
patus,  shell  glands  and  antennal  (green)  glands  in  the  Crustacea, 
and  tubules  (Malpighian  tubules)  connected  with  the  intestine  in 
arachnids  and  insects. 

The  sexual  organs,  which  empty  through  ducts  which  are 
apparently  modified  nephridia,  are  only  rarely  hermaphroditic. 
In  the  bisexual  species  one  can  usually  distinguish  males  and 
females  by  external  characters,  such  as  coloration,  size  or  form  of 
appendages,  especially  those  used  in  copulation.  The  eggs  are 
usually  large  and  rich  in  yolk,  and  consequently  but  rarely  undergo 
total  segmentation.  In  most  eggs  occurs  that  type  of  partial 
segmentation  called  superficial  (fig.  104).  While  the  surface  of 
the  egg  divides  into  the  cells  which  form  the  blastoderm,  the 
central  yolk  long  remains  undivided— a  condition  of  systematic 
interest  since  it  is  not  known  to  occur  outside  the  Arthropoda. 


408  ABTHROPODA. 

The  cases  of  discoidal  and  unequal  segmentation  are  apparently 
derived  from  the  superficial. 

In  accordance  with  their  high  organization,  reproduction  by 
fission  or  budding  never  occurs,  but  parthenogenesis  and  pa?dogen- 
esis  do.  In  some  parthenogenesis  has  a  certain  relationship  to 
the  life  history.  In  lower  Crustacea  and  in  Aphides  (plant  lice) 
it  allows  the  species  to  spread  rapidly  in  large  numbers  over  suit- 
able feeding  grounds.  Among  the  bees  parthenogenesis  has  a 
relation  to  the  sexes,  since  males  are  only  produced  from  unfer- 
tilized eggs.  Along  with  parthenogenesis — there  may  be  rare  ex- 
ceptions— sexual  reproduction  occurs,  so  that  not  rarely  asexual 
alternates  with  sexual  generation  (heterogony),  though  not  in  such 
a  pronounced  manner  as  in  the  worms. 

The  French  entomologist  Latreille  divided  the  Arthropods  into  four 
classes:  Crustacea,  Myriapoda,  Arachnida,  and  Insecta.  Later  the  dis- 
covery, by  Moseley,  that  Peripatus  possesses  tracheae  led  to  the  creation  of 
anew  class,  Protracheata,  and  the  grouping  of  all  arthropods  into  branchi- 
ate and  tracheate  divisions,  the  branchiates  including  the  Crustacea  alone. 
Later  researches  have  shown  that  these  divisions  are  not  natural  and  that 
tracheae  have  had  different  origins,  the  spiders  being  nearer  to  the  crus- 
tacea  than  to  the  insects,  and  that  Crustacea  and  insecta  have  come  from 
the  annelids  through  different  lines.  Similarly  the  myriapods  have  been 
divided,  one  group,  the  chilopods,  being  closely  related  to  the  true  insects, 
the  other  (diplopods)  being  very  uncertain  in  position. 

Class  I.  Crustacea. 

The  Crustacea  owe  their  name  to  the  fact  that  their  chitinous 
cuticle  is  usually  rendered  hard  and  firm  by  deposits  of  carbonate 
and  phosphate  of  lime  and,  in  contrast  to  that  of  other  arthropods, 
has  lost  much  of  its  elasticity  and  has  become  '  crusty/  Another 
important  characteristic  is  the  habitat  of  the  group;  the  Crustacea 
are  typically  aquatic  and  hence  breathe  by  means  of  gills.  This 
branchial  respiration  persists,  as  in  the  case  of  crayfish,  when  the 
animals  are  taken  from  the  water,  for  they  retain  water  in  the  gill 
chamber  and  hence  for  a  long  time  the  gills  are  wet  by  this  fluid. 
There  are  but  few  exceptions  to  this  rule,  as  some  land  crabs  and 
the  sow  bugs;  these  breathe  air,  either  by  means  of  the  gills  or  by 
special  structures  in  the  gill  chamber  to  be  mentioned  later. 

The  branchiae  or  gills  are  always  placed  where  a  rapid  exchange 
of  water  is  possible.  The  appendages  afford  such  a  position,  and 
hence  one  finds  the  gills  as  thin-skinned  vascular  plumes  or  plates 
(figs.  61,  437)  either  on  the  appendages  or  on  the  body  near  by, 
or  the  whole  appendage  may  take  a  leaf -like,  thin-skinned  shape 


7.    CRUSTACEA. 


409 


and  thus  serve  as  a  gill  (figs.  411,  451).  Besides,  the  whole  body 
surface  may  be  respiratory  and  in  small  forms  may  entirely  replace 
that  of  the  gills,  so  that  these  organs  become  rudimentary  or  may 
entirely  disappear,  there  being  a  diffuse  respiration  with  cor- 
responding results  in  the  circulatory  system.  With  a  localized 
respiration  heart,  arteries,  capillaries,  and  veins  are  well  developed, 
but  with  the  diffuse  respiration  only  the  heart  persists  as  a  reduced 
structure,  or  with  its  disappearance  the  last  traces  of  a  circulatory 
system  are  lost. 

Locomotion  as  well  as  respiration  is  related  to  the  aquatic  life, 
and  these  animals  usually  possess  a  special  form  of  appendage  of 
the  biramous  or  schizopodal  type,  which  at  once  differentiates  these 
forms  from  other  arthropods.  While  in  the  latter,  as  every  insect 
shows,  the  joints  of  the  limb  follow  in  a  single  sequence,  the  crus- 
tacean appendage  has  a  two-jointed  base  (basiopodite),  followed 


FIG.  410.— Copepod  appendages.  I-IV,  Diaptomus  castor;  7,  a  pair  of  schizopodal  feet; 
//,  second  right  antenna;  7//,  right  mandible;  IV,  right  maxilla;  F,  right  mandi- 
ble of  Cyclops  coronatus.  I,  #,  joints  of  basiopodite;  t,  endopodite;  a,  exopodite. 

by  two  many-jointed  branches  (fig.  410,  7),  an  inner  or  endopodite 
and  an  outer  or  exopodite. 

The  schizopodal  appendage  occurs  only  when  the  limb  is  used  for 
swimming;  when  it  is  used  for  walking  upon  the  bottom,  as  in  crayfish  and 
crabs,  the  exopodite  is  lost  and  only  the  endopodite  persists  as  the  func- 
tional limb,  which  then  closely  resembles  the  appendages  in  the  so-called 
tracheates.  This  loss  rarely  occurs  on  all  the  appendages;  usually  the 
abdominal  feet  and  the  mouth-parts  retain  the  two-branched  condition. 
Embryology  further  shows  that  even  in  the  case  of  the  crabs  all  the  feet  are 
at  first  schizopodal  and  that  the  walking  legs  lose  the  exopodite  during 
growth.  There  is  some  evidence  to  show  that  the  schizopodal  foot  is  not 
the  primitive  type.  This  is  furnished  by  the  phyllopod  foot  (fig.  411,  77), 


410 


ARTHROPODA. 


and  consists  of  a  medial  axis,  6,  bearing  on  the  inside  six  '  endites,'  t',  and 
on  the  outer  side  two  'exites,'  a  flabellum  or  epipodite,  a,  and  a  gill,  A. 


FIG.  411.— Branchiopod  appendages.  1  and  II  first  and  sixth  legs  of  Branchipus  grubei 
(after  Gerstacker);  111,  fourth  leg  of  Daphnia  simus  (after  Glaus),  a,  flabellum; 
7i,  basis  ;  i,  axon  and  its  endites ;  fe,  gills. 

This  becomes  modified  into  the  schizopodal  form  by  a  loss  of  the  four 
basal  endites  (those  nearest  6)  and  the  development  of  the  two  terminal 
endites  into  exopodite  and  endopodite.  Still  the  schizopodal  condition  is 
so  nearly  universal  among  Crustacea  that  it  must  be  accorded  great  weight 
in  classification. 

The  appendages  furnish  a  further  diagnostic  character  in  that 
two  pairs  of  antennae  are  present  in  the  Crustacea  (see,  however, 
Trilobitae).  Antennae,  it  will  be  remembered,  are  preoral  append- 
ages innervated  from  the  brain.  In  some  cases,  as  many  Ento- 
mostraca,  the  second  pair  may  lose  their  sensory  functions  and 
become  mere  swimming  organs. 

A  carapace,  recalling  the  mantle  of  the  molluscs,  is  widely  dis- 
tributed in  the  Crustacea.  It  arises  as  a  fold  from  the  head,  which 


/.    CRUSTACEA. 


4:11 


may  extend  backwards  as  a  shield,  completely  covering  some  or  all 
of  the  thoracic  segments  (fig.  412),  or  it  extends  right  and  left  on 
the  sides  of  the  body  (fig.  426)  and  produces  two  valves  strikingly 
like  those  of  a  lamellibranch,  the  resemblance  being  strengthened 
in  the  cirripeds  and  ostracodes  by  the  extensive  calcification. 

Concerning  the  internal  organs  but  few  general  remarks  can 
be  made.  Salivary  glands  are  wholly  absent;  on  the  other  hand 
the  stomodeum  is  usually  widened  into  a  strong  chewing  '  stomach/ 
and  behind  this  empty  the  ducts  of  the  so-called  liver  (better 


ttx 


FIG.  412. 


FIG.  413. 


PIG  412.— Apus  cancriformis.    (After  Ludwig-Leunis.)   The  segments  mostly  covered 

by  the  carapace. 
FIG.  413.— Antennal gland  of  Myste.    (After  Grobben.)    Wr,  blood  lacunae;  ea,  external 

opening;  /i,  bladder;  re,  canal;  s,  internal  vesicle. 

hepato-pancreas).  The  liver  itself  differs  widely  from  the  two 
simple  blind  sacs  of  the  Daphnidae  (fig.  420)  to  the  enormous 
livers  of  the  Decapoda  (fig.  439,  A).  Excretory  organs  are  repre- 
sented by  so-called  green  glands  (antennal  glands)  and  shell  glands. 
The  latter,  which  received  their  name  from  the  erroneous  idea 
that  they  produced  the  shell,  open  to  the  exterior  on  either  side  at 
the  bases  of  the  fourth  appendages,  the  maxillae  (fig.  420,  s).  The 
green  gland  opens  similarly  on  the  basis  of  the  second  antennas. 
Both  have  essentially  the  same  structure  (fig.  413);  they  begin  with 
a  terminal  vesicle  (in  the  case  of  the  antennal  gland  in  close  rela- 
tions with  the  reduced  coelom),  which  passes  into  a  slender,  greatly 
coiled  tube.  Their  structure  and  development  lead  to  the  con- 
clusion that  they  are  modified  segmental  organs.  Both  occur 


412  ARTIIROPODA. 

together  only  in  the  larvae;  in  the  adult  one  or  the  other  is  sup- 
pressed. In  some  amphipods  there  are  excretory  diverticula 
developed  from  the  intestine  (fig.  448),  which  resemble  the  Mal- 
pighian  tubes  of  insects,  but  differ  from  them  in  being  of  ento- 
dermal  origin.  In  some  decapods  caeca  occur  in  the  same  region, 
but  nothing  is  known  of  their  function. 

Visual  organs  are  either  represented  by  the  so-called  nauplius 
eye,  consisting  of  a  pigment  spot  with  three  lenses  situated  directly 
on  the  brain,  or  by  a  pair  of  compound  eyes.  The  nauplius  eyes 
are  chiefly  found  in  the  lower,  the  compound  in  the  higher,  groups; 
occasionally  they  coexist  in  the  same  species.  Auditory  (equili- 
bration) organs  (otocysts)  occur  only  in  the  Malacostraca,  either 
in  the  base  of  the  first  antennae  or  in  the  endopodite  of  the  last 
abdominal  feet  (fig.  434,  o).  These  are  rarely  vesicular,  but  are 


FIG.  414.— Otocyst  of  crayfish,    as,  auditory  ridge;  ?i,  nerve. 

usually  grooves  (fig.  414),  bearing  at  the  base  a  row  of  chitinous 
sense  hairs,  the  crista  acustica,  connected  below  with  an  auditory 
nerve,  while  their  free  ends  extend  between  a  cluster  of  otoliths. 

At  ecdysis  these  otocysts  with  their  sensory  hairs  and  otoliths  are 
cast  off.  If  a  crayfish  which  has  just  molted  be  placed  in  perfectly  clean 
water,  the  otocyst  will  remain  without  otoliths;  but  if  some  easily  recog- 
nizable substance,  like  uric  acid  crystals,  be  placed  in  the  water,  some  of 
these  will  soon  be  found  in  the  sac,  thus  proving  that  the  otoliths  are 
introduced  from  the  outside. 

Crustacea  are  only  exceptionally  hermaphroditic.  The  sperma- 
tozoa are  noticeable  for  their  great  size,  in  many  ostracodes  equal- 
ling the  body  in  length.  Except  in  the  Cirripedia  the  spermatozoa 
lack  a  flagellum  and  are  immobile.  Their  round  or  elongate  body 
is  covered  with  rigid  processes  reminding  one  of  the  pseudopodia 
of  Actinosphcerium  (fig.  36,  y,  6).  They  are  frequently  enclosed 
in  spermatophores  (fig.  422). 

The  typical  development  of  a  crustacean  includes  a  metamor- 
phosis, and  where  direct  development  occurs  the  metamorphosis  is 


/.    CRUSTACEA. 


413 


either  suppressed  or,  as  is  easily  shown,  the  corresponding  stages 
are  passed  in  the  egg.  Two  of  the  larval  stages  are  especially  im- 
portant, the  nauplius  and  the  zoea.  The  nauplius  (figs.  7,  429) 
consists  of  three  segments  covered  by  a  dorsal  shield  and  bearing 
below  three  pairs  of  appendages.  The  first  pair,  developing  later 
to  the  first  antennae,  are  simple;  the  others,  corresponding  to  the 


FIG.  415.— Zoea  of  Cai-cinus  mcenas.    (After  Faxon.)    7t,  heart;  i,  intestine;  1-VIL, 
cephalic  appendages. 

second  antennas  and  mandibles,  are  schizopodal.  Internally  there 
is  a  three-chambered  alimentary  tract,  a  supraoesophageal  ganglion 
on  which  is  an  unpaired  eye,  and  a  ventral  chain.  The  nauplius 
is  almost  universal  among  the  lower  Crustacea,  and  some  writers 
believe  that  it  represents  an  ancestral  form  from  which  the  crus- 
tacea  have  descended,  a  view  open  to  much  objection. 

The  zoea  is  more  complex.     It  consists  (fig.  415)  of  cephalo- 
thorax  and  abdomen,  the  latter  without  appendages,  the  former 


414  ARTHROPODA. 

with  several  pairs  of  schizopodal  swimming  feet.  There  are  two 
large  compound  eyes  and,  dorsal  to  the  intestine,  a  heart.  Fre- 
quently the  carapace  is  armed  with  very  long  spines  projecting 
from  front,  back,  and  sides,  which  are  intended  as  protection  from 
enemies. 

Nauplius  and  zoea  are  of  systematic  importance,  since  they 
rarely  both  appear  in  the  life  cycle  of  one  individual.  The  nau- 
plius  is  characteristic  of  the  lower  Crustacea — the  t  Entomostraca.' 
The  zoea,  on  the  other  hand,  has  never  been  noticed  in  the  En  to- 
rn ostraca,  but  occurs  in  many  Malacostraca.  A  nauplius  appears 
in  only  a  few  Malacostraca,  like  the  schizopods  and  Peneus,  and 
there  precedes  the  zoea  stage.  It  must  not  be  forgotten  that 
many  forms  among  both  Entomostraca  and  Malacostraca  have  no 
zoeal  or  nauplius  stage. 

Frequently  the  lower  Crustacea  are  united  under  the  name 
Entomostraca,  but,  aside  from  the  nauplius  stage  and  the  posses- 
sion of  a  shell  gland,  the  only  characters  of  the  group  are  negative. 

Sub  Class  I.   TriloUtce. 

The  most  important  fossils  of  the  class  of  Crustacea  are  the 
Trilobites  which  appeared  in  the  Cambrian  and  died  out  in  the 
Permian,  being  extremely  abundant  in  the  Silurian.  The  body 
(fig.  416)  consists  of  head  and  trunk,  the  latter  segmented.  In 
the  young  the  segments  are  very  few,  but  increase  in  number  with 
age  (10-29,  according  to  the  species).  The  hinder  segments  fre- 
quently differ  from  the  rest  and  form  an  abdomen  OTpygidium. 
Dorsally  the  animal  is  divided  by  two  grooves  into  three  lobes, 
marking  off  in  the  head  a  glabella  and  two  gence;  in  the  trunk 
rhachis  and  two  pleurm.  On  the  head  there  are  usually  a  pair  of 
compound  eyes,  which  in  the  young  were  frequently  ventral,  but 
are  brought  to  the  dorsal  surface  with  growth.  For  many  years 
little  was  known  of  the  under  surface,  but  lately  specimens  of 
Triarthrus  becki  (fig.  417)  from  the  Utica  slate  have  revealed  the 
appendages.  On  the  head  are  a  pair  of  simple  antennae,  and  four 
pairs  of  schizopodal  feet,  the  bases  of  which  acted  as  jaws.  It  is 
a  question  whether  the  first  pair  of  jaw  feet  correspond  to  the  sec- 
ond antennas  or  whether  these  have  been  lost  in  the  group.  The 
trunk  segments  bear  biramous  feet. 

In  some  respects  the  trilobites  resemble  the  Xiphosura  (infra), 
but  the  possession  of  antennas  and  biramous  feet  place  them  among 
the  Crustacea.  Here  their  position  is  very  uncertain.  We  have 


L    CRUSTACEA:  PHTLLOPODA. 


415 


little  knowledge  of  but  one  species,  and  this  with  its  single  pair 
of  antennae  differs  from  all  recent  Crustacea. 


FIG.  416.  FIG.  417. 

FIG.  416.—  Paradoxides  bohemicus.    (From  Zittel.) 

FIG.  W.—Triarthru*  becki,  ventral  surface,  restored.  (After  Beecher.)  The  head 
bears  one  pair  of  antennae  and  four  pairs  of  biramous  feet,  the  basal  joints  serv- 
ing as  maxillae.  Trunk  with  biramous  feet. 

Sub  Class  II.  Phyllopoda. 

The  Phyllopoda  are  clearly  the  most  primitive  of  Crustacea. 
The  name  is  derived  from  the  leaf-like  feet  (p.  410),  which  occur 
upon  the  thoracic  region.  More  anteriorly  the  appendages  are 
schizopodal,  the  second  pair  of  antennae  often  being  efficient  swim- 
ming organs.  The  number  of  body  segments  varies  between  very 
wide  limits,  there  being  less  than  a  dozen  in  the  Cladocera,  while, 
if  Savigny's  law  (p.  401)  holds  true,  there  are  over  sixty  in  some 
Apodidae.  In  most  forms  (the  Branchipodidae  excepted)  a  cara- 
pace is  developed  by  a  backward  growth  from  the  head.  This 
forms  a  broad  oval  shell  covering  most  of  the  body  in  the  Apodidae 
(fig.  412);  in  the  Estheriidae  and  Cladocera  it  is  divided  into  right 
and  left  halves  hinged  together  in  the  mid-dorsal  line,  thus  giving 
these  animals  the  appearance  of  bivalve  molluscs. 

These  forms  have,  besides  the  unpaired  nauplius  eye,  a  pair  of 
compound  eyes  which  in  the  compressed  forms  are  frequently 
fused,  although  distinct  in  the  young  and  retaining  the  double 


416 


ARTHttOPODA. 


optic  nerve  throughout  life.  The  liver  is  present  in  the  shape  of 
simple  ca3ca;  the  heart,  elongate,  chambered,  and  with  many  ostia 
in  the  Branchiopoda,  a  short  sac  with  only  a  pair  of  ostia  in  the 
Cladocera  (fig.  420,  U),  lies  dorsal  to  the  intestine.  The  shell 
gland  is  well  developed. 

In  development  summer  and  winter  eggs  are  distinguished.  The  sum- 
mer eggs  form  a  single  polar  globule  and  develop  partheuogenetically. 
The  winter  eggs  form  two  polar  globules  and  require  fertilization.  The 
thin-shelled  summer  eggs  are  carried  about  by  the  mother  in  a  brood 
pouch  and  hatch  in  a  relatively  short  time.  The  thick-shelled  winter  eggs 
are  cast  off  and  fall  to  the  bottom,  where  they  require  a  long  time  for 
development.  They  may  be  dried  or  frozen  without  injury,  and  at  least  in 
some  cases  drying  is  necessary  to  their  development.  This  feature  explains 
the  appearance  in  early  spring  of  large  numbers  of  Branchipus  and 
Estheria  in  snow  pools  which  are  dry  throughout  the  summer.  On  our 
western  plains  and  in  Europe  Apus  occurs  in  the  same  way.  These  pecu- 
liarities of  reproduction  are  readily  understood  when  we  recollect  that  the 
phyllopods  are  largely  inhabitants  of  fresh  water.  The  winter  eggs  pre- 
serve the  species  through  times  of  drought  and  cold;  the  summer  eggs  are 
for  the  rapid  increase  of  the  species  during  the  wet  season.  The  same 
relations  also  explain  the  fact  that  males  are  rare  and  only  appear  at 
intervals,  indeed  are  not  known  in  many  species. 

Order  I.  Branchiopoda. 

The  Branchiopoda  are  relatively  large  with  numerous  segments,  leaf- 
like  appendages,  long,  chambered  heart,  and  lack  swimming  antennae. 
"With  few   exceptions    they  are  inhabitants    of 
fresh  water.   According  to  the  development  of  the 
carapace  they  are  subdivided  into  three  families. 

1.  APODID^E.     Body  depressed,  with  large  oval 
undivided  carapace.     Eggs  carried  in  brood  cap- 
sules formed  by  a  pair  of  appendages.      Apus 
(fig.  412)  and  Lepidurus  occur  in  Europe,  Green- 
land, and  the  United  States  west  of  the  Missouri. 
Protocaris  of  the  Cambrian  rocks  is  apparently 
an  Apodid. 

2.  BRANCHIPID^E.    Body  without  carapace,  the 
second  antennae  of  the  male  large  and  modified 
into  an  organ  for  clasping  the  female.    The  female 
carries  the  summer  eggs  in  a  wide  '  uterus '  in 
the  abdomen.     Branchipus  lives  in  fresh  water, 
Artemia  in  brine,  and  in  certain  species  one  has 

FIG  41*  —  4 DM*  equalis*  been  transformed  into  the  other  by  changing  the 
'  (After^Packard.)  water  from  fresh  to  salt  or  the  reverse.  Branchi- 
pus vernalis  (fig.  419)  occurs  in  snow-water  pools  in  our  northern  states, 
Artemia  in  Great  Salt  Lake,  around  salt  works  or  in  tubs  of  brine  near 
the  ocean. 


/.    CRUSTACEA:   COPEPODA.  417 

3.  ESTHERIID.E.  Body  laterally  compressed  and  enclosed  in  a  bivalve 
shell,  compound  eyes  fused;  male  very  rare.  Estheria*  Limnadia*  in 
fresh  water. 

Order  II.  Cladocera. 

Like  the  estheriids  the  small  Cladocera  have  the  body  enclosed  in  a 
bivalve  carapace,  which,  in  some  instances,  is  small  and  reaches  back  only 
over  the  first  trunk  segments,  in  others  is  large,  enclosing  the  body,  with  a 
notch  for  the  protrusion  of  the  head,  while  behind  it  terminates  in  a  sharp 
spine.  The  head  bears  a  pair  of  large  swimming  antennae  and  a  much 
smaller  first  pair  bearing  olfactory  bristles  and,  in  the  male,  hooks  for 


FIG.  419.— Branchipus  vernalis,*  fairy  shrimp.    (After  Packard.) 

clasping  the  female.  The  body  consists  of  few  segments,  the  heart  is  a 
simple  sac,  and  the  fused  faceted  eyes,  with  paired  optic  nerves,  are  capa- 
ble of  motion  in  a  special  optic  capsule. 

The  young  eggs  in  the  sexual  organs  always  occur  in  groups  of  four 
(fig.  420).  Of  these  but  one  grows  into  an  egg,  the  others  breaking  down 
and  serving  this  as  nourishment.  Larger  eggs  with  more  yolk  occur  when 
several  of  these  groups  (2-12  fuse  to  form  a  single  egg.  The  summer 
eggs  arise  from  a  single  group,  the  winter  eggs  from  several  groups  of 
primordial  ova. 

In  all  Cladocera  the  space  between  the  back  of  the  animal  and  the 
shell  serves  as  a  brood  pouch.  In  some  cases  this  pouch  contains  an 
albuminous  fluid  secreted  by  the  mother,  which  nourishes  the  embryo. 
The  larger  winter  eggs — one  or  two  in  number — frequently  remain  for 
awhile  in  the  brood  chamber  and  are  there  enveloped  in  a  peculiar  shell, 
the.ephippium,  consisting  of  two  chitinous  plates,  like  watch  crystals,  their 
edges  closely  appressed. 

DAPHNID^:.  Shell  weft  developed;  Daplinia*  (fig.  420),  Bosmina* 
POLYPHEMID^E.  Shell  small,  only  functioning  as  a  brood  case;  head  with 
an  enormous  eye  and  large  swimming  antenna;  no  phyllopodous  feet; 
marine  and  lacustrine.  Leptodora  hyalina  *  appears  at  night,  sometimes 
in  great  numbers,  in  some  of  our  lakes.  Evadne,*  marine. 

Sub  Class  III.   Copepoda. 

A  general  description  of  the  copepods  can  only  apply  to  the 
non-parasitic  forms,  since  many  of  the  parasites  have  become  so 


418 


ARTHROPODA. 


FIG.  420.— Daphnia  pulex.  6,  brood  chambers  with  embryos;  <?,  brain  with  nauplms 
eye;  go,  optic  ganglion;  /i,  heart;  o,  ovary;  s,  shell  gland.  The  eggs  arise  at  fc,  and 
separate,  forming  in  groups  of  four,  as  at  e,  of  which  one  becomes  the  egg,  while 
the  others  abort  (o)  and  form  food.  The  egg  then  passes  to  the  brood  chamber. 
1,  2,  first  and  second  antennae;  5,  mandible  (maxilla  rudimentary  and  invisible); 
5-9,  lege.  g,  brain;  go,  optic  ganglion;  h,  heart. 


/.    CRUSTACEA:    COPEPODA. 


419 


degenerate  (figs.  6,  423)  as  to  be  recognized  even  as  arthropods 
only  by  a  knowledge  of  the  development.  The  sixteen  somites  of 
the  body  are  nearly  equally  divided  among  the  three  regions — 


i 


FIG.  421.— Evadne  (orig.),  showing  the  brood  pouch  filled  with  eggs  and  young.  aa, 
second  antenna;  ao,  adhesive  organ;  b,  brain;  /,  furca;  7i,  heart;  i,  intestine;  I, 
liver;  s,  shell  gland. 


FIG.  422.—Dinptomus  castor,  b,  ventral  nerve  cord;  0,  brain  with  nauplius  eye;  hr 
heart,  beneath  it  the  ovary  and  digestive  tract;  sp,  spermatophores ;  J,  2,  first  and 
second  antennae;  3,  mandibles;  4,  maxillae;  5,  maxilliped;  6-10,  swimming  feet. 

head  (6),  thorax  (5),  and  abdomen  (5) — of  the  animal.  (In  Cyclops 
the  first  thoracic  segment  is  fused  with  the  head,  the  first  two 
abdominal  segments  are  fused — fig.  7.)  The  terminal  abdominal 


420  ARTHROPODA. 

segment  is  two-forked,  forming  the  '  furca.'  While  the  abdomen 
lacks  appendages,  the  thorax  bears  typical  biramous  appendages, 
consisting  of  a  two-jointed  basiopodite,  the  basal  joint  being  fre- 
quently united  with  its  fellow  of  the  pair  for  common  motion  (fig. 
410,  /).  Exopodite  and  endopodite,  usually  three-jointed,  are 
fringed  with  bristles.  Usually  the  fifth  pair  of  thoracic  append- 
ages are  not  so  well  developed,  and  in  some  cases  are  represented 
by  two  bunches  of  bristles. 

The  two  pairs  of  antennae  are  frequently  similar  in  size  (whence  the  old 
name  Cyclops  quadricornis).  The  first  pair  are  always  uniserial  and  in  the 
males  may  be  hooked  near  the  base  for  clasping;  the  second  are  some- 
times biramous  (fig.  410,  II).  The  mandible  (fig.  410,  III,  V)  is  instruc- 
tive, since  a  study  of  several  species  shows  that  it  is  derived  from  a  schiz- 
opodal  condition  and  that  the  first  basal  joint  alone  is  used  for  chewing, 
the  rest  being  reduced  to  a  palpus  of  varying  development.  Both  basal 
joints  of  the  maxillaB  (fig.  410,  IV)  can  be  used  in  eating.  Two  inaxilli- 
peds  (formerly  regarded  as  the  separated  branches  of  an  appendage)  mark 
the  termination  of  the  head  (fig.  422,  .5);  both  are  hooked  for  holding  the 
food. 

The  internal  anatomy  is  simple.  There  is  no  liver,  and  the 
straight  intestine  (fig.  422)  runs  without  marked  changes  in  size 
to  the  anus  between  the  branches  of  the  furca.  The  visual  organ 
is  the  unpaired  nauplius  eye  (which  has  given  the  name  to  one 
genus,  Cyclops).  It  lies  directly  on  the  brain.  The  ventral  chain 
has  its  ganglia  irregularly  distributed.  Gills  are  always  absent,  as 
are  usually  the  heart  and  blood-vessels.  Only  in  a  few  parasitic 
forms  are  there  tubes  which  have  been  interpreted  as  parts  of  a  vas- 
cular system;  in  some  free  forms  there  is  a  short  saccular  slowly 
pulsating  heart.  The  gonads  are  unpaired  in  both  sexes,  but  the 
sexual  ducts,  which  open  at  the  base  of  the  abdomen,  are  paired. 
The  females  possess  a  receptaculum  seminis  distinct  from  the  ovi- 
ducts, to  which  the  male  attaches  the  spermatophores  packed  with 
sperm  (fig.  422,  sp).  As  the  eggs  leave  the  oviduct  they  are  fer- 
tilized by  the  sperm  issuing  from  the  spermatophores,  and  num- 
bers are  enclosed  in  a  gelatinous  substance,  thus  producing  bundles 
of  eggs,  the  so-called  egg-sacs,  attached  to  the  abdomen,  by  which 
one  can  easily  recognize  the  females  (fig.  7).  A  nauplius  hatches 
from  the  egg,  and  by  budding  segments  and  appendages  at  the 
hinder  end,  and  by  a  change  of  the  nauplius  appendages  into 
antennae  and  mandibles,  passes  through  a  ( cyclops-stage '  into  the 
adult. 

The  Copepoda  have  clearly  descended  from  some  phyllopod- 
like  form.     The  poorly  developed  ventral  chain,  the  loss,  partial 


/.    CRUSTACEA:   COPEPODA. 


421 


or  complete,  of  a  circulatory  system,  and  the  absence  of  gills  are 
afl  evidences  against  the  view  which  would  consider  them  primitive. 

Order  I.  Eucopepoda. 

The  forms  to  which  the  foregoing  description  will  apply  are  the  Euco- 
pepoda, and  include  many  species,  which  often  occur  in  enormous  numbers 
in  both  fresh  and  salt  water,  forming  the  larger  proportion  of  the  plank- 
ton. They  thus  furnish  the  most  important  food  supply  not  only  for  fishes 


FlG.  423. 


FIG.  424. 


FIG.  423. — Female  Lernceocera  esocina.  (From  Lang,  after  Glaus.)  A,  armlike  proc- 
esses of  anterior  end;  d,  digestive  tract;  es,  egg-sacs;  od,  oviduct;  *i-£4,  rudi- 
mentary thoracic  appendages. 

FlG.  424.— Argulus  foliaceus.  (From  Ludwig-Leunis.)  a,  sting;  a',  antenna ;  b, 
mouth;  c,  intestine  with  liver;  d,  abdomen;  pm1,  pms,  first  and  second  maxilli- 
peds ;  pl-p*i  biramous  feet  of  thorax. 

but  for  those  giants  among  mammals,  the  baleen  whales.  Cetochilus  sep- 
tentrionalis  occurs  at  times  in  such  myriads  that  the  sea  for  long  distances 
is  colored  red. 

The  CYCLOPID.E,  with  no  heart  and  paired  egg  sacs,  are  fresh-water 
forms;  Cyclops*  (fig.  7).  CALANID^E,  fresh  water  and  marine;  heart  pres- 
ent, single  egg-sac.  Diaptomus,*  fresh  water  (fig.  422);  Cetochilus,*  Pon- 
tilla*  marine.  HARPACTIDJS,  creeping  forms,  mostly  marine;  Cantho- 
camptus*  fresh  water.  The  CORYC^ID^E,  which  are  half  parasitic  and 
include  the  wonderfully  iridescent  Sapphirina  *  (upon  pelagic  animals,  as 


422  ARTHROPODA. 

Salpa},  and  the  NOTODELPHID^,  parasitic  in  the  gills  of  ascidians,  form  a 
transition  to  the  next  order. 

Order  II.  Siphonostomata  (Parasita.) 

There  are  also  Copepoda  to  which  the  account  in  large  type  will  not 
apply,  animals  of  such  strange  appearance  that  many  of  them  were  long 
regarded  as  parasitic  worms  (figs.  6,  423,  425).  Their  mandibles  are 
altered  to  piercing  bristles  and  enclosed  in  a  piercing  proboscis  formed  of 
upper  and  lower  lips.  With  this  sucking  organ  they  bore  into  the  skin  or 
gills  of  fishes.  They  have  cylindrical  forms  or  bodies  of  the  most  bizarre 
shapes,  in  which  frequently  no  segmentation  is  visible,  while  the  append- 
ages are  rudimentary  or  even  entirely  lost.  Indeed  one  would  not  recog- 
nize them  as  arthropods  save  for  the  following  features  : 

(1)  Most  of  them  have  the  typical  Copepod  egg-sacs  (sometimes  elongate 
and  spirally  coiled)  attached  to  the  hinder  end.  (2)  In  the  course  of  years 
a  complete  series  of  intermediate  forms  has  been  found,  allowing  one  to 
trace,  step  by  step,  the  alterations  of  form  from  that  of  the  free-living 
species  to  that  of  the  most  modified  parasites.  (3)  Ontogeny  is  convincing. 
Most  parasitic  Copepoda  leave  the  egg  as  a  nauplius  and  pass  through  a 
Cyclops-stage  before  attaching  themselves  to  fishes  and  becoming  the  highly 
degenerate  parasites.  These  parasites  are  always  females.  The  males 
scarcely  pass  the  Cyclops-stage,  copulate  with  the  females 
and  then  die,  or  if  they  pass  through  the  metamorpho- 
sis, they  remain  small  and  different  in  appearance. 
They  occur  attached  to  the  female  near  the  genital 
openings.  There  is  thus  here  a  marked  sexual  dimor- 
phism. 

The  ARGULID^E  (sometimes,  but  without  warrant, 
made  a  distinct  sub  order,  Branchiura)  are  fresh-water 
forms  with  compound  eyes,  liver  lobes,  and  the  second 
maxillipeds  metamorphosed  into  suckers.  Argulus  * 
FIG.  425.  —  Lerncea  (fig.  424)  causes  considerable  mortality  to  fish.  The 
branchiaiis*«>rig.)  marine  aud  brackish-water  CALIGID.E  (Caligus  *)  have 
similar  habits.  LERN^EOPODID^:.  Fish  parasites  with  maxillae  united  into 
an  adhesive  organ.  Achtheres  *  (fig.  6),  parasitic  on  perch.  LERN^EID^E  ; 
worm-like  parasites.  Lerncea  branchialis,*  common  on  gills  of  cod ; 
Lernceocera*  (fig.  423),  on  pike  ;  Penella.* 

Sub  Class  1 V.   Ostracoda. 

Like  the  Cladocera  and  the  Estheriidae  the  Ostracoda  are  en- 
closed in  a  bivalve  shell,  which,  when  closed,  includes  not  only 
the  body  but  the  head  and  appendages  as  well,  these  being  pro- 
truded when  the  shell  is  opened.  The  valves  are  closed  by  an 
adductor  muscle,  opened  by  a  hinge  ligament  like  that  of  lamel- 
libranchs.  This  resemblance  to  the  molluscs  is  heightened  by 
lines  of  growth  upon  the  shell. 


/.    CRUSTACEA:   CIRRIPEDIA. 


423 


The  antennae,  the  first  simple,  the  second  frequently  two- 
branched,  are  used  for  swimming  and  creeping,  and  are  bent  back- 
wards and  provided  with  numerous  joints  and  hairs.  The 
following  appendages  (mandible,  maxillae,  and  three  pairs  of  legs) 


FIG.  426. — Cypris  fa sciatus,  adult  female.    (After  Glaus.)    I-IV,  appendages;  c,  furca; 
e,  eye;  /,  liver;  m,  adductor  muscle  of  shell;  o,  ovary;  s,  shell  gland. 

vary  greatly  from  genus  to  genus.  The  internal  structure  is  also 
variable.  The  Ostracoda  are  largely  bottom  forms  and  live  in  fresh 
and  brackish  water  as  well  as  in  the  sea. 

CYPRIDINID^E.  First  two  pairs  of  legs  maxillary  in  character,  the  last 
developed  into  a  hook  for  cleansing  the  shell;  heart  v^esent;  marine. 
Cypridina*  CYPRIDID.E.  First  pair  of  legs  maxillary  in  character ; 
heart  lacking;  fresh  water.  Cypris,*  Candona.* 

Sub  Class  V.    Cirripedia. 

The  cirripeds,  or  barnacles,  differ  from  all  other  Crustacea  in 
that  they  have  lost  their  locomotor  powers  and  live  attached  to 
rocks,  floating  timber,  and  the  like.  In  some  cases  they  attach 
themselves  to  other  animals,  as  crabs  and  molluscs,  or,  as  in  the 
case  of  Coronula,  to  whales.  This  leads 
in  Anelasma  and  the  Rhizocephala  to  a 
true  parasitism,  the  barnacle  not  only 
attaching  itself  to  an  animal  but  sucking 
its  juices  as  food. 

The  attachment  is  by  the  dorsal  sur- 
face in  the  neighborhood  of  the  head, 
and  is  initiated  by  the  first  antennae,  in 

which  is  a  cement  gland  secreting  a  no.  427.-flaZajitw/iameri,*  acorn 
rapidly  hardening  cement.  The  region  Da™ne)'  Formed  of  rosteum' 

nf    fivatirm     in    +Vio    "RalQnirJco     (Grr      /fO^N       lateralia,    and      carina,    the 

nxation   in  tne   tfaiamdae    (fig.   4xJ7)     opercuium  of  scuta  («)  and 
lies  in  the  plane  of  the  head;  in  the    terga  (t)- 
Lepadidae  it  is  drawn  out  into  a  long  muscular  stalk  (fig.   114). 
To  this  attached  life  are  related  all  the  peculiarities  of  structure. 


424  ARTEROPODA. 

It  is  clear  that  a  fixed  animal  has  greater  need  of  protection  than 
one  which  can  flee  from  its  enemies,  therefore  we  find  not  only  a 
right  and  left  mantle  and  a  shell  capable  of  complete  closure, 
like  that  of  an  ostracode,  but  also  in  this  calcified  plates,  the  scuta 
and  terga  (figs.  114,  427,  s,  t),  the  first  cephalic,  the  other  pos- 
terior, in  position. 

Between  the  pairs  of  these  is  the  gap  through  which  the  feet 
are  protruded.  Besides  there  are  other  calcified  portions,  one  of 
which,  the  carina  (fig.  114,  c),  corresponds  to  the  hinge-line  of 
the  ostracode  and  in  some  Lepads  is  supplemented  by  a  farther 
unpaired  piece,  the  rostrum.  In  the  Balanidae  the  rostrum  and 
carina  are  much  stronger,  while  between  them  other  paired  pieces, 
the  lateralia,  are  intercalated.  Lateralia,  rostrum,  and  carina  arise 
from  a  base  (usually  calcareous)  and  form  a  capsule,  closed  above 
by  a  double  valve  formed  of  the  paired  scuta  and  terga,  between 
which,  when  open,  the  animal  can  be  seen  (fig.  427). 

The  body  in  both  lepads  and  balanids  has  essentially  the  same 
structure.  It  is  flexed  ventrally,  so  that  mouth  and  vent  are  near 
each  other,  and  bears  six  pairs  of  feathered  feet,  or  cirri,  which, 
when  extended,  become  widely  separated  and  form  a  most  efficient 
means  of  straining  small  organisms  from  the  water  and  conveying 
them  to  the  mouth.  These  feet  are  biramous,  with  their  branches 
ringed  and  thickly  haired.  Behind  them  is  a  rudimentary  abdo- 
men and  an  elongate  penis;  while  the  mouth  is  surrounded  by  a 
pair  of  mandibles  and  two  pairs  of  maxillae. 

In  internal  structure  the  most  noticeable  feature  is  that  the 
animals,  in  contrast  to  almost  all  other  arthropods,  are  hermaphro- 
ditic, a  condition  possibly  correlated  with  their  sedentary  life  and 
the  consequent  need  of  self -impregnation.  Yet  it  is  to  be  remem- 
bered that  the  common  forms  have  a  long 
penis,  so  that  these  animals,  crowded 
closely  together,  can  fertilize  each  other. 
In  cases  of  several  species  which  live 
solitary  complementary  males  occur. 
These  are  very  small,  purely  male  forms, 
with  extremely  simple  structure  (fig. 
428),  which  live  inside  the  mantle  cavity 
near  the  genital  openings.  The  un- 

Se^mented    ^°^   ls    enclosed    in    a    sac   (a 

lobes;  m ,  muscles ;oc,  ocellus:  soft-skinned  shell),  and  anchored  by  the 

p,  penis;  t,  testis;  vs,  seminal  n 

vesicle.  antennas.     The  long  penis  protrudes  from 

the  mantle.     In  the  genus  Scalpellum  there  are  purely  hermaph- 


/.    CRUSTACEA:   CIRRIPEDIA,  LEPADID^S,  BALANID^E. 


roditic  species,  hermaphroditic  species  with  complemental  males, 
and  purely  dioecious  species. 

Since  the  hard  shells  of  the  barnacles  resemble  those  of  the  molluscs,  it 
is  not  to  be  wondered  that  these  forms  were  long  regarded  as  belonging  to 
that  group.  It  was  not  until  the  development  (fig.  429)  was  studied  that 


oe 


FIG.  429.— Nauplius  (A)  and  Cypris  (B)  stages  of  Sacculina  carcini.  (After  Delage.) 
1,  2,  antennae ;  5,  mandible  ;  /,  cirrhous  foot ;  m,  muscles ;  oc,  nauplius  eye  ;  ou, 
anlage  of  ovary. 

the  error  was  corrected.  A  large  nauplius  comes  from  the  egg  and  later 
is  metamorphosed  into  a  second  larval  stage  with  bivalve  shell  which, 
from  its  appearance,  is  called  the  cypris-stage.  This  becomes  fixed  and 
develops  into  the  adult,  losing  the  compound  eyes  and  retaining  the  nau- 
plius eye. 

Order  I.  Lepadidae. 

Stalked  cirripeds,  with  shell  largely  formed  of  scuta,  terga,  and  carina ; 
other  parts  may  be  added.  Lepas  anatifera* 
(fig.  114)  is  the  goose  barnacle,  which  owes 
its  common  name  to  a  mediaeval  myth 
which  claimed  that  the  Irish  (or  bernicle) 
goose  developed  from  these  animals.  L. 
fascicularis,*(tig.  430),  abundant  floating  on 
the  eastern  coast.  Anelasma  squalicola,  a 
thin-skinned  barnacle,  is  parasitic  on  sharks 
and  forms  a  transition  to  the  Rhizocephala. 
Order  II.  Balanidae. 

Sessile  cirripeds  with  calcareous  shell 
formed  of  carina,  rostrum,  and  lateralia; 
scuta  and  terga  forming  the  valves  (fig.  427). 
Balanus  ~balanoides*  common  on  our  coast. 


Coronula  dwdemata,  attached  to  the  skin  FlG 
of  whales. 


fascicularis,* 
goose  barnacle.    (From  Smith.) 


426  ARTHROPOD  A. 


Order  III.  Rhizocephala. 

These  forms  differ  so  much  from  the  other  cirripeds  as  to  demand  sepa- 
rate mention.  They  are  parasitic  on  the  abdomens  of  various  decapod 
crabs  and  consist  of  a  stalk  which  penetrates  the  body  of  the  host  and  a 
body  which  remains  outside.  The  stalk,  which  branches  in  a  root-like  man- 


Fio.  431.— Sacculina  carcini  attached  to  Carcinus  mcenas,  whose  abdomen  is  extended, 
ru,  shell  opening;  r,  network  of  roots  ramifying  the  crab;  s,  stalk  ;  a,  o,  d,  anten- 
nula,  eye  and  anus  of  the  crab. 

ner,  penetrates  the  cephalothorax  and  absorbs  its  juices.  Since  the  stalk 
furnishes  the  food,  an  alimentary  canal  is  absent.  The  body  lacks  all  ap- 
pendages, is  enclosed  by  a  soft-skinned  mantle,  and  is  almost  entirely 
filled  with  the  gonads.  Since  these  forms  lack,  as  adults,  all  arthropodan 
features,  their  position  is  only  settled  by  their  development,  which  shows 
(fig.  429)  no  great  difference  from  that  of  other  cirripeds.  These  forms 
are  rare  on  the  American  coast.  Sacculina,  Peltogaster* 

Two  more  orders,  ABDOMINALIA  and  APODA,  parasitic  in  the  mantle 
and  shells  of  molluscs  and  other  cirripeds,  scarcely  need  mention. 

Sub  Class  V.  Malacostraca. 

The  Malacostraca  are  sharply  marked  off  from  the  other  Crus- 
tacea by  having  a  body  which  consists  of  twenty  segments,  of  which 
seven  are  abdominal  (Nebalia  has  twenty-one,  eight  abdominal). 
The  excretory  organs  are  represented  by  the  antennal  glands,  and 
shell  glands  are  lacking  except  in  some  Isopoda.  The  male  geni- 
tal ducts  open  on  the  thirteenth,  the  female  on  the  eleventh, 
segment. 


/.   CRUSTACEA:  LEPTOSTRACA.  427 

Legion  I.   Leptostraca. 

The  Leptostraca  connect  the  Phyllopoda  with  the  higher 
groups.  They  have  twenty-one  somites,  eight  abdominal,  eight 
thoracic,  and  five  cephalic,  and  this  and  the  openings  of  the  genital 
ducts  ally  them  to  the  Malacostraca.  On  the  other  hand  the 
bivalve  carapace  covering  the  cephalothorax  and  part  of  the  abdo- 
men, and  the  leaf-like  thoracic  feet,  are  phyllopodan.  They  have 
an  antennal  gland  and  a  rudimentary  shell  gland ;  an  elongate  heart 
which  extends  through  cephalothorax  and  abdomen;  and  com- 


Fie.  432.— Nebalia  Znpes.*    (After  Sars.)    7t,  heart;  i,  intestine;  o,  ovary;  a,  adductor 
of  carapace  ;  b,  brain  ;  r,  rostrum. 

pound  stalked  eyes.     The  few  species  are  all  marine  and  belong  to 
the  genus  Nebalia.     N.  Mpes  *  (fig.  432). 

Legion  II.    Thoracostraca  (Podophthalmia). 

The  names  given  this  division  have  reference,  first,  to  the  fact 
that  the  head  and  more  or  fewer  of  the  thoracic  segments  are  united 
into  an  immovable  part,  covered  by  a  firm  carapace;  second,  that 
the  compound  eyes  (except  in  Cumacea)  are  placed  at  the  ends  of 
movable  eye  stalks,  a  condition  which  has  possibly  arisen  from 
the  inflexibility  of  the  anterior  part  of  the  body.  The  first  five 
appendages  are  always  two  pairs  of  antennae,  a  pair  of  mandibles, 
and  two  pairs  of  maxillae.  The  remaining  pairs  vary  greatly  in 
character  and  from  one  to  three  may  be  modified  into  maxillipeds, 
while  the  abdominal  somites  except  the  last  (telson)  usually  bear 
appendages,  at  least  in  the  female.  There  is  usually  a  metamor- 
phosis in  development  in  which  a  nauplius-stage  may  appear,  most 
frequently  in  the  lower  forms  (schizopods),  but  even  in  the  deca- 
pods (Peneus). 


428 


ARTEROPODA. 


Order  I.  Schizopoda. 

These  are  small  forms,  mostly  marine,  in  which  the  cephalo- 
thorax  is  covered  by  a  carapace  with  which  some  or  all  of  the 


FIG.  433.— Amphithoe.      (From  Gerstacker.)   a",  a2,  first  and  second  antennae ;  aw,  eye; 
VlI-XIIIi  the  seven  free  thoracic  segments  ;  1-7,  abdominal  segments. 

thoracic  somites  are  firmly  united.  The  eight  thoracic  feet  retain 
throughout  life  a  biramous  condition  and  are  used  in  swimming. 
The  posterior  pair  of  abdominal  feet  together  with  the  telson  form 


FIG.  434.— Mi/sis  elonqata.    (From  Gerstacker.)    a,  /3,  first  and  second  antennae;  a,  ex- 
pedite; cm,  eye;  «,  endopodite;  o,  otocyst ;  1-7,  abdominal  somites. 

a  caudal  '  fin '  by  means  of  which  the  animal  can  swim  backwards. 
The  delicate  skin  permits  of  diffuse  respiration,  and  gills  are  fre- 
quently lacking.  In  some  genera  plates  from  the  legs  of  the  female 
enclose  a  brood  case  beneath  the  cephalothorax,  thus  giving  these 
forms  the  common  name  of  opossum  shrimps. 


I.    CRUSTACEA:  STOMATOPODA.  429 

The  MYSIDID.E  are  the  most  widely  distributed,  several  species  of  My  sis 
(fig.  434)  occurring  on  our  coasts  and  one  in  the  Great  Lakes.  In  these  the 
endopodite  of  the  sixth  abdominal  appendage  contains  an  otocyst,  with  a 
calcic  fluoride  otolith.  Other  families  are  the  EUPHAUSIID^E  and  LOPHO- 
GASTRID.E  of  the  deeper  seas. 

Order  II.  Stomatopoda. 

In  structure  of  the  cephalothorax  these  forms,  known  as  mantis 
shrimps  (from  a  resemblance  to  the  insect,  the  praying  mantis), 
have  not  advanced  as  far  as  the  schizopods,  since  the  last  three 
thoracic  somites  remain  free  and  are  not  covered  by  the  carapace. 


FIG.  435.— Squilla  mantis,  at,  at',  first  and  second  antennae ;  /,  sixth  abdominal  feet; 
fc,  gills ;  p,  schizopodal  thoracic  feet ;  pr,  pr',  raptorial  feet ;  ps,  pleopoda ;  sa< 
telson. 

The  appendages,  however,  are  more  differentiated,  since  only  the 
three  posterior  thoracic  feet  are  biramous  and  natatory.  The 
four  in  front  of  these  are  prehensile  and  bear  a  pincer  formed  of 
the  last  two  joints,  the  last  being  slender  and  usually  toothed  and 
closing  in  a  groove  of  the  penult  joint  like  a  knife  blade  in  the 
handle.  The  first  of  these  raptorial  feet  are  the  largest  and  are 
used  in  capturing  fishes,  etc.  Since  the  thoracic  feet  are  of  little 
service  for  locomotion,  the  abdomen  is  long  and  stout,  especially 
the  caudal  fin.  The  five  anterior  abdominal  feet  bear  the  gills,  and 
correspondingly  the  elongate  heart  with  many  ostia  extends  into 
the  abdomen.  The  transparent  pelagic  larvae  were  formerly  re- 
garded as  adults  and  described  as  Alima  and  Erichthus.  Squilla 
empusa  lives  on  our  east  coast,  Gonodactylus  in  Florida.  They  are 
burrowing  animals  and  deposit  their  eggs  in  their  holes. 

Order  III.  Decapoda. 

The  Decapoda  is  the  most  important  group  of  Crustacea,  since 
it  contains  the  shrimps,  lobsters,  crayfish,  and  crabs.  It  agrees 
with  the  Schizopoda  in  having  a  cephalothorax  composed  of  thirteen 
fused  somites,  but  differs  in  the  structure  and  function  of  the 
thoracic  extremities.  Only  the  last  five  pairs  (whence  the  name 
Decapoda)  are  locomotor.  These  lose  the  exopodite  during  de- 


430 


ARTHROPODA. 


velopment  and  become  strong  walking  legs,  terminated  either  with 
claws  or  pincers  (chelae).  Usually  the  first  pair  is  distinguished 
from  the  others  by  its  size  and  by  being  chelate,  and  becomes  not 
locomotor  but  grasping  in  function.  In  the  development  of  a 
chela  the  penult  joint  sends  out  a  strong  process,  the  ( thumb/ 


FIG.  436.— Erichthus  stage  of  Squilla  (orig.). 

which  extends  as  far  as  the  last  joint  (the  e  finger '),  which  closes 
against  it. 

The  mouth  parts — a  pair  of  mandibles,  two  pairs  of  maxillae,  and  three 
pairs  of  maxillipeds  (fig.  404)— lie  in  front  of  the  first  pair  of  legs.  The 
maxillipeds  (7,  6,  5)show  clearly  a  biramous  condition,  while  the  maxillae 
(4,  3)  retain  considerable  of  the  original  phyllopod  character.  In  the  man- 
dibles (2)  there  is  always  a  strong  basal  joint,  the  edge  of  which  serves  as  a 
jaw,  while  this  may  bear  additional  joints,  the  palpus.  Behind  the  mouth 
are  a  pair  of  scales,  the  paragnaths  or  metastoma,  formerly  regarded  as 
appendages.  The  antennae  are  usually  distinguished  from  their  size  as 
antennae  (second  pair)  and  antennulce  (first  pair,  fig.  404).  They  have  large 
basal  portions,  which  in  the  antennulae  bear  two  many-jointed  flagella, 


/.    CRUSTACEA:  DEC  APOD  A.  431 

while  the  antennae  proper  have  but  a  single  though  usually  much  larger 
flagellum.  On  the  basal  joint  of  the  antennulae  is  the  auditory  organ 
(p.  412),  while  the  green  gland  opens  on  the  basal  joint  of  the  antennae 
^flg.  439,  gd). 

When  the  abdomen  is  not  rudimentary  (as  in  the  crabs)  the  appendages 
of  the  sixth  abdominal  segment  together  with  the  telson  form  a  strong 
caudal  fin  (fig.  439);  the  other  appendages  (fig.  404,  7)  are  small,  bira- 
mous  organs  to  which,  in  the  female,  the  eggs  are  attached.  In  the  female 
the  first  pair  is  reduced,  but  in  the  male  except  in  Palinuridae  this  pair  is 
well  developed,  curiously  modified,  and  serves  as  a  copulatory  (introm it- 
tent)  organ.  The  condition  of  these  appendages  as  well  as  the  openings  of 
the  genital  ducts — on  the  base  of  the  third  walking  foot  in  the  female, 
the  fifth  in  the  male— serve  at  once  to  distinguish  the  sexes.  Frequently 
also  the  males  have  the  larger  pincers. 

The  thickness  of  the  integument  prevents  diffuse  respiration 
and  accounts  for  the  numerous  gills  (fig.  437)  which  are  attached 


pdb.Q  pdfc.13 


FIG  437.— Gills  of  Astacus  exposed  by  cutting  away  the  branchiostegite.  pdb,  plb. 
podo-  and  pleurobranchia  of  the  corresponding  segments;  r,  rostrum;  1,  stalked 
eyes;  £,  3,  antennae ;  A-e,  mandibles  and  maxillae;  7-9,  maxillipeds;  10,  #,  bases  of 
thoracic  feet;  15,  first  pleopod. 

to  the  bases  of  the  appendages  (maxillipeds  and  walking  feet)  or 
to  the  sides  of  the  body  near  them.  (In  the  Thalassinidse — forms 
near  the  Astacidae — the  gills  are  on  the  abdominal  appendages). 
These  gills  are  not  visible  externally,  for  the  carapace  extends 
down  on  the  sides  of  the  body  as  a  fold  (branchiostegite)  over 
them,  thus  enclosing  them  in  a  branchial  chamber.  A  process  of 
the  second  maxillae — the  scapliognathite — plays  in  this  branchial 
chamber  and  pumps  the  water  over  the  gills,  the  water  flowing  out 
near  the  mouth.  All  decapods  can  live  some  time  out  of  water,  a 
fact  readily  explained  when  we  remember  that  they  retain  some 
water  in  the  gill  chamber,  which  keeps  the  gills  in  a  moist  con- 
dition. In  some  of  the  tropical  land  crabs  which  live  almost  ex- 
clusively on  land  there  is  a  true  aerial  respiration,  the  lining  of 
the  gill  chamber  becoming  modified  into  a  kind  of  lung  traversed 


432 


ARTHROPODA. 


by  numerous  blood-vessels.  In  Birgus  latro  the  gill  chamber  is 
divided  into  two  portions  (fig.  438),  the  upper  part  being  pulmo- 
nary, the  lower  containing  the  reduced  gills. 


FIG.  438.— Diagrammatic  section  through  Birgus  latro,  showing  lungs.  (From  Lang, 
after  Semper.)  a,,  a4,  afferent  blood-vessels :  ah,  pulmonary  chamber;  ek,  el,  el'% 
efferent  blood-vessels ;  ft,  heart;  k,  gills ;  kd,  branchiostegite ;  p,  pericardium. 

C 


bl... 


FIG.  439.— Anatomy  of  Crayfish  (Astacus).  A,  dorsal  surface  removed ;  B,  scheme 
of  circulation  ;  C,  viscera  removed,  showing  green  gland  and  nervous  system.  a, 
anus ;  oa,  hepatic  artery ;  ae,  antenna ;  az,  antennula,  also  sternal  artery ;  am, 
muscles  of  stomach ;  «o,  ophthalmic  artery  ;  op,  abdominal  artery  ;  av,  ventral 
artery;  bl,  urinary  bladder;  ftr,  gill  arteries;  c,  oesophageal  commissures;  gd, 
green  gland;  0n',  brain  ;  .qr?i2-13,  ganglia  of  ventral  chain;  ft,  heart;  hd,  intes- 
tine ;  fc,  mandibular  muscles ;  I,  l\  liver  and  its  duct ;  w,  stomach ;  o,  otocyst ; 
oes,  oesophagus  ;  on,  optic  nerve ;  pc,  pericardium  ;  sgn,  sympathetic  nerve  ;  t,  t', 
unpaired  and  paired  portions  of  testes ;  t?,  ventral  blood  sinus ;  vd,  vas  deferens ; 
vdr,  vems  from  gills  to  heart. 


/.    CRUSTACEA:  DEC  APOD  A. 


433 


Correlated  to  this  localized  respiration  is  the  nearly  closed  cir- 
culatory system  (figs.  439,  A,  B).  The  heart  (/*),  a  compact 
pentagonal  organ,  receives  its  blood  from  the  uericardial  sinus 
(pc)  through  three  pairs  of  ostia,  and  forces  it  out  through  five 
arteries  to  the  capillary  regions  of  the  body.  The  venous  blood 
collects  in  a  large  venous  sinus  at  the  base  of  the  gills  (v),  passes 
thence  through  gills,  and  is  returned  by  several  branchial  veins 
(vbr)  to  the  pericardium. 

The  alimentary  canal  is  straight  and  has  only  one  conspicuous 
enlargement,  the  so-called  stomach  (fig.  439,  A9  m),  divided  into 
two  portions,  an  anterior  sac  (cardiac  pouch),  lined  with  chitinous 
folds  and  teeth  and  serving  to  chew  the  food  and  bearing  in  its 
walls  the  so-called  '  crab-stones/  which  are  masses  of  calcic  carbon- 
ate stored  up  to  harden  the  armor  rapidly  after  the  molt.  The 
second  or  pyloric  portion  of  the  stomach  is  guarded  by  hairs  and 
serves  as  a  strainer,  allowing  only  food  sufficiently  comminuted 
to  pass.  The  two  liver  lobes — voluminous  masses  of  branched 
glandular  tubes  (I)  open  just  behind  the 
stomach. 

The  two  antennal  glands  (fig.  439,  C, 
gel),  each  provided  with  a  large  urinary 
^bladder  ( bl),  are  dirty  green  in  color,  whence 
the  name  green  glands  often  applied  to 
them.  The  gonads  (figs.  440)  lie  close 
beneath  the  heart,  those  of  the  two  sides 


FIG.  440.  FIG.  441. 

FIG.  440.— Reproductive  organs  of  (A)  female  and  (B)  male  crayfish.  (From  Hux- 
ley.) od,  oviduct;  od',  its  opening  on  llth  appendage;  ou,  ovary;  f,  testes;  vd, 
vas  deferens ;  vd',  its  opening  on  13th  appendage 

FIG.  441.— Nervous  system  of  crab,  Carcinus.  (From  Gegenbaur.)  a,  antennal  nerves ; 
c,  cesophageal  commissures;  gi,  fused  ventral  chain  perforated  for  sternal 
artery  ;  gs,  brain  ;  o,  optic  nerve. 


434  ARTHROPODA. 

being  united  behind,  while  their  ducts  remain  separate.  The 
structure  of  the  nervous  system  is  in  part  dependent  upon  that 
of  the  abdomen.  In  the  Macrura  (fig.  439,  C)  the  ventral  chain 
consists  of  six  ganglia  in  the  thorax,  six  in  the  abdomen,  but  in 
the  Brachyura  (fig.  441)  these  all  flow  together  in  a  common  mass, 
connected  with  the  brain  by  two  long  oasophageal  commissures. 

The  development  of  most  decapods  is  interesting  from  the  number  of 
larval  forms.  As  a  rule  a  zoea  (fig.  415)  is  hatched  from  the  egg  ;  this 
passes  next  into  a  Mysis-stage  (fig.  442)  in  which  head,  thorax,  and  abdo- 
men are  distinct,  the  thorax  bearing  biramous  feet  like  those  of  schizo- 
pods— a  proof  of  the  origin  of  the  simple  feet  from  the  biramous  type. 
In  the  crabs  (Brachyura)  the  Mysis-stage  is  replaced  by  a  Megalops  (fig. 
443),  in  which  the  abdomen  is  well  developed  but  the  feet  have  lost  their 


FIG.  442.  FIG.  443. 

FIG.  442.—  Phyllosoma  larva  (Mysis-stage)  of  Palinurus.  (After  Gerstacker.)  A,  ab- 
domen ;  C',  head :  T,  thorax ;  a  and  i,  exopodites  and  endopodites  of  thoracic  feet. 

FIG.  443.— Megalops  larva  of  Portunus.  (From  Lang,  after  Glaus.)  2,  antennae ;  IV- 
VIIi\  thoracic  appendages  ;  a2-a6,  abdominal  somites  (a*  is  the  seventh). 

biramous  character.  In  some  prawns  (Peneus)  the  series  is  rendered  more 
complete  by  the  appearance  of  a  nauplius  and  a  metanauplius  with  many 
appendages,  before  the  zoeal  stage.  In  the  crayfish  and  many  land  crabs 
the  metamorphosis  has  been  lost,  but  the  lobster  leaves  the  egg  in  the 
Mysis-stage.  Differences  may  occur  even  in  the  same  species;  thus  in  the 
European  Palcemonetes  varians  the  embryo,  in  the  sea,  leaves  the  egg  as  a 
zoea  ;  in  fresh  water  in  the  Mysis-stage. 

Sub  Order  I.  MACRURA.  Abdomen  well  developed  ;  antennae  long  ; 
ventral  nerve  chain  elongate ;  no  megalops-stage  in  development. 
CARIDE A.  Body  compressed ;  no  sutures  on  carapace ;  feet  weak,  ex- 
ternal maxillipeds  pediform;  a  large  scale  on  the  second  antennae.  In  the 
PENEID^E  there  are  weak  exopodites.  Peneus*  Sicyonia*  PAL^EMO- 
NID^E,  mandibles  bifid  at  tip.  Palcemon,  Alpheus,*  Hippolyte,*  Panda- 
lus.*  In  the  CRANGONID.E  the  mandible  is  simple.  Crangon*  Sabinea* 


/.    CRUSTACEA:  DEC  APOD  A.  435 

ASTACOIDEA.    Carapace  crossed  by  a  transverse  groove.     The  ASTACID.E 
have  well-developed  chelae.     Cambarus*  includes  the  crayfish   of   the 


A  B 

FIG.  444.— A,  Crangon  vulgaris  *;  B,  Pandulus  montagui.* 


FIG.  445.— Eupagurus  bernhardus,  hermit  crab.    (From  Emerton.) 
eastern  states;  those  of  the  Pacific  coast  and  Europe  belong  to  Astacus* 
The  lobsters  belong  to  Homarus*    PALINURID^J   (Loricata),  no  chelae, 


436 


ARTHROPODA. 


body  with  heavy  armor;  larva  leaf -like  and  transparent  'glass  crabs/ 
culled  Phyllosomae  (fig.  442).  Palinurus*  spiny  lobster.  PAGURIDEA, 
hermit  crabs;  abdomen  reduced,  soft-skinned,  and  hidden  for  protection 
in  a  snail  shell  which  the  animal  carries  about,  which  habit  has  resulted 


FIG.  446.— A,  Platyonichus  ocellatus,*  lady  crab ;  B,  Libinia  emarginata*  spider  crab 

(From  Emerton.) 

in  a  spiral  twisting  of  the  abdomen.  Some  hermits  (Eupagurus)  carry 
sea  anemones  or  hydroids  on  their  shell,  cases  of  symbiosis  (p.  170). 
Eupagurus*  Clibanarius*  Allied  is  Birgus,  the  palm  crab  of  the  East 


/.    CRUSTACEA:   CUMACEA.  437 

Indies,  which  is  said  to  climb  palm  trees  for  the  cocoanuts,  which  it  eats. 
Its  respiratory  organs  have  been  referred  to  on  p.  432. 

Sub  Order  II.  BRACHYURA.  Body  depressed;  abdomen  rudimentary 
and  folded  in  a  groove  under  the  cephalothorax;  antenna  short;  never 
more  than  one  pair  of  feet  chelate;  ventral  nerve  cord  concentrated  (fig. 
441).  Omitting  some  inconspicuous  groups  like  the  porcellain  crabs  (PoR- 
CELLANIDJ2),  the  HiPPiD^:,  and  the  LITHODID.E,  which  are  united  as  a  group 
of  Schizosomi  from  the  fact  that  the  last  thoracic  segment  is  free  from  the 
carapace  and  its  appendages  are  rudimentary,  the  sub  order  is  usually 
divided  as  follows:  LEUCOSOIDEA  (Oxystomata).  Body  oval  or  triangular, 
area  of  mouth  parts  triangular,  the  apex  anterior.  Calappa,  Matuta* 
Hepatus  *  of  warmer  seas.  OXYRHYNCHA  (Maioidea).  Cephalothorax 
triangular,  narrowed  in  front;  mouth  area  (as  in  the  following  tribes) 
quadrilateral.  Mostly  tropical.  Hyas,*  Libinia*  Pugettia*  spider  crabs. 
CYCLOMETOPA.  Body  broader  than  long,  regularly  arcuate  in  front. 
CANCRID.E,  with  last  pair  of  feet  pointed.  Cancer*  shore  crab;  Pano- 
peus,*  mud  crab.  PORTUNID^E,  with  last  pair  of  feet  flattened  paddles. 
Platyonichus  *;  Neptunus  liastatus*  when  thin-skinned  after  molting,  is  the 
'soft-shell  crab'  of  the  markets.  CATOMETOPA.  Front  of  carapace 
nearly  straight;  body  from  above  nearly  quadrilateral;  Gelasimus*  the 
fiddler  crabs  of  our  warm  shores;  Pinnotheres  ostreum*  common  in 
oysters;  GECARCINID^E  (Z7ca,  etc.),  land  crabs  of  the  tropics,  which  only  go, 
to  the  sea  at  the  reproductive  season  to  lay  their  eggs. 

Order  IV.  Cumacea. 

Small  marine  forms  with  sessile  eyes,  three  or  four  free  thoracic  somites; 
appendages  biramous;  a  brood  sac  beneath  the  cephalothorax.  Of  interest 
because  combining  arthrostracan  and  thoracostracan  features.  Diastylis 

(fig.  447). 


FiG.  447.— Diastylis  quadrispinosus. 

Especial  interest  also  centres  in  the  little  known  Anaspides  tasmanice 
from  lakes  in  Tasmania,  which  unites  schizopod  and  amphipod  characters. 
It  has  the  stalked  eyes,  caudal  fin,  and  biramous  feet  of  a  schizopod; 
otocysts  in  the  antennulge  like  a  decapod;  but  agrees  with  the  amphipods 
in  shape  of  body  and  in  free  thoracic  segments.  The  epipodial  plates 
are  paralleled  elsewhere  only  in  carboniferous  species,  with  which  these 
forms  apparently  are  closely  allied. 


438 


ARTHROPODA. 


Legion  III.  Arthrostraca  (EdriopUthalmata). 

Although  the  head  of  the  Arthrostracan  consists  of  six  seg- 
ments, it  is  remarkably  short.  It  bears  six  pairs  of  appendages, 
one  of  the  normal  thoracic  pair  being  added  to  it  as  maxillipeds. 
Eyes,  when  present,  are  aggregates  of  ocelli  situated  on  the  sides 
of  the  head.  There  are  seven  thoracic  segments,  the  appendages 
of  which  are  walking  feet  which  lack  exopodites.  The  abdominal 
appendages,  when  present,  are  always  biramous,  the  telson  never 
bearing  appendages,  and  in  the  Amphipods  is  greatly  reduced, 
sometimes  being  split  nearly  its  whole  length. 

The  nervous  system  (figs.  75,  448)  is  of  the  ladder  type.  The 
alimentary  canal  is  straight  and  has  an  anterior  enlargement,  the 


FIG.  448.— Male  Orchestia  cavimana.  (After  Nebeski.)  a',  a2,  antennae  ;  ao,  aop, 
anterior  and  posterior  aortse ;  c,  heart ;  d,  digestive  tract ;  0,  brain  and  eye ;  ft, 
testes  ;  /c,  gills;  7c/,  maxilliped ;  Z,  liver;  ra,  excretory  organ ;  ?i,  ventral  nerve  cord; 
o,  rudimentary  ovary;  vd,  vas  deferens ;  I-  VII,  thoracic  feet;  1-3.  anterior,  A-6', 
posterior  abdominal  feet. 

chewing  stomach,  behind  which  empty  one  or  more  pairs  of  long 
liver  tubes,  while  in  a  few  Amphipods  a  pair  of  excretory  tubes, 
the  so-called  Malpighian  tubules,  empty  into  the  intestine  near  its 
end.  Respiratory  and  circulatory  systems  vary  so  that  they  are 
best  described  in  connexion  with  the  two  orders. 

Order  I.  Amphipoda. 

The  Amphipods  are  almost  exclusively  aquatic,  a  few  species 
living  on  the  shore  near  high-tide  mark.  A  few  live  in  fresh 
water  (Gammarus,  Allorchestes),  the  majority  being  marine.  On 
land  they  move  by  a  leaping  motion,  whence  the  common  name, 


L    CRUSTACEA:  AMPHIPODA. 


439 


beach  fleas.     In  swimming  the  abdomen  is  alternately  bent  against 
the  breast  and  then  forcibly  straightened. 

The  body  is  usually  strongly  compressed  from  side  to  side. 
The  thoracic  feet  generally  bear  large  epineural  plates  (fig.  433), 
which  extend  the  sides  of  the  body 
downward,  while  on  the  inner  side 
delicate  gills  or  gill  sacs  (fig.  449, 
br)  arise  from  their  bases.  In  the 
female  brood  lamellae  (brl)  are 
added — broad  chitinous  plates 
which  enclose  a  brood  chamber 
beneath  the  body  in  which  eggs  or 
young  are  carried.  'The  three  an- 
terior pairs  of  abdominal  feet  are 
two-branched,  richly  haired,  and 
serve  to  create  currents  of  water  Fl«-  449  -Cross-section  of  Amphipod 

(Corophium).    (From  Lang,  after  De- 

which  pass  forward  over  the  gills. 

The      remaining   abdominal   feet, 

though   biramous,  are   short   and 

stout   and   form  springing   organs 

explains  why  the  abdominal  part  of  the  heart  is  degenerate  and 

only  the  anterior  thoracic  portion  with  three  pairs  of  ostia  persists. 


thoracic  leg;  bm,  ventral 
chtae;  brl,  brood 


lage.)    b/, 

nerve  cord;  br,  branc 

lamella;  d<  intestine;  7i,  heart;  I,  liver: 

ov,  eggs  in  brood  chamber. 


The   position   of   the   gills 


Sub  Order  I.  HYPERINA.  Large  head  and  eyes;  strong  prehensile  feet. 
Live  attached  to  other  pelagic  animals  on  which  they  feed.  Hyperia 
medusarum  *  lives  on  the  jelly  fish  Cyanea;  Plironima,*  warmer  seas. 

Sub  Order  II.  GAMMARINA.     Head  much  smaller;  abdomen  well  devel- 
oped; are  mostly  free  swimmers.     Numerous  species  in  the  sea.     Cfam- 


Fig.  450.— Gammarus  ornatus.*    (From  Smith.) 

marus  *  occurs  in  shallow  water,  some  being  fluviatile;  Orchestia  *  above 

tide  marks.     Chelura  terebrans  *  destroys  piles  and  other  submerged  wood. 

Sub  Order  III.  L^EMODIPODA.     Parasitic  or  semi-parasitic  forms  in 

which  the  first  (second)  somite  is  fused  to  the  head;  appendages  are  lacking 


440 


ARTHROPODA. 


from  some  of  the  thoracic  segments  and  the  abdomen  is  reduced.    Species 
of  Caprella*  are  common  on  hydroids.     Cyamus  ceti  is  parasitic  on  whales. 

Order  II.  Isopoda. 

The  Isopoda  are  readily  distinguished  from  the  Amphipoda  by 
their  depressed  (i.e.  horizontally  flattened)  bodies.  The  feet  are 
adapted  for  creeping,  and  a  brood  pouch  is  formed  as  in  the  Am- 
phipoda, but  gills  are  lacking  here  since  some  of  the  abdominal 
feet  are  modified  for  respiration  (fig.  451,  k).  In  the  abdomen, 
the  somites  of  which  exhibit  a  great  tendency  to  fusion,  the  telson, 
as  in  all  Malacostraca,  is  without  appendages;  the  sixth  somite 


FIG.  451. 


FIG.  452. 


FIG.  451.— Asellus  aquaticus.  (From  Ludwig-Leunis.)  a1,  a2,  antennae;  />r,  brood 
pouch;  fc,  pleopoda  modified  to  gills;  md,  mandibles-  p1-/)7,  thoracic  feet; 
paHxt'i  abdominal  feet  (pleopoda);  I-V1,  head;  VII-XIII,  thoracic  segments; 
XIV-  XX,  abdominal  segments,  partly  fused. 

FIG.  152,—Cymothoa  emarginata.    (After  Gerstacker.)    p8,  sixth  pleopod. 

bears,  in  the  walking  forms,  long  forked  appendages  (fig.  451);  in 
the  swimming  species  (fig.  452)  they  are  flattened  and,  with  the 
telson,  make  a  swimming  organ.  The  five  anterior  pairs  of  pleo- 
poda are  modified  for  respiration,  by  the  expansion  of  the  endop- 
odites  into  thin-walled  plates,  while  the  exopodites  and  the  whole 
first  pair  serve  as  opercula  or  gill  covers.  As  a  result  of  this  posi- 
tion of  the  gills  the  heart  (usually  with  two  pairs  of  ostia)  is  ab- 
dominal in  position. 

In  the  terrestrial  species  the  gills  are  adapted  for  breathing  damp  air. 
In  Porcellio  and  Armadillidum  the  first  or  first  and  second  opercula  are 
permeated  with  a  system  of  air  tubes,  which  physiologically,  though  not 
morphologically,  are  comparable  to  the  tracheae  of  insects. 

In  the  Isopoda  the  tendency  to  parasitism  is  greater  than  in  the 
Amphipoda.  Many  swimming  forms  attach  themselves  to  fishes  and 
feed  by  boring  with  their  mouth  parts,  which  are  modified  for  the  purpose, 


/.    CRUSTACEA:  1SOPODA. 


441 


into  the  skin.  The  Bopyridae  live  in  the  branchial  chamber  of  shrimps. 
Cryptoniscus  is  a  shapeless  sac  which  attaches  itself  to  the  stalk  of  Sacoii- 
lina  (p.  426),  and,  after  causing  the  death  of  this  parasite,  uses  its  network 
of  « roots '  for  its  own  nourishment.  The  Entoniscidae  (fig.  453)  attack 


FIG.  453.— Entoniscus  porcellance.  (From  Gerstacker,  after  Miiller.)  A,  male;  2?,  female; 
C,  heart;  he,  liver;  la,  brood  lamellae;  ov,  ovary. 


FIG.  454.— J,  Idotea  irrorata  *;  B,  Limnoria  lignorum  *;  <7,  ^ga  psora  *  ('  salve  bug  '); 
D,  Leptochela  algivola*    (After  Harger.) 

Decapoda  and,  pressing  the  skin  before  them,  penetrate  the  interior.    Their 
strange  shape  is  largely  due  to  the  lobe-like  brood  lamellae.     They  are 


442  ARTHROPODA. 

usually  hermaphroditic,  but  have  besides  complemental  dwarf  males  (fig. 
453,  A). 

Sub  Order  I.  ANISOPODA.  Six  free  thoracic  segments;  heart  tho- 
racic; first  thoracic  foot  (on  head)  chelate;  abdomen  with  swimming  feet. 
A  group  intermediate  between  Amphipoda  and  other  Isopoda.  Tanais,* 
Leptochela  *  (fig.  454). 

Sub  Order  II.  EUISOPODA.  Seven  free  thoracic  segments.  ONISCID.E; 
terrestrial,  familiarly  known  as  sow  bugs;  Ligia,  on  seashore;  Porcellio* 
Oniscus*  ArmadilUdum*  'pill  bug.'  ASELLID.E  (fig.  451),  fresh  water. 
SPILEROMID.E,  head  broad,  body  rounded  and  convex;  Sphwroma*  Lim- 
noria  lignorum  *  (fig.  454),  the  gribble,  attacks  submerged  wood  and  is 
nearly  as  destructive  as  Teredo.  IDOTEIDJE,  free-living,  marine,  with  usually 
elongate  bodies;  Idotea,*  Ccecidotea*  BoPYRnxE,  parasitic  on  Caridea; 
body  of  female  disc-like,  asymmetrical,  without  eyes;  Bopyrus*  CYMO- 
THOID^E,  parasitic  on  fishes  or  in  their  mouths.  Cymothoa*  Mga* 
Cirolana* 

Sub  Order  III,  ENTONISCIDA,  parasites  whose  general  features  are 
described  above.  Entoniscus,  Cryptoniscus. 

Class  II.  Acerata. 

The  animals  comprising  this  group  were  formerly  divided 
among  the  tracheates  (p.  408)  and  the  Crustacea,  but  more  recent 
studies  show  that,  although  differing  widely  in  respiration,  the 
forms  included  are  closely  allied  in  structure  and  development  and 
present  many  differences  from  both  Crustacea  and  from  other 
tracheates  (Insecta).  The  former  views  were  based  upon  a  con- 
fusion between  analogy  and  homology,  it  being  thought  that 
tracheae  wherever  found  were  homologous  structures. 

In  the  Acerata  the  body  is  usually  divided  into  two  regions, 
cephalothorax  and  abdomen,  though  in  some  cases  (mites)  the 
two  regions  become  fused.  The  cephalothorax  consists  of  six 
somites  which  always  bear  appendages,  and  these  appendages  are 
arranged  in  a  circle  around  the  mouth,  the  basal  joints  of  one  or 
more  pairs  frequently  serving  as  jaws.  None  of  these  appendages 
are  like  antennae  (whence  the  name  of  the  group).  The  abdomen 
consists  of  a  varying  number  of  somites,  all  of  which  may  be  free, 
or,  again,  may  be  fused  into  a  common  mass.  These  abdominal 
somites  bear  appendages  in  the  embryo,  but  in  the  adults  (except 
the  Xiphosura)  these  are  usually  lost  or  so  modified  that  their 
existence  is  only  recognized  by  a  study  of  development. 

The  alimentary  canal  is  straight,  without  marked  enlargements, 
and  lacks  a  chewing  stomach.  The  liver  is  large  and  opens  into 
the  intestine  by  two  or  more  pairs  of  ducts.  The  nervous  system 
has  some  or  all  of  its  ventral  ganglia  arranged  in  a  ring  around  the 


//.    ACER  AT  A:    GIGANTOSTRACA. 


443 


oesophagus,  and  in  many  forms  is  enclosed  in  the  ventral  artery. 

Excretory  organs,  in  the  shape  of  neph- 
ridia,  are  frequently  present  and  open  to 
the  exterior  at  the  base  of  the  second  or 
the  fifth  pair  of  appendages.  Malpighian 
tubes  may  occur,  but  these,  unlike  those 
of  other  tracheates,  are  entodermal  in 
origin  and  hence  not  homologous  with 
them. 


FIG.  455.  FIG.  456. 

FIG.  455.— Digestive  tract  of  Ctenida  ccementaria.  (From  Lang,  after  Dug6s.)  a,  ab- 
domen ;  an,  anus;  da,  rff,  diverticula  ('liver')  of  midgut ;  g,  brain;  v6,  rectal 
bladder  (stercoral  pocket) ;  vm,  excretory  tubules. 

FIG.  456.— Lung  book  of  Zilla  cadophyla.  (After  Bertkau.)  a,  a  lung  leaf  separated 
from  the  other  leaves,  6  ;  st,  spiracle. 

The  respiratory  organs  are  either  gills,  lungs,  or  tracheae.  The 
gills  are  borne  on  some  of  the  abdominal  appendages.  The  lungs 
are  sacs  on  the  anterior  abdominal  somites  opening  by  narrow  slits 
(fig.  461)  to  the  exterior.  The  anterior  wall  of  each  lung  sac  is 
made  up  of  thin  plates  arranged  like  the  leaves  of  a  book,  and  em- 
bryology shows  that  these  lung  books  are  gill  books  drawn  into  the 
ventral  surface  of  the  abdomen.  The  tracheae  in  development 
pass  through  a  gill-stage  and  a  lung-stage,  the  tracheal  tubes  being 
outgrowths  of  the  spaces  between  the  lung  leaves  which  penetrate 
all  parts  of  the  body. 

The  reproductive  openings  are  on  the  basal  somite  of  the  abdo- 
men. The  spermatozoa  are  motile.  The  development  is  direct, 
there  being  no  metamorphosis. 

Sub  Class  I.    Gigantostraca. 

Marine  forms  with  gills  on  the  2-6  abdominal  appendages; 
bases  of  five  pairs  of  cephalothoracic  feet  masticatory ;  a  pair  of 
medinn  ocelli  and  a  pair  of  compound  eyes  on  the  cephalothorax. 


444 


ARTHKOPODA. 


Order  I.  Xiphosura. 

Cephalothorax  large  ;  abdomen  terminated  by  a  long  spiniform  telson. 
Limulus  polyphemus  of  our  east  coast,  commonly  known  as  king  crab 


FIG.  457.  FIG.  458. 

Fio.  457.— Limulus  polyphemus.*  horseshoe  crab  (orig.). 
FIG.  458.— Ventral    surface    of  Limulus   moluccanus.    (From   Ludwig-Leunis.) 


chelicerse  ;  2-5,  walking  feet ;  6',  pushing  foot ;  6a,  flabellum ;  7,  genital"  operculum 
8,  gills  (there  should  be  five) ;  P,  base    '     ' 


of  telson. 


or  horseshoe  crab.  Other  species  on  eastern  shore  of  eastern  continent. 
They  burrow  beneath  the  sand  and  mud  of  the  bottom  and  feed  on  worms. 
In  the  spring  they  come  to  the  shore  to  lay  eggs. 

Order  II.  Eurypterida. 

Extinct  Silurian  and  Devonian  forms  with  small  cephalothorax  and 
large  twelve-jointed  abdomen.  The  animals  are  intermediate  between  the 
xiphosures  and  the  scorpions.  Eurypterus;  Pterygotus.  some  species 
seven  feet  long. 

Sub  Class  II.  Arachnida. 

Under  this  name  are  included  a  number  of  orders  of  greater  or 
less  extent  which  can  be  arranged  around  the  spiders,  or  Aranea, 
as  a  centre.  There  is  considerable  modification  of  form,  and  the 
following  account  applies  only  to  the  more  typical  groups.  In 
these  the  cephalothorax  and  abdomen  are  separated  by  a  distinct 
line,  and  since  the  abdominal  appendages  almost  entirely  disappear 
in  the  adult,  the  number  of  somites  can  only  be  ascertained  where 
their  boundaries  are  evident.  The  number  varies  between  six  in 
the  phalangids  and  thirteen  in  the  scorpions. 

The  cephalothorax  is,  except  in  the  Solpugidae,  a  single  piece- 


//.   ACEEATA:  ARACHNIDA.  445 

which  bears  six  pairs  of  appendages;  the  four  posterior  pairs,  con- 
sisting typically  of  seven  joints,  are  locomotor,  so  that  the  posses- 
sion of  eight  legs  is  as  characteristic  for 
an  arachnid  as  ten  for  a  decapod  or  six 
for  a  hexapod.  The  first  pair  of  append- 
ages, the  chelicerce  (fig.  459),  are  preoral, 
the  second,  m  pedipaipi,  beside  that  open- 
ing. The  chelicerae  are  short  and  con- 
sist of  two  or  three  joints,  the  terminal 
joint  either  folding  back  upon  the  other 
or,  pincer-like,  meeting  an  opposable 
thumb.  In  the  spiders  the  last  ioint  or 

n          ...  ,   .     .  .     .    *  ,  FIG.  459.— Mouth  parts  of  Epeira. 

claw  is  forced  into  the  prey,  introducing     i,  cheiicera;  *,  pedipaipi;  p 

»  , ,        ,         -,     .    .    !         palpus;  J,  basal  plate. 

poison   irom   a   sac   in  the  basal  joint. 

The  pedipaipi  are  elongate,  leg-like,  their  basal  joints  often  form- 
ing a  lip,  the  other  joints  forming  the  palpus,  which  may  end  with 
a  claw  or  a  pincer. 

The  question  has  often  been  discussed  as  to  whether  the  chelicerae  are 
the  homologues  of  the  antennas  of  other  arthropods.  The  embryological 
evidence,  which  cannot  be  detailed  here,  is  in  favor  of  their  equivalence  to 
the  second  antenna  of  the  Crustacea,  and  to  the  mandibles  of  insects. 

Since  the  Arachnida  usually  suck  their  food,  the  oesophagus  is 
frequently  widened  to  a  sucking  stomach,  behind  which  comes  the 
true  stomach,  with  which,  as  well  as  with  the  intestine,  a  number 
of  so-called  liver  tubes  may  arise  (fig.  455,  da,  dt).  These  may 
be  restricted  to  the  abdomen  alone,  as  in  the  scorpions.  The 
hinder  part  of  the  intestine  is  often  enlarged  into  a  rectal  vesicle 
(stercoral  pocket),  just  in  front  of  which  the  excretory  tubules  (so- 
called  Malpighian  tubules)  empty.  These  resemble  the  true  Mal- 
pighian  tubes  of  insects  in  function,  but  differ  in  being  entodermal 
in  origin.  Besides  there  also  occur,  coxal  glands  (modified  ne- 
phridia),  of  which  only  one  pair  comes  to  development,  and  this 
may  lose  its  external  opening  on  the  base  of  the  appendage. 

The  oesophagus  is  always  closely  surrounded  by  a  nerve  ring 
composed  of  brain  above  and  of  part  of  the  ventral  chain  on  the 
sides  and  below,  the  thoracic  and  more  or  fewer  of  the  abdominal 
ganglia  entering  into  its  composition  (fig.  405,  D).  Of  sense  organs, 
besides  tactile  hairs,  only  the  eyes  (fig.  406),  2-12  in  number,  are 
well  known.  Hearing  is  well  developed,  but  it  is  uncertain  whether 
certain  hairs  on  the  legs  and  palpi  are  the  seats  of  the  recognition 
of  sound.  The  function  of  the  '  lyriform  organs/  which  occur  in 
the  skin  of  body  and  legs  in  several  groups,  is  unknown. 


446 


ARTHROPODA. 


The  respiratory  organs  already  alluded  to  (p.  443)  have  their 
spiracles,  always  few  in  number,  on  the  anterior  ventral  part  of 
the  abdomen  and,  it  is  stated,  sometimes  on  the  cephalothorax. 
The  internal  organs  are  the  lungs  and  the  tracheae.  A  lung  is  a 
rounded  sac  just  inside  the  spiracle  and  consists  of  numerous  leaves 
on  the  anterior  wall  of  the  lung  sac.  Each  leaf  is  covered  on  each 
side  by  a  thin  layer  of  chitin  and  contains  a  blood  space  in  its  in- 
terior, while  between  the  leaves  are  flattened  spaces  into  which  the 
air  enters  (fig.  456).  The  tracheae,  on  the 
other  hand,  are  branched  tubes  arising  from 
the  abdominal  spiracles  and  penetrating 
the  abdomen  (fig.  460).  These  are  lined 
with  chitin,  and  to  strengthen  them  with- 
out undue  thickness  this  lining  is  thrown 
into  folds,  usually  arranged  in  a  spiral. 
In  the  scorpions  and  tetrapneumonous 
Araneina  only  lungs  occur.  In  other 
spiders  one  pair  of  lungs  is  replaced  by 
tracheae,  while  in  most  other  arachnids  only 
tracheae  occur.  (The  smaller  mites  and 
FIG.  460.— Beginning  of  paired  parasites  lack  specialized  respiratory  or- 

trachese  of  Anyphozna  uccen-  ,         .         ,    ,  ,,  . 

taatn.    (After  Bertkau.)  st,  gans    and    circulatory   organs    as    well.) 
These  facts,  aside  from  embryological  con- 

ditions,  show  that  lungs  and  tracheae  are  morphologically  equiva- 
lent. The  localization  of  respiration  in  the  abdomen  has  resulted 
in  having  the  heart  in  the  same  region.  It  is  noticeable  that,  as 
the  tracheae  are  developed,  the  circulatory  vessels  are  reduced.  In 
the  scorpions,  which  have  only  lungs,  the  circulation  is  most 
nearly  complete. 

In  development  the  arachnidan  tracheae  arise  from  the  abdominal 
appendages,  as  do  the  lungs.  (In  the  Solpugidse  and  some  mites  cepha- 
lothoracic  tracheae  occur,  but  nothing  is  known  of  their  development.) 
This  fact  shows  that  they  are  entirely  different  in  origin  from  the  tracheae 
of  insects,  while  numberless  details  show  that  these  structures  are  only  to 
be  compared  with  the  gills  of  Limulus. 

The  gonads  (only  the  Tardigrades  are  hermaphroditic)  are 
abdominal  in  position  and  open  by  paired  ducts  (sometimes  with  a 
single  mouth)  on  the  first  abdominal  somite.  In  most  cases  the 
animals  are  oviparous,  but  the  scorpions  and  many  mites  bear  liv- 
ing young.  In  many  instances  the  mothers  care  for  their  eggs  and 
young,  the  scorpions  carrying  their  families  on  their  bodies.  Only 
rarely  is  there  a  metamorphosis,  and  then  in  the  aberrant  forms 


//.   ACERATA:  SCORPIONIDA. 


447 


like  the  Linguatulida  and  Acarina,  where  the  young  have  but  two 
or  three  pairs  of  appendages,  acquiring  the  others  later. 

Legion  I.  Arthrogastrida. 

Arachnida  in  which  the  abdominal  somites  are  distinct. 
Order  I.  Scorpionida. 

The  scorpions  bear  a  superficial  resemblance  to  crayfish  and  for 
a  long  time  were  associated  with  them,  since  (fig.  402)  they  have 
four  pairs  of  walking  feet  (3-6),  while  the  pedipalpi  (2)  are  large 
and  bear  pincers.  The  chelicerae  are  also  chelate.  The  pedipalpi 
and  the  two  anterior  pairs  of  legs  have  the  basal  joint  expanded 
for  chewing.  The  peculiarities  of  the  abdomen  mark  the  group 
off  from  all  other  arachnids.  It  consists  of  seven  broader  somites 
attached  by  their  whole  width  to  the  cephalothorax  and  behind 
six  narrower  somites,  forming  a  tail  or  postabdomen.  The  last 
somite  is  bent  ventrally  in  a  sharp  spine  and  contains  two  large 
poison  glands.  It  is  the  «  sting '  of  the  animal,  which,  in  the  case 
of  the  small  species,  causes  painful  wounds  in  man ;  and  in  the 
large  tropical  species  is,  perhaps,  fatal.  Usually  scorpions  feed 
upon  insects,  which  they  seize  with  the  pincers,  and,  arching  the 


FIG.  461.— Under  surface  of  scorpion,  showing  the  combs  and  the  outlines  of  the  lung 
sacs  with  their  spiracles  (orig.). 

tail  over  the  back,  kill  with  the  sting.  On  the  ventral  surface  of 
the  second  abdominal  somite  (fig.  461)  are  a  pair  of  appendages, 
the  combs  or  pectines;  rods  with  teeth  on  one  side  of  uncertain 
function.  They  are  clearly  appendages  with  modified  gill  leaves, 
and  from  their  nearness  to  the  sexual  opening  and  their  rich  nerve 
supply  are  supposed  to  be  stimulating  organs  in  copulation.  The 


448  ARTHROPODA. 

next  four  segments  bear  spiracles  which  lead  to  four  pairs  of  lung 
sacs.  The  heart  is  abdominal  and  the  '  liver 9  diverticula  are  con- 
fined to  the  same  region.  The  large  number  of  abdominal  ganglia 
distinct  from  the  oesophageal  ring  is  also  characteristic.  From 
three  to  six  pairs  of  eyes  occur. 

The  scorpions  are  inhabitants  of  warm  regions,  ranging  north  with  us 
to  the  Carolinas  and  Nebraska.  Buthus*  Centrums.* 

Order  II.  Phrynoidea  (Pedipalpi,  Thelyphonida). 

The  thoracic  segments  are  fused,  and  of  the  appendages  only 
the  last  three  are  walking  feet,  the  third  pair  having  the  last 
joint  (tarsus)  developed  into  a  long  many- jointed  tactile  flagel- 


FlO.  462.— Phrynus  (Phrynichus)  reniformis.    (From  Schmarda.) 

him.  The  chelicerae  are  strong  and  spined,  but  end  in  a  claw,  not 
in  a  pincer.  The  chelicerae  are  also  clawed  and  are  possibly  poison 
organs,  since  the  bite  of  these  animals  is  feared.  The  abdomen 
consists  of  eleven  or  twelve  somites  and  contains  two  pairs  of  lungs. 
There  are  eight  eyes — two  large  ones  in  the  middle  of  the  cephalo- 
thorax,  and  three  small  ones  on  either  side. 

The  species  are  tropical.  Phrynus  (fig.  462)  has  a  simple  abdomen  ; 
Thelyphonus*  (fig.  405,  D)  has  a  short  postabdomen  which  bears  a  long, 
many-jointed  thread.  One  species  in  the  southwestern  United  States. 

Order  III.  Microthelyphonida. 

Small  animals  as  yet  known  only  from  Texas,  Sicily,  Paraguay, 
and  Siam.  They  have  a  general  resemblance  to  a  scorpion,  the 
chelicerae  are  three-jointed  and  chelate,  the  pedipalpi  simple,  neither 
these  nor  any  of  the  legs  having  chewing  lamellae.  The  head  is 
distinct  from  two  *  thoracic  segments/  the  abdomen  is  eleven' 
jointed  and  is  terminated  by  a  long  many-jointed  caudal  flagellum. 


//.    ACES  ATA:   SOLPUGIDA. 


449 


Lung  sacs,  which  are  true  appendages  without  lung  leaves,  occur 
on  abdominal  segments  four  to  six,  and  are  eversible.     The  ovary 


FIG.  463.— Koenenia  wheeler  i.*    (From  Wheeler.) 

is  unpaired,  the  testes  paired.  There  is  a  circumcesophageal  nerve 
ring  and  a  single  abdominal  ganglion.  No  Malpighian  tubes 
occur.  Kcenenia.* 

Order  IV.  Solpugida  (Solifugae). 

In  these  the  cephalothorax  is  broken  up  into  a  head  bearing  the 
chelicerae,  pedipalpi,  and  the  first  pair  of  legs;  and  three  posterior 
free  somites,  each  bearing  a  pair  of  legs,  thus  giving  these  forms  a 
certain  resemblance  to  the  Hexapoda  (infra).  The  chelicerae  are 
strong  and  chelate,  the  pedipalpi  are  simple  and  are  used  in  walk- 
ing, while  the  first  pair  of  legs  are  tactile.  Respiration  occurs  by 
four  pairs  of  tracheae,  the  first  of  which  opens  between  the  first  and 


450  ARTHKOPODA. 

second  ( thoracic '  somites,  a  condition  which  deserves  embryologi- 
cal  investigation.  The  abdomen  consists  of  nine  or  ten  somites, 
and  the  head  bears  two  ocelli. 

As  the  name  implies,  the  Solpugidae  are  nocturnal,  living  by  day  in 
holes  in  the  sand  and  searching  for  their  prey  at  night.  In  the  Old  World 
they  are  reputed  as  poisonous,  but  no  poison  glands  occur.  "Warmer  parts 
of  U.  S.  Solpuga,*  Galeodes*  Datames  *  (fig.  464). 


FIG.  464.  FIG.  465. 

FIG.  464.— Datames  formidibilis*    (After  Putnam.) 

FIG.  465.— Chelifer  bravaisi.    (From  Schmarda.)    1,  cheliceree;  8,  pedipalpi. 

Order  V.  Pseudoscorpii. 

These  small  forms  resemble  the  true  scorpions  in  the  chelate 
cheliceraB  and  pedipalpi  (fig.  465),  and  in  the  abdomen  joined  by 
its  whole  breadth  to  the  thorax.  They  differ  in  the  lack  of  post- 
abdomen  and  sting.  They  breathe  by  tracheae;  have  from  two  to 
four  ocelli,  and  spinning  glands  opening  on  the  second  abdominal 
somite. 

These  animals,  2-3  mm.  long,  live  in  moss,  etc.,  and  among  old  and 
dusty  books,  where  they  feed  on  mites  and  minute  insects.  Their  bodies 
are  flattened  and  they  run  side  wise.  Chelifer,*  Obisium,*  Chernes.* 

Order  VI.  Phalangida. 

The  abdomen  in  the  harvestman,  or  'daddy  long  legs/  is  less 
evidently  segmented  than  in  the  forms  already  mentioned,  nor  is 
it  sharply  distinct  from  the  cephalothorax.  The  small  body  bears 
four  pairs  of  exceedingly  long  legs;  the  cheliceraB  are  drawn  out 


//.   ACERATA:  AEANEINA.  451 

in  long  horny  processes;  the  pedipalpi  are  tactile  organs  as  in  the 
true  spiders.     The  males  possess  a  long  penis,  and  the  females  a 


FIG.  466.— A  phalangid  laying  eggs.    (After  Henking.) 

long  ovipositor  (fig.   466).      They  have  two  or  four  ocelli  and 
breathe  by  tracheae. 

These  largely  nocturnal  animals  are  predaceous,  feeding  upon  small 
mites.  In  structure  they  fqrm  in  some  ways  an  approach  to  the 
Acarina.  Phalangium*  Liobunum* 

Legion  II.  Splicer ogastrida. 

Arachnida  with  the  abdominal  somites  fused  so  that  no  traces 
of  segmentation  remain. 

Order  I.  Araneina. 

In  the  spiders  the  soft-skinned 
body  is  divided  by  a  deep  con- 
striction into  cephalothorax  and 
abdomen  (fig.  467).  The  four  pairs 
of  legs  are  adapted  for  springing 
or  for  walking,  the  hinder  pair 
being  also  accessory  to  the  spin- 
ning. It  bears  a  comb-like  claw 
with  which  several  threads  are 
combined  into  a  stronger  cable. 
The  chelicera  bears  a  sharp  claw 
(fig.  459),  traversed  by  the  duct 
of  the  poison  gland  with  which  the 
prey  is  killed,  although  but  few 

(species     Of    LatrodecteS,     fig.    468,  FlO.  467.— Epeira  tnsularis*  round- web 

the  tarantula,  and  the  bird  spiders, 

Mygalidae)  can  injure  man.      The  pedipalpi  are  used  as  feeling 


452 


ARTUKOPODA. 


organs  and  with  the  basal  maxillary  process  to  comminute  the 
food.  In  the  male  the  pedipalpi  have  the  terminal  joint  swollen 
to  a  pear-shaped  structure  (fig.  469)  by  which  the  sexes  are  easily 


FIG.  468.  FIG.  469  FIG.  470. 

FIG.  468.— Latrodectes  macfmis,*  poison  spider.    (After  Marx.) 
FIG.  469.-  Pedipalp  of  Pardosa  uncnta.    (After  Emerton.) 

FIG.  470.— Spinnerets  of  Epeira  diadema.     (After  War  burton.)     1,  2,  3,  first,  second, 
and  third  spinnerets;  /,  threads. 

distinguished.  This  is  used  to  convey  the  spermatozoa  to  the 
female,  a  rather  dangerous  process,  as  the  male  is  apt  to  be  killed 
by  the  much  stronger  mate. 

At  the  hinder  end  of  the  abdomen,  just  in  front  of  the  anus, 
are  the  spinnerets,  which  are  reduced  appendages,  as  is  shown  by 
their  paired  arrangement  and  their  jointing  (fig.  470),  as  well  as 
by  development.  They  are  truncate  and  have  at  the  tip  a  '  spin- 
ning field'  from  which  numerous  minute,  two-jointed  spinning 
tubes,  resembling  hairs,  arise,  each  of  which  is  the  end  of  a  duct 
of  a  silk  gland.  Different  kinds  of  glands,  producing  silk  for  differ- 
ent purposes,  occur.  The  number  of  spinnerets  varies  between  two 
and  three  pairs,  and  in  front  of  these  may  be  an  unpaired  spinning 
region,  the  cribrellum,  so  that  hundreds  or  even  thousands  (Epei- 
ridae)  of  glands  may  be  present. 

The  secretion  of  the  glands  hardens  in  contact  with  the  air,  and  the 
single  threads  are  united  by  the  combs  of  the  hinder  feet,  into  a  larger  cord 
which  can  be  regulated  in  size  according  to  the  number  of  glands  which 
are  active.  Yet  the  largest  cord  is  finer  than  the  finest  silkworm  silk, 
hence  it  is  often  used  for  the  cross-hairs  of  telescopes.  The  spider  silk  has 
many  uses;  it  is  used  to  line  the  nests,  to  form  cocoons  for  the  eggs,  as  a 
means  of  descent  from  high  places,  and  to  form  the  well-known  webs. 

The  nervous  system  consists  of  a  brain  and  a  circumoesophageal  ring, 
and,  in  the  Mygalidae,  a  single  abdominal  ganglion.  The  arrangement  of 
the  six  or  eight  ocelli  and  the  relative  lengths  of  the  legs  are  matters  of 
systematic  importance.  Two  pairs  of  respiratory  organs  occur.  In  the 
Tetrapneumones  there  are  two  pair  of  lungs,  but  in  the  Dipneumones  the 


//.   ACERATA:  ACARINA.  453 

hinder  pair  are  replaced  by  tracheae,  which  may  open  by  separate  spiracles 
(Tetrasticta)  or  by  a  common  opening  (Tristicta,  fig.  460). 

Sub  Order  I.  TETRAPNEUMONES.  Four  lungs,  four  spinnents  and 
eight  eyes  in  two  rows.  The  MYGALnxsare  the  most  important  group,  large- 
forms  which  spring  upon  their  prey,  capturing  even  small  birds  and  mice. 
To  the  genus Mygale*  belong  the  spiders  (commonly  but  erroneously  called 
tarantulas)  which  occur  in  banana  bunches.  Here  also  belong  the  trap- 
door spiders,  Cteniza,*  of  the  southwest,  which  excavate  burrows  in  the 


FIG.  471.— Cteniza  ccementaria  in  its  tube,  closing  the  lid.    o,  eyes ;  b,  inside  of  lid! 
with  places  for  the  claws ;  c,  egg  cocoon. 

soil,  line   them   with   silk,    and  close   them  with  a  hinged  lid  (fig.  471). 
Atypus.* 

Sub  Order  II.  DIPNEUMONES.  One  pair  of  lungs,  one  of  trachea; 
six  spinnerets.  Here  belong  most  of  the  native  and  numerous  tropical 
species.  Some  (VAGABUND^E)  use  their  webs  only  to  line  the  nests  and 
enclose  the  eggs,  which  are  either  hidden  away  or  carried  about  attached  to 
the  body,  while  they  spring  upon  or  chase  their  prey.  SEDENTARIA  are- 
the  web  builders,  their  webs  varying  widely  in  structure.  Of  the  first 
group  the  SALTIGRADA  include  forms  which  jump  upon  their  prey  (Attus,* 
PhidippuS)*  Habrocentrum*),  and  the  CITIGRADA  (Lycosa,*  Dolomedes,* 
Trochosa  *),  which  run  their  prey  down.  Among  these  is  the  true  Taran- 
tula, T.  apulice  of  Italy,  whose  bite  was  once  believed  to  cause  a  frenzy  only 
to  be  cured  by  peculiar  music  (' Tarantello ').  The  Sedentaria  are  divided, 
according  to  the  web-building  habits-  The  ORBITELARLE  or  orb  weavers 
(Epeira*  Argiope*)  form  vertical  webs  which  in  many  instances  are  com- 
plete circles.  The  RETITELARLE  (Theridium*  Erigone  *)  build  irregular 
webs.  The  species  of  Latrodectes  *  are  reputed  poisonous  to  man  (fig. 
468).  The  TUBITELARI.E  build  horizontal  webs  with  a  tube  to  the  mar- 
gin in  which  they  lay  in  wait  for  insects. 

Order  II.  Acarina. 

The  mites,  partly  from  parasitism,  partly  from  other  conditions 
of  life,  have  become,  in  some  instances,  considerably  modified. 
With  the  fusion  of  cephalothorax  and  abdomen  the  last  traces  of 
segmentation  in  the  body  are  lost.  Yet  they  retain  the  six  pairs 
of  appendages — four  pairs  of  legs  which  at  once  distinguish  them 
from  the  parasitic  hexapods;  and  two  pairs  of  mouth  parts,  modi- 
fied  into  a  sucking  beak.  This  consists  of  a  tube  formed  by  the 


454 


ABTHROPODA. 


basal  joints  of  the  pedipaJpi,  in  which  the  chelicerae,  either  chelate, 
clawed,  or  stylet-like,  play. 

Since  the  mites  are  small  and  half  or  wholly  parasitic,  they  are  much 
simplified  in  structure.  Frequently  heart  and  tracheae  are  lacking.  The 
larva  as  it  escapes  from  the  egg  lacks  the  last  pair  of  legs  and  then  closely 
resembles  certain  imperfectly  segmented  parasitic  insects  like  the  lice. 

The  red  mites  or  TROMBIDIID^E  and  the  water  mites,  HYDRACHNID.E  (Hy- 
draclma*  Atax  *),  are  free-living  in  the  adult  condition,  but  parasitic  as 
young.  The  IXODID^:  or  ticks (Ixodes*},  live  in  woods  or  on  bushes,  attack 
man  and  other  mammals,  burrowing  beneath  the  skin,  sucking  the  blood  un- 
til they  become  enormously  swollen  and  fall  off.  The  much  smaller  males 


FIG.  472. 


Fm.  473. 


FIG.  472. — Sarcoptes  scabei,  female  itch  mite.    (After  Leuckart ) 

FIG.  473. — Demodex  folticulorum,  follicle  mite.    (From  Ludwig-Leunis.) 

are  attached  to  the  females  and  take  no  food.  Argas  persicus,  of  eastern 
lands,  with  habits  like  a  bedbug,  is  poisonous.  The  GAMASID^E  are  para- 
sitic, species  of  Gamasus  *  occurring  on  beetles  and  Dermanyss-us*  on  bats. 
The  ACARID.E  include  permanent  parasites  like  Sarcoptes  scabei*'  (fig.  472), 
the  cause  of  the  'itch,'  and  the  closely  allied  cheese  mite.  The  follicle 
mite,  Demodex  folliculorum,*  lives  in  the  sebaceous  glands  of  various 
mammals,  including  man  (fig.  473). 

Order  III.  Linguatulida. 

Elongate  mites  like  Demodex  lead  to  the  Linguatulida,  which 
as  adults  live  in  the  frontal  sinuses  of  carnivorous  mammals,  as  en- 
cysted young  in  the  liver  of  herbivorous  forms,  especially  rodents. 
The  body  is  long,  flattened  and  ringed,  and  hence  somewhat  tape- 
worm-like (fig.  112).  The  adults  have  the  mouth  at  the  base  of 
a  chitinous  capsule,  and  on  either  side  are  two  hooks  regarded  as 
the  claws  of  the  first  and  second  legs.  Inside  the  body  is  a  spa- 
cious cavity  traversed  by  the  alimentary  canal  which  is  without 
appendages.  The  nervous  system  is  largely  a  circumcesophageal 


//.  ACERATA:  LINGUATULIDA,  TARDIGRADA. 


455 


ring;  the  sexual   organs  are  very  complicated,  the  males  having 
the  openings  in  front,  the  females  at  the  hinder  end. 

The  presence  of  these  parasites  in  animals  causes  a  profuse  catarrh, 
and  the  eggs  pass  out  with  the  mucus.    Falling  on  vegetation,  these  are 


FIG.  474.  FIG.  475. 

FIG.  474.— Larva  of  Pentastomum  proboscideum.  (After  Stiles.)  rf,  stomach;  c,  gland 
cells ;  m,  mouth  ;  st,  stylet ;  ?/,  posterior  larval  hooks  ;  J,  2,  legs. 

FIG.  475.— Macrobiotus  hufelandi,  water  bear.  (After  drawings  by  Greef  and  Plate.) 
I-IV^  legs  ;  d,  accessory  glands ;  m,  stomach ;  mfc,  mouth  capsule  ;  ov,  ovary  ;  sp, 
salivary  glands  ;  st,  stylets ;  vm,  excretory  tubules ;  blood  cells  in  the  body. 

liable  to  be  eaten  by  various  animals.  The  larvae  (tig.  474)  have  a  boring 
apparatus  in  front  and  two  pairs  of  legs,  the  latter  being  lost  in  the 
metamorphosis  except  for  the  hooks.  It  is  by  no  means  certain  that 
these  are  degenerate  arachnids.  The  points  in  favor  of  such  a  position 
are  about  equally  balanced  by  those  against.  Pentastomum. 

Usually  associated  with  the  Arachnida  are  two  other  groups  of  very 
doubtful  position,  which  until  more  definite  knowledge  is  obtained,  may 
remain  near  them. 

Tardigrada. 

These  are  minute  fresh-water  forms,  known  to  microscopists  as 
4  water  bears '  (fig.  475),  which  owe  their  name  to  their  slow  motions. 
They  have  four  pairs  of  short,  hooked  legs,  their  sole  Arachnidan  charac- 
ter. The  genital  ducts  empty  into  the  rectum ;  the  nervous  system  has 
four  ventral  ganglia ;  heart  and  respiratory  organs  are  lacking.  In  de- 
velopment they  are  remarkable  for  the  large  ccelomic  pouches.  In  the 


456 


ARTHROPODA. 


feet  are  glands  recalling  nephridia  in  their  history.      It  is  possible  that 
these  animals  are  to  be  placed  among  the  Coelhelminthes.    Macrobiotus* 

Pycnogonida  (Pantopoda). 

These  marine  animals  have  a  cylindrical  body,  with  a  tubular  probos- 
cis in  front  and  an  abdominal  appendage  behind,  and  four  pairs  of  very 
long  legs.  In  front  of  the  legs  is  a  pair  of  small  chelate  appendages  and 
usually  a  pair  more  like  pedipalpi.  In  the  male  there  is  an  additional 
pair  of  '  ovigerous '  legs  to  which  the  eggs  are  attached  after  being 
deposited  by  the  female,  thus  giving  a  total  of  seven  appendages,  a  num- 


FIG.  476.— Nymphon   stroemii  *  (orig.).    c,  chelicerse  ;  o,  ovigerous  legs ;  p,  pedipalpi  ? 

r,  rostrum. 

ber  not  reached  in  any  arachnid.  Diverticula  of  the  stomach  extend  into- 
the  legs ;  a  heart  is  present,  but  respiratory  organs  are  lacking.  The- 
Pycnogonids,  which  creep  slowly  over  seaweeds  and  hydroids,  may  be  (1) 
a  distinct  group  of  arthroproda,  or  (2)  modified  arachnids,  or  (3),  and  less 
probable,  Crustacea.  Nymphon*  Phoxichilidium*  Colossendeis.* 

Class  III.  Malacopoda  (Protracheata). 

These  forms,  including  only  a  single  family  PERIPATID^E,  show 
a  strange  mixture   of   annelid  and  arthropodan   (or  '  tracheate ') 


Fio.  477.— Peripatus  capensis.    (From  Balfour,  after  Moseley.) 

characters,  so  that  they  are  usually  regarded  as  representatives  of 
the  stock,  early  separated  from  the  annelids,  from  which  the  Insecta 
have  descended.  They  recall  the  annelids  by  the  presence  of 
nephridia,  so  characteristic  of  that  group,  which  begin  by  a  closed 
vesicle  (reduced  coelom),  pursue  a  short  course,  and  expand  into  a 
urinary  bladder  before  opening  at  the  bases  of  the  legs  (fig.  478, 
so).  On  the  other  hand  they  possess  tracheae,  long  unbranched 


///.  MALACOPODA. 


457 


tubes  which  arise  in  numbers  from  the  spiracles,  which  are  irregu- 
larly distributed  in  each  somite  (fig.  478,  tr). 


FIG.  478.— Anatomy  of  female  Peripatus  opened  dorsally.  (From  figures  of  Moseler 
and  Balfour.)  a,  anus ;  at,  antennae  ;  6m,  ventral  nerve  cords;  d,  digestive  tract; 
go,  genital  opening ;  o,  ovary ;  ogr,  brain  ;  p,  pharynx ;  sd,  slime  gland ;  so,  ne- 
phridia ;  sp,  salivary  gland;  tr,  tracheae  ;  w,  uterus. 

The  soft-skinned  body,  which  shows  no  external  ringing,  bears 
the  legs,  each  terminated  by  claws.  These  legs  somewhat  resemble 
the  annelidan  parapodia  in  that  they  are  not  jointed  and  are  not 
sharply  separated  from  the  trunk.  Each  segment  bears  legs,  while 
the  head  is  provided  with  three  pairs  of  appendages:  a  pair  of 
ringed  antennae,  a  pair  of  mandibles,  which  lie  in  the  oral  cavity, 
and  a  pair  of  mouth  papillae,  at  the  tips  of  which  are  the  openings 
of  the  slime  glands,  the  sticky  secretion  of  which  is  squirted  out 
and  serves  to  capture  insects  (fig.  478,  sd). 

The  nervous  system  consists  of  a  pair  of  cerebral  ganglia  (og), 
supplying  the  antennae  and  a  pair  of  very  primitive  eyes;  and  a 
pair  of  ventral  cords  (bin),  swollen  slightly  in  each  segment,  which 


458 


ARTHROPOD  A. 


connect  dorsal  to  the  anus  and  are  connected  in  the  trunk  by 
numerous  non -segmental  commissures. 

The  description  may  be  completed  by  saying  that  the  straight  aliment- 
ary canal  (p  and  d)  bears  only  salivary  glands  (sp)  ;  that  it  is  accompanied 
throughout  by  a  dorsal  heart  ;  that  the  gonads  (the  sexes  are  separate) 
open  just  in  front  of  the  anus  (#o),  their  ducts  being  modified  nephridia. 
The  animals  are  viviparous,  live  in  decaying  wood,  hide  by  day  and  hunt 
their  prey  at  night.  The  several  species  have  a  wide  but  discontinuous 
distribution  (South  America,  Cape  of  Good  Hope,  New  Zealand,  etc.),  an 
indication  of  great  antiquity.  Recently  the  forms  have  been  divided  into 
several  genera,  Peripatus,  Peripatopsis,  Opisthopatus^  etc. 


Class  IV.  Insecta. 

The  Insecta  is  a  distinct  group  marked  off  from  all  other 
arthropods  by  several  important  characters. 
The  appendages  show  no  signs  of  a  schizo- 
podal  condition.  The  head  is  always  a 
distinct  region,  bearing  a  single  pair  of 
antennae,  a  pair  of"  mandibles,  and  two  pairs 
of  maxillae,  the  posterior  pair  often  being 
fused  into  a  lower  lip  or  labium. 

The  respiratory  organs  are  trachea  (figs. 
479,   480),  which  resemble  the  trachea  of 


FIG.  479. 


FIG.  480. 


FIG.  479 —Tracheal  system  of  Machilis.    (From  Lang,  after  Oudemans.)    fc,  head; 

J-JJI,  thoracic  somites;  s,  spiracles;  1-10,  abdominal  somites. 
FIG.  480.— Portion  of  trachea  of  caterpillar.    (From  Gegenbaur.)    A,  mam  trunk; 

B,  C,  D,  branches;  a,  epithelium  with  nuclei,  b;  d,  air  in  tracheal  tube. 


IV.   1NSECTA.  459 

man  only  in  that  they  are  tubes  filled  with  air,  and  kept  from 
collapse  by  firm  walls.  They  open  to  the  exterior  by  openings 
(spiracles,  stigmata)  on  the  sides  of  the  body.  They  are  inpushings 
of  the  skin  and  consequently  have  the  same  structure,  an  epithe- 
lium and  an  outer  chitinous  layer.  The  latter  lines  the  lumen 
of  the  tubes,  and  since  it  must  be  thin  to  permit  the  passage 
•of  gases  (oxygen,  carbon  dioxide),  and  at  the  same  time  firm, 
to  keep  the  tubes  open,  it  is  thrown  into  folds  which  usually 
pursue  a  spiral  course.  The  turns  of  the  spiral  are  so  close 
that  it  gives  the  tubes  a  ringed  appearance.  Inside  the  spiracles 
the  tracheae  branch  repeatedly  until  they  end  in  the  tissues  in 
fine  tracheal  capillaries.  In  general  it  may  be  said  that  each 
segment  has  a  right  and  a  left  spiracle  and  corresponding  tracheal 
systems  (fig.  59),  but  this  scheme  is  complete  in  no  known  species, 
for  there  are  always  some  segments  (especially  in  the  head)  which 
lack  these  organs  and  are  supplied  from  adjacent  segments  (fig. 
479).  Again,  the  tracheae  may  be  connected  by  longitudinal  trunks 
(fig.  494,  ib),  so  that  spiracles  occur  in  only  a  part  of  the  segments, 
these  supplying  the  whole  system.  Although  the  tracheae  are  for 
aerial  respiration,  there  are  aquatic  insects,  but  these  also  breathe 
air,  since  they  carry  air  about  with  them  entangled  among  the 
hairs  which  surround  the  spiracles.  Then,  too,  aquatic  larvae  often 
have  tracheal  gills,  thin-walled  processes  of  the  integument  which 
project  into  the  water  and  are  penetrated  by  numerous  tracheal 
twigs  (fig.  495). 

The  alimentary  tract  always  has  excretory  organs,  the  Mal- 
pighian  tubules,  connected  with  it.  These  vary  in  number  be- 
tween wide  limits,  but  are  always  placed  at  the  junction  of  the 
rectum  with  the  rest  of  the  track  They  diifer  from  the  physiolog- 
ically similar  tubes  of  the  Arachnida  in  being  of  ectodermal  origin, 
so  that  no  homology  can  be  traced  between  them.  The  gonads 
are  always  paired  and  placed  dorsal  to  the  intestine,  while  the 
ducts  (at  least  in  some  cases  modified  nephridia)  open  ventrally 
at  the  hinder  end  of  the  body.  The  spermatozoa  are  motile. 

In  the  subdivision  of  the  '  tracbeate  '  arthropods  a  group  of  Myriapoda 
is  usually  recognized,  containing  forms  known  as  centipedes  and  '  galley 
worms.'  These  two  types  are  in  reality  very  different.  The  centipedes 
(Chilopoda)  show  in  all  structural  features  close  relationships  to  the  Hex- 
apoda,  while  the  other  group,  Diplopoda,  differ  in  almost  every  respect, 
except  the  presence  of  numerous  walking  legs,  from  the  Chilopoda. 
Hence,  since  the  object  of  classification  is  to  show  resemblances  and  dif- 
ferences, the  group  of  Myriapoda  has  been  dismembered,  the  Chilopoda 


460 


ABTIIROPODA. 


being  considered  here,  the  Diplopoda  as  a  distinct  class  at  the  end  of  the 
group  of  Arthropoda. 

Sub  Class  I.    Chilopoda. 

The  most  striking  characteristic  of  the  chilopods  is  their  long, 
flattened  bodies,  each  of  the  numerous  somites  bearing  a  pair  of 


FIG.  481.— Diagram  of  transverse  section  of  a  centipede  (orig.).    d,  digestive  tract; 
gonad;  n,  nerve  cord;  s,  spiracle  and  tracheae. 


FIG.  482.  FIG.  483. 

FIG.  482.— Mouth  parts  of  Scolopendra  morsitans.    1,  antennae  ;  2,  mandibles;  3,  max- 
illae ;  A,  second  maxillae  (labium) ;  5,  poison  feet. 
FIG.  483.— Scolopendra  morsitans,  centipede.    (After  Schmarda.) 

six-  or  seven-jointed  limbs.     The  head  bears  a  pair  of  long  antennae 
and  usually  numerous  ocelli,  which  only  in  Scutigera  show  a  ten- 


IV.   INSECTA:   HEXAPODA.  461 

dency  to  become  compound.  The  mouth  parts  (fig.  482)  are  a 
pair  of  mandibles  and  two  pairs  of  maxillae,  both  united  in  the 
median  line.  Besides,  the  first  pair  of  legs  (fig.  482,  5),  with  their 
fused  bases,  extend  forward  beneath  the  head  and  form  the  poison 
claws.  Their  terminal  joints  are  sharp  and  contain  the  ducts  of 
poison  glands. 

The  spiracles  (at  least  a  pair  to  every  other  somite  except  those 
of  the  head)  are  lateral  in  position  in  the  soft  integument  between 
the  dorsal  and  ventral  plates  (fig.  481).  The  heart  is  elongate,  with 
chambers  in  each  somite  (fig.  66);  there  are  two  large  Malpighian 
tubes,  and  the  nervous  system  is  elongate,  with  ganglia  in  each 
.somite.  The  gonads  are  dorsal  to  the  intestine  and  are  unpaired, 
while  the  single  duct  opens  ventrally  in  the  preanal  somite. 

The  LITHOBIID.E,  with  15  leg-bearing  somites,  have  certain  dorsal  plates 
enlarged  and  overlapping  the  succeeding  somites  ;  Lithobius,*  common 
under  stones,  etc.  SCOLOPENDRID.E,  centipedes;  at  least  17  legs  and  5 
ocelli ;  Scolopendra*  in  warmer  regions  (fig.  483).  GEOPHILHLE,  not  less 
than  30  pairs  of  legs,  spiracles  2  less  than  legs.  Geophilus*  SCUTIGE- 
RiDyE,  legs  very  long,  15  leg-bearing  segments,  but  only  8  dorsal  plates. 
jSoutigera.* 

Sub  Class  II.   Hexapoda. 

The  Hexapoda  is  by  far  the  largest  division  of  the  Arthropods, 
since  it  contains  at  least  ten  times  as  many  known  species  as  all 
the  rest.  The  number  is  so  large  that  it  cannot  be  given  with 
accuracy;  an  estimate  is  250,000.  Since  the  tropics,  which  have 
not  been  exhaustively  studied,  are  very  rich  in  insects,  it  is  con- 
ceivable that  there  are  at  least  a  million  different  species  in  the 
world.  On  the  other  hand  great  uniformity  of  structure  exists, 
all  adhering  with  great  fidelity  to  plan  of  structure,  regional  divi- 
sions, and  number  of  appendages  under  the  most  diverse  conditions, 
so  that  the  difference  between  the  most  extreme  forms  is  far  less 
than  that  in  Crustacea  or  Arachnida.  But  while  hexapods  thus 
lose  in  morphological  interest,  they  gain  in  their  life  relations,  in 
the  way  that  they  are  injurious  or  beneficial  to  man,  in  their  breed- 
ing habits,  and  in  their  intellectual  and  social  relations.  From  the 
evolutionary  standpoint  they  show  marked  adaptations  to  environ- 
ment, and  the  large  number  of  species  is  only  possible  by  taking 
advantage  of  every  opportunity  in  nature. 

Of  systematic  importance  are  the  regional  division  of  the  body 
and  the  number  and  character  of  the  appendages.  In  the  body 
three  regions  are  distinguished,  often  separated  by  marked  con- 


462 


ARTHROPODA. 


strictions:   head,  thorax,  and  abdomen.     The  number  of  abdomi- 
nal somites,  varies  with  the   order   and   even  with   the  family, 


FIG.  484.—  Schematic  section  of  a  hexapod  through  the  thorax  (orig.).  ex,  coxa;  d, 
digestive  tract:  /,  femur;  7i,  heart;  n,  notum;  pi,  pleuron  ;  st,  sternum;  (,  tibia; 
(a,  tarsus  ;  tr,  trochanter. 

ranging  between  eleven  (in  some   larvae  and  embryos  twelve)  in 
the  Orthoptera  and  five  in  many  Diptera.     Each  cuticular  abdo- 

minal segment  consists  of  two  plates, 
tergite  (dorsal)  and  sternite  (ventral), 
united  on  the  sides  by  a  softer  mem- 
brane which  contains  the  spiracles. 
Head  and  thorax,  on  the  other  hand, 
have  a  constant  number  of  somites. 
The  thorax  is  plainly  divided  into  three 
segments,  pro-,  meso-  and  met  at  h  or  ax  , 
each  composed  of  three  elements,  an 
unpaired  dorsal  portion,  notum;  a  pair 
of  lateral  plates,  pleura,  and  an  unpaired 
ventral  sternum  (fig.  484).  For  sim- 
Fio.485.-Head  of  a  grasshopper,  plicity  one  speaks  of  pronotum,  meso- 


'  ifabiasi;  pafprrv';  sternum,  etc.,  to  indicate  the  portions  of 

labrum;    wid,    mandible  ;  'mpl  fV,  ~    e*vnarnfp    apo-mpnt«         TTif>    Vi^nrl    ia   a 
maxillary  palpi;  mx,  maxilla;  tjl€  ts* 

o,  occiput;  v,  vertex.  continuous  capsule  in  which  the  follow- 

ing parts  are  recognized:  in  front  and  dorsal  clypeus  and  frons; 
dorsal  and  posterior  a  vertex  and  an  occiput;  laterally  gence,  ven- 
trally  a  gula.  The  appendages  show  that  the  head  is  composed  of 
at  least  four  somites. 

The  view  that  the  head  consists  of  six  somites  is  based  on  the  existence 
of  two  more  segments  without  appendages  in  the  embryo,  a  preantennal 
and  a  postantennal  (intercalary,  premandibular),  as  well  as  the  knowledge 
that  the  brain,  in  which  formerly  only  antennal  ganglia  were  recognized, 
consists  of  three  pairs  of  ganglia  (proto-,  deuto-,  and  trito-cerebrum). 


IV.   INSECTA:  HEXAPODA.  463 

The  appendages  (fig.  484),  seven  pairs,  are  confined  to  the  head 
and  thorax  (see,  however,  infra).  The  three  thoracic  segments 
bear  three  pairs  of  legs,  whence  the  name  Hexapoda.  The  legs  are 
inserted  between  pleura  and  sterna  and  begin  with  a  short  coxa 
(c),  followed  by  a  trochanter  (tr),  also  short.  The  two  following 
joints  are  long,  the  first,  the  femur  (/<?),  being  large  and  contain- 
ing the  muscles;  the  next,  tibia  (t),  being  more  slender;  the  foot, 
or  tarsus  (ta),  is  composed  of  a  series  of  joints,  the  last  bearing  a 
pair  of  claws. 

The  first  of  the  cephalic  appendages,  the  antennae,  are  the 
most  leg-like,  but  normally  are  never  clawed.  They  spring  from 
the  frons  above  the  mouth  and  are  innervated  from  the  brain. 
The  number  and  shape  of  the  antennal  joints  varies  with  the 
group,  and  according  as  the  single  joints  are  lengthened  or  short- 
ened, narrowed  or  expanded,  or  provided  with  appendages,  etc., 
different  kinds  of  antennae — knobbed,  club-shaped,  toothed,  feath- 
ered, etc.-2— are  recognized,  distinctions  of  great  value  in  classi- 
fication. 

The  morphology  of  the  three  pairs  of  mouth  parts,  the  mandi- 
bles (md),  maxillae  (mx),  and  second  maxillae,  or  labium  (la,  figs. 
486-489),  is  more  interesting.  The  labium,  formed  of  united 
right  and  left  appendages,  lies  behind  the  mouth  and  forms  the 
lower  lip,  and  is  in  contrast  to  the  upper  lip,  or  labrum  (lr)> 
which,  however,  is  not  appendicular  in  character.  Both  labium 
and  labrum  may  bear  unpaired  processes  on  their  oral  surfaces,  an 
epipharynx  above,  a  hypopharynx  below  the  mouth,  neither  of 
them  true  appendages. 

The  different  kinds  of  food  necessitate  differences  in  the  char- 
acter of  the  mouth  parts, — chewing,  licking,  sucking,  or  piercing 
— all  referable  back  to  the  chewing  kind,  and  these  in  turn  are 
modified  legs.  In  the  description  of  the  chewing  type  it  is  well 
to  begin  with  the  maxillae  (fig.  486),  because  of  their  easy  com- 
parison with  the  other  mouth  parts  and  with  the  legs  as  well. 
These  begin  with  a  triangular  joint,  the  cardo  (c),  which  is  fol- 
lowed by  a  larger  stipes  (st).  The  stipes  in  turn  supports  two 
chewing  lobes,  the  inner,  or  lacinia  (li),  and  an  outer,  or  galea  (le), 
these  being  processes  segmented  off  from  the  stipes.  In  the 
Orthoptera  and  Coleoptera  only  the  lacinia  is  sharp-pointed  ;  the 
galea  may  either  form  a  sheath  for  the  lacinia,  or,  as  in  many  beetles 
(fig.  514),  it  may  be  tactile  and  jointed  again.  The  stipes  also 
bears  the  maxillary  palpus  (pm),  consisting  of  from  three  to  six 
similar  joints,  and  is  the  mostly  leg-like  part  of  the  appendage. 


464 


ARTHROPODA. 


The  labium  arises  as  a  pair  of  processes  which  early  approach  each 
other  and  fuse  behind  the  mouth.  All  the  parts  of  the  maxilla 
may  be  recognized,  only  it  must  be  remembered  that  the  basal 
parts  of  the  two  sides  are  fused.  The  united  cardines  form  an 
under  chin,  the  submentum,  the  stipites  a  chin  or  mentum,  which 
in  the  Orthoptera  is  cleft,  a  result  of  incomplete  fusion.  This 
may  bear  inner  and  outer  processes,  the  glosscv  (gl)  and  the  para- 
glosses  (pg)  respectively,  and  the  labial  palpus.  The  mandible  con- 


FIG.  486.  FIG.  487. 

Fio.  486. — Chewing  mouth  parts  of  cockroach  (Periplaneta  orientalis).  The  letter- 
ing is  the  same  in  figs.  486-489.  c,  cardo ;  gl,  glossa ;  /it/,  hypopharynx ;  I,  lobe ; 
le,  li,  external  and  internal  lobes  of  maxilla;  Zr,  labrum ;  m,  memtum ;  md,  man- 
dible ;  mx,  maxilla ;  p,  pm,  maxillary  palpus  ;  pg,  paraglossa  ;  pi,  labial  palpus  ; 
sm,  submentum;  st,  stipes. 

Fio.  487.— Licking  mouth  parts  of  bumble  bee  (Bombus  terrestris). 

sists  of  merely  the  basal  joint,  altered  for  biting,  while  the  rest  of 
the  appendage,  common  in  Crustacea  as  the  mandibular  palpus,  is 
lacking. 

The  licking  mouth  parts,  like  those  of  the  bees  (fig.  487),  stand 
next  to  those  already  described,  there  being  many  transitional 
stages.  Labrum  and  mandibles  retain  their  primitive  condition, 
while  maxillae  and  labium  are  greatly  elongate,  are  connected  at 
the  bases,  and  can  be  folded  away  beneath  the  head  or  extended  at 
will.  The  small  submentum  is  followed  by  an  elongate  mentum 


IV.   INSECTA:  HEXAPODA. 


465 


which  bears  the  unpaired  tongue  or  glossa  (gl),  which  corresponds 
to  the  fused  glossae  (or  to  the  hypopharynx?)  of  the  first  type  and 
which  is  used  for  sucking  honey  and  hence  has  the  form  of  a 
nearly  closed  tube.  Beside  it  lie  the  rudimentary  paraglossse  (pg) 
and  the  well-developed  palpi.  Similarly  the  maxillae  have  small 
cardines  and  palpi,  while  the  stipites  and  the  undivided  lobe  (/) 
are  long  and  well  developed. 

The  piercing  mouth  parts  of  the  flies  (Diptera)  and  bugs 
(Ehynchota)  can  be  compared  with  those  of  the  bees  in  so  far  as 
the  labium  forms  the  groundwork  of  the  whole  (fig.  488).  The 


FIG.  488.  FIG.  489. 

PIG.  488.— Sucking  mouth  parts  of  mosquito,  Culex  pipiens.     (After  Muhr.)    The 

groove  of  labium  opened  by  removing  labrum;  the  stylets  separated. 
FIG.  489.— Sucking  mouth  parts  of  a  butterfly.  (After  Savigny.)  ma:',  ma;",  shows  how 

right  and  left  maxillae  unite  into  a  tube;  right  labial  palpus  (pi)  with  hairs 

removed. 

beak  (rostrum,  haustellum)  of  these  animals  corresponds  to  the 
labium;  it  is  a  grooved  structure,  either  fleshy  and  flexible,  or  stiff 
.and  jointed.  The  edges  of  the  groove  are  inrolled  so  that  there 
remains  a  narrow  dorsal  slit,  which  can  be  closed  by  the  slender 
upper  lip  (Ir).  The  tube  formed  of  these  parts  contains  four 
stylets,  toothed  or  with  retrorse  hooks  at  the  tip.  These  are  the 


466  ARTHROPODA. 

mandibles  and  maxillae,  and  a  fifth  stylet,  the  hypopharynx  (liy) 
can  be  present.  Palpi,  which  only  occur  in  the  Diptera,  belong  to 
the  maxillae  (p).  Reduction  in  number  of  stylets  to  four  or  three, 
or  their  complete  absence  (some  flies),  is  brought  about  by  fusion 
or  by  degeneration.  The  haustellum  serves  as  a  case  for  the  suck- 
ing tube,  which  in  the  Ehynchota  is  formed  by  the  united  maxillae, 
in  the  Diptera  by  labrum  and  hypopharynx. 

The  proboscis,  or  haustellum  (the  so-called  tongue),  of  the 
Lepidoptera  (fig.  489)  is  a  long  tube  coiled  like  a  watch  spring 
beneath  the  head.  It  consists  of  two  long  grooved  maxillary  galea 
firmly  united  by  their  edges.  The  maxillary  palpi  are  well  de- 
veloped in  the  moths;  elsewhere  they  show  all  stages  of  reduction 
to  complete  disappearance.  Labium  and  labrum  are  reduced  to 
small  triangular  plates  at  the  base  of  the  proboscis,  the  labium 
bearing  a  pair  of  hairy  palpi  (pi).  The  mandibles  are  represented 
by  small  plates  or  bunches  of  hair.  These  conditions  gain  in  in- 
terest when  we  remember  that  in  the  larva  the  mandibles  are 
strong  biting  organs,  while  the  maxillae  are  small  hooks,  and  the 
labium  is  better  developed  only  in  those  parts  connected  with  the 
silk  glands,  a  beautiful  example  of  relations  of  structure  to  life 
conditions. 

In  contrast  to  the  other  regions,  the  abdomen  lacks  appendages  in  the 
adults.  Only  in  the  lower  group  of  Thysanura  are  small  lobes  present, 
behind  and  in  the  same  line  with  the  thoracic  feet,  which  may  be  regarded 
as  abdominal  feet.  Apparently,  too,  the  appendages  of  the  last  segment, 
the  stylets  and  cerci,  are  modified  limbs,  but  the  parts  (gonapophyses) 
used  in  copulation  and  oviposition  are  different  in  character.  False  feet, 
or  pro-feet,  occur  on  the  abdomen  of  the  larvae  of  the  Lepidoptera  and  the 
Tenthredinidae,  but  since  these  are  fleshy  unjointed  processes,  it  is 
doubtful  whether  these  are  true  abdominal  limbs,  like  those  of  other 
Arthropoda,  or  are  structures  independently  acquired. 

Besides  ventral  appendages  the  insects  usually  have  two  pairs 
of  dorsal  outgrowths  upon  the  meso-  and  metathorax,  the  wings. 
They  are  lateral  folds  of  the  chitinous  coat  of  the  notum  and  con- 
tain on  their  interior  extensions  of  the  blood  sinuses  and  of  the 
tracheae,  which  are  protected  by  thickenings  of  the  chitin,  causing 
the  network  of  '  veins'  or  <  nervures '  in  the  wing.  Both  wings 
may  be  elastic,  flexible,  and  adapted  for  flight,  or  the  hinder  pair 
may  alone  partake  of  this  character  (true  wings  or  alae),  while  the 
first  pair  may  be  thick  and  parchment-like  wing  covers,  or  elytra, 
under  which  the  true  wings  are  concealed  when  at  rest.  When 
only  the  base  of  the  wing  is  thus  thickened  hemelytra  result. 
Between  the  origins  of  the  anterior  wings  is  frequently  a  chitinous. 


IV.   IN  SECT  A:  HEX  APOD  A. 


467 


plate,  the  scutellum,  while  between  the  hinder  wings  is  a  similar 
postscutellum.  In  many  insects  one  pair  of  wings  is  lacking,  the 
anterior  pair  being  retained  in  the  Diptera,  the  posterior  in  the 
Strepsiptera;  these  are  clearly  cases  of  degeneration.  The  entire 
absence  of  wings  may  occur  from  two  causes;  wings  have  apparently 
never  been  developed  in  some  (primary  lack  of  wings  of  the 
Apterygota),  while  there  are  others  in  which  we  must  believe  that 
wings  once  present  have  been  lost,  because  nearly  related  forms — 
bugs,  lice,  etc. — have  wings,  or  because  certain  individuals  (male 
cockroaches,  sexual  ants  and  termites)  are  winged  (figs.  506,  528, 
529).  The  prothorax  of  all  recent  insects  is  wingless,  but  in  some 
of  the  Archiptera  of  the  coal  period  wing  rudiments  occurred  on 
this  somite. 

As  a  result  of  differences  in  food  the  alimentary  canal  (figs.  490, 
491)  varies  greatly.  The  ectodermal 
stomodaeum  begins  with  a  pharynx, 
which  in  the  sucking  insects  is  a 
sucking  apparatus  with  radial  mus- 
cles. The  oesophagus,  which  follows, 
may  be  widened  to  a  crop  (ingluvies), 
or  it  may  have  a  caecal  outgrowth 
which  in  the  butterflies  may  take  the 
shape  of  a  stalked  vesicle  (falsely 
'  sucking  stomach').  Also  ectoder- 
mal is  the  gizzard  (km,pv),  or  pro- 
ventriculus,  the  chitinous  lining  of 
which  is  toothed  for  grinding  the 
food.  The  true  stomach,  of  ento- 
dermal  origin  (m,  cd),  frequently 
bears  blind  sacs  or  gastric  caeca  (ap) ; 
in  general  it  is  short  and  its  junction 
with  the  hinder  ectodermal  portion, 
the  proctodeum,  is  marked  by  the 
entrance  of  the  Malpighian  tubules 
(vasa  Malpighii,  vm).  The  latter, 
excretory  in  fuuction,  arise  from  the 
proctodeal  region.  The  latter  is 
usually  differentiated  into  a  small  in- 
testine and  a  two-regional  (colon  and 
rectum)  large  intestine.  The  rectum 
may  have  enlargements  called  rectal 
glands.  True  glands,  however,  occur  only  at  the  beginning  and 


FIG.  490.— Alimentary  tract  of  Card- 
bus  auratus.  (From  Lang,  after 
Dufour.)  av,  anal  vesicle ;  arf,  anal 
gland  ;  cd,  stomach  with  caeca  ;  ed^ 
hind  gut;  m,  ingluvies  (crop);  fc, 
head;  oe,  oesophagus;  pv,  proven- 
triculu s (gizzard ):  r, rectum;  vm, 
Malpighian  tubules. 


468  ARTHEOPODA. 

end  of  the  alimentary  tract ;  into  the  mouth  empty  from  one  to 
four  pairs  of  salivary  glands  (sp) ;  at  the  anus  are  defensive  anal 
glands  with  their  malodorous  secretions  of  a  protective  character. 
The  alimentary  tract  with  the  other  viscera  is  enveloped  in  the 
fat  body,  a  soft  mass  which  contains,  besides  fat  cells  and  connec- 
tive tissue,  concretions  of  uric  acid. 

The  nervous  system  (fig.  405)  has  the  ventral  cord,  especially 
in  primitive  forms  (Apterygota,  Archiptera,  Orthoptera,  fig.  491), 


rtg 


FIQ.  491.—  Viscera  of  male  cockroach  (Perip/cmefa  orientalis).  (Partly  after  Huxley.) 
/-///,  segments  of  thorax  and  corresponding  legs;  1-10,  abdominal  segments;  a, 
anus;  <»(/,  ventral  ganglia;  ap,  gastric  ceeca  ;  at,  antenna;  W,  salivary  bladder; 
?/,  sexual  opening;  h,  heart;  fc»\  crop;  km,  gizzard;  /,  labial  palpus;  m,  stomach 
(the  arrow  shows  the  connexion  between  m  and  km},  also  maxillary  palpus  ;  mg, 
male  genitalia  ;  oe,  oesophagus  ;  oy,  brain;  7-,  rectum;  sp,  salivary  gland;  tg, 
thoracic  ganglia;  iig,  infracesophageal  ganglion  ;  inn,  Malpighian  tubules. 

nnd  nearly  all  larvse  (fig.  59),  long  and  composed  of  numerous 
separate  pairs  of  ganglia.  In  beetles,  moths,  bees  (fig.  494),  and 
flies  the  cord  is  shortened  and  the  ganglia  are  in  part  fused.  The 
brain  arises  by  the  fusion  of  three  pairs  of  ganglia  (proto-,  deuto-, 
and  tritocerebrum),  and  is,  especially  in  the  adult,  very  complex. 
It  is  connected  on  either  side  with  a  large  optic  ganglion  the  size 
•of  which  is  correlated  to  that  of  the  eyes.  In  the  adult  condition 
the  Hexapoda  have  a  single  pair  of  highly  developed  compound  eyes 
{figs.  407,  408),  which  not  infrequently  occupy  nearly  the  whole 
•of  the  top  of  the  head.  Between  and  in  front  of  these  small  and 
simple  ocelli,  usually  three  in  number,  frequently  occur,  especially 
in  insects  which  are  strong  fliers.  These  are  either  lacking  or 
poorly  developed  in  the  larvae,  while  the  compound  eyes  are  fre- 
quently replaced  by  groups  of  from  two  to  six  closely  crowded 
ocelli.  Of  other  sense  organs  only  the  tactile  hairs  of  the  skin  are 
known  with  certainty,  while  similar  hairs  on  the  antennas  and 
about  the  mouth  are  supposed  to  be  organs  of  smell  and  taste,  since- 
these  senses  are  known  to  be  well  developed.  The  tympanal 
organs  of  the  Orthoptera  are  the  only  structures  which  can  be  with 


IV.   INSECT  A:  HEX  APOD  A. 


469 


much  probability  connected  with  hearing.  These  are  thin  drum- 
like  parts  of  the  chitin,  framed  in  thicker  portions  (figs.  492,  493), 
beneath  which  is  a  tracheal  vesicle,  with  a  nerve  ending  in  a  '  crista 
acustica.'  The  power  of  producing  sound  is  widely  distributed  and 
often  highly  developed,  the  organs  for  this  purpose  varying  widely 
in  character.  Stridulating  organs  are  formed  by  ridges  on  wings 
and  legs,  which  are  rubbed  against  each  other  or  against  similar 
ridges  on  the  body.  Humming  is  produced  by  the  action  of  the. 


FIQ.  492. 


Fia.  493. 


FIG.  492.— Side  view  of  grasshopper,    s,  spiracles  ;  f,  tympanic  organ. 
FIG.  493.— Anterior  tibia  of  a  Locustid  with  tympanum,  t.    (From  Hatschek,  after 
Fischer.) 

wings  or  by  the  passage  of  air  through  the  spiracles,  which  are 
often  provided  with  vibrating  membranes  which  also  serve  to  close 
these  openings. 

The  tracheae  (figs.  479,  494)  are  usually  united,  just  inside  the 
spiracles,  by  longitudinal  trunks  from  which  fine  branches  extend, 
enveloping  and  penetrating  all  the  organs  with  delicate  silvery 
threads.  This  connexion  of  tracheae  renders  it  possible  for  the 
spiracles  of  some  segments  to  disappear.  The  spiracles  of  the 
abdomen  are  the  most  constant,  usually  occurring  in  the  soft  mem- 
brane between  the  sternites  and  tergites;  the  thorax  at  most  has 
but  two  pairs,  the  head  none.  In  insects  with  good  powers  of 
night  many  of  the  tracheal  trunks  are  expanded  to  large  air  sacs, 
which  may  be  of  value  as  reservoirs  of  air,  so  that  the  ordinary 
respiratory  motions  are  less  necessary  during  flight. 

An  interesting  adaptation  of  the  tracheal  system  to  aquatic  life  occurs 
in  the  Iarva3  of  many  Archiptera  (Odonata  and  Mayflies)  and  Neuroptera, 
and  even  among  Lepidoptera  (Paraponyx)  and  Coleoptera  (Gyrinida3). 
The  spiracles  here  are  usually  closed,  and  the  taking  of  oxygen  occurs 
either  through  the  skin  or  by  means  of  so-called  tracheal  gills — bushy  or 
leaf-like  appendages  of  the  surface  or  the  rectum,  richly  permeated  by 
tracheal  branches  (fig.  495).  In  such  cases  the  tracheal  system  has  two 
portions,  one  which  receives  oxygen  from  and  gives  off  carbon  dioxide  to 
the  water  ;  the  other  which  supplies  the  tissues  with  oxygen  and  receives 
carbon  dioxide. 


470 


ARTHROPODA. 


Since  the  tracheae,  with  their  fine  branches,  supply  the  tissues 
directly  with  oxygen,  the  blood-vascular  system  is  rudimentary. 
Directly  under  the  back  lies  the  elongate  tubular  heart  in  a  special 


FIG.  494. 


FIG.  495. 


FIG.  494.— Anatomy  of  honey  bee.  (From  Lang,  after  Leuckart.)  ",  antennae:  au, 
eye  ;  6,  legs;  cm,  chyle  stomach;  ed,  rectum  ;  hm,  honey  stomach  iproventriculus) , 
rd,  rectal  glands;  s£,  spiracles ;  tb,  tracheal  chambers  with  tracheae ;  v/n,  Mal- 
pighian  tubules. 

FIG.  495.— Abdomen  of  Ephemera  larva  (from  Gegenbaur)  with  tracheal  gills,  c;  a, 
tracheal  trunks ;  b,  intestine  ;  d,  caudal  bristles  (cerci). 

pericardial  sinus.  This  is  a  part  of  the  haemoccele  cut  off  from 
the  gastric  portion  of  this  space  by  an  incomplete  partition  in 
which,  right  and  left,  are  the  wing  muscles  (alee  cordis)  of  the 
heart.  The  heart  receives  its  blood  through  lateral  ostia  (eight  or 
fewer)  from  the  pericardial  sinus  or  (Orthoptera)  through  ventral 
openings  from  the  large  haemocoele.  The  blood  passes  forward 
through  an  anterior  aorta  into  the  haemocoele  and  thence  back  to 
the  pericardial  sinus.  The  arrangement  of  the  viscera,  fat  bodies, 
and  muscles  gives  a  certain  regularity  to  the  circulation,  especially 
in  the  appendages.  Accessory  pulsating  ampullae  in  the  bases  of 


IV.   INSECTA:  HEXAPODA. 


471 


the  antennae  (Orthoptera)  help  in  the  flow  of  the  blood.  It  is 
noteworthy  that  many  beetles  (Meloidae  and  Coccinellidae)  squirt 
blood  through  the  jointing  membranes  of  the  legs  as  a  means  of 
protection. 

The  Hexapoda  are  dkscious.  The  gonads  consist  of  a  few  or 
many  ovarial  or  testicular  tubules  (figs.  496,  497),  the  latter  some- 
times coiled  into  small  oval  bodies.  Ovaries  and  testes  are  paired 
and  lie,  right  and  left,  in  the  abdomen.  Their  paired  ducts  (ovi- 


FIG.  496. 


FIG.  497. 


FIG.  496.— Male  genitalia  of  Melolontha.  (From  Gegenbaur,  after  Fabre.)  gl,  accessory 

glands;  t,  testes;  vd,  vas  deferens;  vs,  seminal  vesicles. 
FIG.  497.— Genitalia  of  female  Hydrobius.    (From  Gegenbaur,    after  Stein.)    be,  bursa 

copulatrix;  gl,  tubular  glands;  o,  ovarial  tubes;  ov,  oviduct  with  glands;  rs,  re- 

ceptaculum  seminis;  v,  vagina. 

ducts,  vasa  deferentia)  open  separately  in  the  Ephemerida,  but  in 
all  other  Hexapoda  there  is  a  single  ventral  unpaired  sexual  open- 
ing just  in  front  of  the  anus.  This  arises  as  a  median  invagination 
of  the  ectoderm  (hence  lined  with  chitin),  which  extends  inwards 
and  meets  the  genital  ducts  (modified  nephridia),  and  forms  the 
ductus  ejaculatorius  of  the  male,  the  vagina  of  the  female.  Aside 
from  many  accessory  glands,  the  sexual  apparatus  shows  the  follow- 
ing differentiations:  in  the  male  vesiculaa  seminales,  as  widenings 
or  diverticula  of  the  vasa  deferentia ;  in  the  female  the  receptaculum 
seminis  and  the  bursa  copulatrix.  The  latter  may  be  either  the 
vagina  or  a  blind  sac  arising  from  it,  or  a  special  invagination  of 
the  ectoderm,  emptying  into  the  vagina  by  an  internal  canal.  It 
receives  the  penis.  The  receptaculum  seminis,  a  stalked  vesicle 
connected  with  the  vaginia  or  the  bursa,  has  a  special  biological 
interest.  In  insects  which  copulate  but  once  during  life  it  retains 
the  spermatozoa  for  a  long  time — four  years  in  bees — in  a  living 
condition.  As  the  eggs  are  laid  they  are  impregnated  by  sperma- 


472  AETHROPODA. 

tozoa  from  it.  Since  a  firm  shell  or  chorion  is  developed  around 
the  egg  in  the  ovary,  access  of  spermatozoa  is  only  possible  by  the 
existence  of  a  micropylar  apparatus,  a  system  of  tubes  penetrating 
the  chorion  at  one  end  of  the  egg. 

Oviposition  occurs  in  many  insects  by  means  of  an  ovipositor 
which  may  project  free  from  the  body  (fig.  509)  or  may  be  re- 
tracted into  it.  It  consists  of  four  or  (Orthoptera)  six  parts  or 
gonapophyses  developed  from  the  eighth  and  ninth  abdominal 
segment,  which  form  a  tube.  In  many  Hymenoptera  this  struc- 
ture has  become  modified  into  a  sting  (aculeus),  and  is  provided 
with  poison  glands,  making  it  an  efficient  weapon  of  defence. 
From  its  nature  the  sting  is  of  necessity  confined  to  the  females. 
In  the  males  there  is  usually  a  protrusible  penis  which  is  frequently 
composed  of  the  same  parts  as  the  ovipositor ;  in  others  of  metamor- 
phosed somites.  Further  sexual  differences  lie  in  the  form  of  the 
antennae,  shape  and  color  of  the  wings,  modifications  of  the  eyes, 
etc. 

In  many  insects  the  eggs  may  develop  parthenogenetically. 
Plant  lice  and  scale  insects  reproduce  for  generations  asexually,  and 
parthenogenesis  is  widely  distributed  among  Hymenoptera,  Lepi- 
doptera,  and  Neuroptera.  The  conditions  among  the  bees  are 
especially  interesting,  since  here  the  determination  of  sex  rests  with 
the  existence  or  non-existence  of  fertilization  (pp.  142,  487). 
Much  rarer  than  the  ordinary  parthenogenesis  is  that  special  form, 
known  as  paedogenesis,  which  occurs  only  in  certain  Diptera  like 
Miastor.  In  the  female  Miastor  larva  (fig.  498)  the  eggs  develop 


FIQ.  498.— Larva  of  a  Cecidomyid  with  psedogenetic  daughter  larvae.  (From  Hatschek, 

after  Pagenstecher.) 

before  the  appearance  of  the  ducts,  so  that  the  young  can  only 
escape  by  rupture  of  the  mother.  After  several  paedogenetic 
generations  there  appear  at  last  larvae  which  pupate  and  produce 
adult  male  and  female  flies. 

With  the  exception  of  these  paedogenetic  forms,  the  Pupipara, 
many  Aphidae  and  a  few  other  viviparous  species,  the  Hexapoda 
are  oviparous.  The  development  begins,  after  oviposition,  by  a 
superficial  segmentation  of  the  egg.  Later  there  appear  two  em- 
bryonic structures,  the  yolk  sac  and  the  amnion;  the  first,  in  con- 
trast to  the  vertebrate  structure  with  the  same  name,  is  dorsal. 


IV.   INSECTA:  HEX  APOD  A. 


The  amnion  is  a  thin  layer  of  cells  which  covers  the  ventral  surface 
and  arises  in  a  manner  similar  to  the  vertebrate  amnion ;  folds  aris- 
ing from  the  blastoderm  in  front  and  behind,  right  and  left  of 
the  embryo,  fuse  with  one  another  and  produce  a  double  envelope, 
an  inner  amnion,  an  outer  serosa. 

With  the  rupture  of  the  amnion  and  egg  shell,  the  postembry- 
onic  development  begins.  This  differs  so  in  the  different  orders 
that  ametabolous,  hemimetabolous,  and  holometabolous  insects  are 
recognized,  i.e.,  insects  with  direct  development  without  meta- 
morphosis, those  with  partial  and  those  with  complete  metamor- 
phosis. The  ametabolous  young  is  closely  like  the  adult,  so  that 
it  only  has  to  grow,  with  periodic  ecdyses,  and  to  mature  its  re- 
productive organs.  Since  no  insect  has  wings  when  it  leaves  the 
egg,  this  direct  development  is  possible  only  in  wingless  forms  like 
the  Apterygota  and  Apt  era. 

All  winged  insects,  on  the  other  hand  have  a  more  or  less  pro- 
nounced metamorphosis,  the  final  cause  of  which  is  the  necessity 
of  developing  wings.  This  view  holds  although  there  are  wing- 
less insects  with  a  complete  metamorphosis.  These  forms  (fleas, 
wingless  moths,  and  ants)  have  undoubtedly  sprung  from  winged 
species  and  have  inherited  from  them  the  metamorphosis  which 
has  been  retained  after  the  wings  were  lost.  In  incomplete 
metamorphosis  the  differences  between  the  newly  hatched  young 
and  the  adult,  or  imago,  gradually 
disappear  (fig.  499).  At  the  second 
molt  the  wings  often  appear  as  small 
folds  in  the  chitinous  wall  of  meso- 
and  metathorax ;  they  grow  with  each 
ecdysis,  until  at  last,  in  size,  form, 
and  movability,  they  are  functional 
wings.  The  chitinous  coat  of  each 
wing  pad  (fig.  499,  B,  1,  2}  encloses 
the  compressed  and  folded  wing  of 
the  next  stage.  Since  the  larvae  by  _ 

J  FIG.  499.— Hemimetabolous  develop- 
their    lack    OI    Wings    are    placed    in      ment  of  Perlanigra.    (From  Hux- 

different     circumstances     from     the 

adult,  the  differences  between  the  two 

may  be  increased  by  the  development  of   special  larval   organs. 

Thus  the  aquatic  larvae  of  the  May  flies  and  dragon  flies  differ  from 

the  adults  not  only  in  the  absence  of  wings,  but  by  the  different 

form  and  the  tracheal  gills,  which  are  almost  always  lost  at  the  last 

molt  (fig.  495). 


A,  wingless  larva;  B,  larva 
with  wing  pads,  l.  2 ;  C,  adult ;  /, 
II,  III,  thoracic  segments. 


474 


ARTHROPODA. 


Increase  in  the  differences  of  environment  and  the  correlated 
increase  in  larval  characters  lead  to  complete  metamorphosis.  In 
order  to  profit  as  much  as  possible  by  its  adaptation  to  its  environ- 
ment the  larva  retains  its  shape  as  long  as  possible ;  the  gradual 
change  is  suppressed  and  the  alteration  in  form  necessary  to  the 
metamorphosis  is  postponed  until  the  end  of  the  larval  life,  to  the 
period  between  the  last  two  molts.  In  this  interval  there  is  such 
.an  energetic  transformation  of  the  organism  that  the  performance 
•of  ordinary  vital  functions,  especially  motion  and  feeding,  is  in- 
terfered with  or  rendered  impossible.  This  last  stage  therefore 
becomes  a  period  of  rest,  the  pupal  stage,  upon  the  existence  of 
which  great  weight  must  be  laid  in  the  definition  of  complete  met- 
amorphosis. The  more  complete  the  condition  of  rest  the  more 
pronounced  is  the  holometabolous  development.  From  this  point 
of  view  different  types  of  pupae  are  distinguished :  pupae  liberae, 
pupae  obtectae,  and  pupae  coarctatae.  In  a  free  pupa  (pupa  libera) 
the  appendages  stand  out  from  the  body  (fig.  500),  so  that  not 


FIG.  500.— Larva  and  pupa  of  May  beetle,    a',  a",  fore  and  hind  wings  ;  an,  anus ; 
at,  antennae;  o,  eyes;  p'-p'",  legs  ;  s£,  spiracles. 

only  the  segmentation  of  the  body  but  the  antennae,  legs,  wings, 
and  often  the  mouth  parts  of  the  imago  are  visible.  Such  pupae 
have  a  certain  power  of  motion,  as,  for  instance,  the  pupae  of  many 
Neuroptera  and  mosquitos,  the  latter  rising  and  falling  in  the 
water.  The  covered  pupae  (pupae  obtectae)  at  the  moment  of 
pupation  have  free  appendages  which  with  the  hardening  of  the 
chitin  become  closely  appressed  to  the  body,  so  that  even  by  close 
inspection  only  indistinct  contours  can  be  seen  (fig.  501).  Motion 
is  confined  to  bending  of  the  whole  body,  as  is  familiar  in  the 
pupae  of  moths  and  butterflies.  The  pupae  coarctatae  are  without 
motion  because  here  the  pupa  (in  structure  a  pupa  libera)  is  en- 
closed in  a  larger  coat,  the  last  larval  skin  (Muscajia). 

The  variations  among  larvae  are  even  greater  than  with  pupae. 
Here  structure  and  jointing  of  the  body  are  so  completely  under 
the  influence  of  environment  that  with  similar  or  different  con- 


IV.   INSECT  A:  HEXAPODA. 


475 


ditions  larvae  widely  remote  from  the  systematic  standpoint  may 
closely  resemble  each  other,  while  those  of  closely  related  species 
may  differ  extremely.  The  leaf -feeding  larvae  of  Lepidoptera  (fig. 


FIG. 501. 


FIG.  502. 


FIG.  503. 


FIG.  501.— Pupa  of  Sphinx  ligustri.  (After  Ludwig-Leunis.)  i,  eye;  2,  head  ;  3,  an- 
tennae; £-6',  thoracic  somites;  7,  hind,  #,  fore  wing;  &,  legs;  J0,  proboscis;  Ji,  ab- 
dominal somites ;  J2,  spiracles. 

FIG.  502.— Larva  of  Sphinx  liyustri.  (After  Ludwig-Leunis.)  71,  caudal  disc  ;  p,  tho- 
racic feet ;  ps,  prolegs. 

FIG.  503.— Larva  (maggot)  of  blowfly,  Musca  vomitoria.    (After  Leuckart.) 

502)  and  Tenthreds  are  brightly  colored,  the  thoracic  appendages 
remaining  small,  and  are  reinforced  by  the  fleshy  ventral  append- 
ages, the  prolegs  or  pedes  spurii.  The  predacious  larvae  of  many 
beetles  and  Neuroptera  have  long  thoracic  legs,  strong  mandibles, 
and  no  prolegs.  Other  beetle  larvae,  which  burrow  in  wood  or 
live  in  the  earth  (fig.  500),  have  plump  whitish  bodies,  with  the 
legs  rudimentary  or  wholly  lacking.  These  lead  to  the  maggot- 
like  larvae,  in  which  the  mouth  parts  are  inconspicuous  and  the 
distinction  between  head  and  thorax  may  vanish.  Such  soft-skinned 
annulated  sacs  occur  in  the  bees  (fig.  59)  and  other  Hymenoptera, 
as  well  as  in  many  flies  (fig.  503) ;  that  is,  in  animals  which  live 
in  an  abundance  of  food  either  because  of  parasitism  or  because 
the  mother  has  provided  plenty. 

From  the  outer  appearance  one  would  gain  the  impression  that 
these  holometabolous  larvae  not  only  lacked  the  wings,  but  that  the 
appendages  of  the  imago  were  entirely  absent  or  had  an  entirely 
different  form;  farther,  that  wings,  and  frequently  antennae,  legs, 
and  mouth  parts,  come  into  existence  at  the  moment  of  pupation, 
and  then  in  remarkable  size  and  completeness.  A  more  accurate 
investigation  shows  that  the  anlagen  of  all  these  structures  are 
formed  long  before  pupation,  often  at  the  first  molt.  The  wings 


476  ARTHROPOD  A. 

of  a  butterfly  are  present  in  the  caterpillar  as  small  folds  or  proc- 
esses of  the  surface  which  increase  in  size  with  each  molt.  That  they 
are  not  visible  externally  is  due  to  the  fact  that  they  are  pushed 


FIG.  504.— Diagram  of  development  of  wings  and  legs  from  the  imaginal  discs  of  a. 
fly  during  metamorphosis.  (After  Lang.)  /i,  larval  hypodermis;  t,  imaginal 
hyppdermis  ;  /,  ?u,  imagin&l  discs  and  legs  and  wings  formed  from  them;  s,  con- 
nexion of  discs  with  hypodermis;  x,  chitinous  larval  skin. 

into  the  body  and  enclosed  in  sacs  opening  to  the  exterior.  Such 
anlagen  are  called  imaginal  discs;  with  their  recognition  the  dis- 
tinctions between  complete  and  incomplete  metamorphosis  in  part 
disappear,  since  in  the  first  the  structures  of  the  imago,  even  if  in 
a  modified  shape,  are  outlined  very  early.  Still  there  remains 
much  to  be  remodelled  during  the  pupal  rest.  The  muscles  must 
be  adapted  to  the  new  locomotor  organs,  the  digestive  tract  to  the 
altered  food,  the  nervous  system  re-formed.  Since  a  great  part  of 
the  previous  structures  must  be  broken  down  to  afford  material 
for  the  reconstruction  of  the  organs,  the  pulpy  nature  of  the  inside 
of  the  pupa  is  easily  understood.  In  a  rapid  degeneration  of  the 
tissues  the  material,  consisting  of  indistinctly  separated  cells,  is- 
so  homogeneous  that  it  was  formerly  thought  that  the  pupa  re- 
turned to  the  indifferent  condition  of  the  egg  (Histolysis  of  flies). 
In  the  classification  four  points  are  of  special  importance:  (1)  The 
segmentation  of  the  body,  in  which  it  is  to  be  noted  whether  the  segments. 
of  thorax  and  abdomen  follow  without  change  of  form,  or  whether  the 
thorax,  by  the  closer  union  of  its  somites,  is  sharply  marked  off  from  both 
head  and  abdomen.  (2)  The  character  of  the  wings,  which  are  either 
lacking  in  the  lower  forms  or  are  delicate  chitinous  structures,  with 
numerous  veins,  the  wings  of  the  two  thoracic  segments  similar.  In  the 
higher  forms  a  degeneration  of  the  wing  veins  or  a  leathery  consistence  of 
the  membrane,  together  with  a  divergent  development,  partial  reduction 
of  antennge  and  posterior  wings  may  occur.  (3)  The  structure  of  the  mouth 
parts,  and  (4)  the  type  of  development,  both  described  above.  From  these 
characters  it  is  easy  to  differentiate  six  orders:  Lepidoptera,  Diptera, 
Aphaniptera,  Rhynchota,  Hymenoptera,  and  Coleoptera.  The  remaining 
forms  were  formerly  divided  among  the  Orthoptera  and  Neuroptera,  but 


IV.   INSECT  A:  HEX  APOD  A,  APTERYGOTA.  477 

these  groups  are  not  considered  natural  and  the  attempt  has  been  made  to 
divide  them  into  more  or  fewer  groups.  Here  the  Pseudoneuroptera  or 
Aphaniptera  are  separated  from  the  Neuroptera,  the  wingless  forms  or 
Apterygota  from  the  Orthoptera. 

Order  I.  Apterygota. 

At  the  bottom  of  the  Hexapoda  come  forms  which  lack  wings 
and  which  show  no  evidence  of  having  descended  from  winged  an- 
cestors. They  are  regarded  as  slightly  modified  descendants  of 
the  ancestral  Hexapod.  Besides  the  lack  of  wings  they  show  many 
primitive  characters;  compound  eyes  are  poorly  developed  or  lack- 
ing; the  tracheal  system,  when  not  degenerate,  consists  of  isolated 
tracheal  bushes,  rarely  connected  by  longitudinal  trunks  (fig. 
479) ;  the  mouth  parts,  resembling  somewhat  those  of  Orthoptera, 
are  for  biting,  though  frequently  rudimentary;  the  development 
is  always  ametabolous. 

Sub  Order  I.  THYSANURA  (Bristle-tails).  Body 
elongate,  with  long  bristles  (cerci)  at  the  hinder  end. 
Lepisma  saccharina,*  silver  fish,  common  among  old 
books  and  papers,  does  considerable  damage.  It  is 
covered  with  shining  scales.  Campodea  *  (fig.  400), 
with  rudimentary  abdominal  appendages.  Machilis* 
lapyx*  with  caudal  forceps. 

Sub  Order  II.  COLLEMBOLA  (Spring-tails).  Com- 
pressed forms  in  which  the  bristles  bent  under  the  body 
serve  as  a  spring,  throwing  the  animals  (one  to  three 
mm.  long)  forwards.  Podura  *;  Anurida  maritima* 
in  tide  pools  ;  Entomobrya  *;  Lipura  *;  Achoreutes  ni- 
valis*  the  snow  flea. 

Order  II.  Archiptera  (Pseudoneuroptera). 
These  represent  the  primitive  forms  of  winged 
insects.  The  elongate  body  consists  of  numerous  Packard'.) (After 
segments  and  usually  bears  the  cerci  of  the  Thysanura.  The 
wings  are  delicate  and  transparent,  supported  by  a  close  net- 
work of  nervures,  both  pairs  being  very  closely  alike.  The 
mouth  parts  are  of  the  typical  biting  kind;  the  maxillae  have 
lacinia  and  galea;  the  labium,  with  glossa  and  paraglossa,  is 
frequently  deeply  cleft.  These  points  of  primitive  structure 
are  correlated  with  a  primitive,  usually  hemimetabolous  de- 
velopment. The  distinction  between  larva  and  imago  is  largely 
one  of  presence  or  absence  of  wings,  although  larval  organs  like 
gills  (Amphibiotica)  may  occur.  Frequently  the  development  is 
direct  when  the  imagines,  as  in  some  Termites  and  the  Psocida?, 
are  wingless. 


478 


ARTIIROPODA. 


The  Archiptera  were  formerly  united  with  the  Neuroptera  on  account 
of  similarities  of  wings.  The  separation  is  due  to  characters  of  mouth 
parts  and  development. 

Sub  Order  I.  CORRODENTIA.  Larvae  distinguished  from  the  im- 
agines by  difference  in  size  and,  in  the  winged  forms,  by  lack  of  wings. 
Best  known  are  the  TERMITID.E  (Isoptera),  or  white  ants,  which  must  not 
be  confused  with  the  true  ants  (Hyrnenoptera),  from  which  they  are  dis- 
tinguished by  the  similar  body  segments,  the  mouth  parts,  and  the  simple 
development.  Like  the  true  ants,  they  have  a  well-developed  social  state. 
A  colony  of  termites,  consisting  usually  of  thousands  of  individuals,  forms 
a  nest  with  numerous  chambers  and  passages.  They  are  nocturnal,  and 
they  burrow,  without  coming  to  the  surface,  through  old  wood  (timbers 
of  houses,  furniture,  picture  frames,  dead  wood  in  the  forest,  etc.).  They 
line  these  chambers  with  a  cement-like  substance  composed  of  refuse  which 
has  passed  through  the  alimentary  canal.  Many  species  build  dome-like 
nests,  ten  or  fifteen  feet  high,  fifteen  to  twenty  or  twenty-five  feet  across, 
of  chewed  earth.  In  a  colony  are  winged  and  wingless  individuals,  the 
latter  with  ametabolous  development  (fig.  506).  The  wingless  forms  have 
the  sexual  organs  rudimentary,  but,  in  contrast  to  ants  and  bees,  may 
belong  to  either  sex.  They  are  frequently  blind,  have  strong  mandibles, 
and  are  of  two  kinds,  the  workers  (c)  and  the  large-headed  soldiers  (d). 
The  winged  forms  are  sexually  functional  (6).  Shortly  after  the  metamor- 
phosis they  swarm,  and  then  the  wings  are  bitten  off  at  the  base  and 
'king'  and  'queen'  either  form  a  new  colony  or  enter  one  already  in 
existence.  After  copulation  the  abdomen  of  the  queen,  by  the  formation 
of  numerous  eggs,  swells  to  an  enormous  size  (e).  Since  the  swarming 


Fio.  506.— Termes  flavipes*  white  ant.    (From  Riley.)    a,  larva  ;  b,  winged  male  ;  c, 
worker  ;  d,  soldier  ;  e,  queen ;  /,  pupa. 

individuals  form  the  prey  of  birds  and  other  animals,  it  often  happens  that 
a  colony  is  left  without  a  royal  couple.  In  such  cases  the  line  is  perpet- 
uated by  reserve  males  and  females,  sexual  animals  which  have  not  com- 


IV.   INSECTA:  HEXAPODA,  ARCHIPTERA. 


470 


pleted  the  metamorphosis  but  are  in  the  wing-pad  stage.  The  termites  are 
able,  by  quantity  and  quality  of  food,  to  modify  the  development  of  the 
larvae  and  to  determine  which  type  of  individual  shall  be  produced.  The 
termites  are  farther  noticeable  for  the  bitter  wars  they  conduct  against 
the  true  ants.  Termes  flavipes  *  in  our  northern  states.  T.  fatalis, 
Africa. 

Allied  to  the  Termites  are  the  often  wingless  PSOCID^,  or  book  lice. 
Trades  divinatorius  *  is  the  book  louse.  Other  species  are  winged  and 
live  in  moss,  etc.  Near  here  also  belong  the  MALLOPHAGA,  which,  like 
lice,  live  upon  mammals  and  especially  on  birds.  Like  true  lice  they  are 
wingless,  but  they  have  biting  mouth  parts.  Trichodectes*  on  the  dog, 
ox,  etc.;  Goniodes,*  Docophorus*  Nirmus,*  etc.,  on  birds. 

Sub  Order  II.  AMPHIBIOTICA.  The  three  families  united  here 
differ  much  in  structure,  but  agree  in  having  aquatic  larvae  with  tracheal 
gills  (fig.  495).  These  are  ventral  bushes  in  the  Perlida3,  wing-like  or 
bushy  appendages  of  the  abdomen  in  the  Ephemeridae,  and 
three-leaved  appendages  in  those  Odonata  which  do  not 
respire  by  tracheal  branches  in  the  rectum.  All  of  these 
larvae  are  predaceous,  especially  the  larvae  as  well  as  the 
adults  of  the  Odonata.  The  Odonate  larvae  have  a  peculiar 
apparatus  for  the  capture  of  prey.  The  mentum  and  sub- 
mentum  of  the  labium  are  greatly  elongate  and  when 
folded  bring  the  tip  like  a  mask  beneath  the  mouth.  The 
^  structure  can  be  suddenly  extended  (fig.  507)  and  grasps- 


FIG.  507. 


FIG.  508. 


FIG.  507.— Larva  of  ^schna  grandis.    (After  Rosel  von  Rosenhof.)    a1,  aa,  wing  pads  j 

m,  mask  ;  st,  spiracles. 
FIG.  508.— Ephemera  vulgata.    (From  Schmarda.)    The  caudal  bristles  incomplete. 

the  food.  PERLID^E  (Plecoptera) ;  hind  wings  the  larger.  Perla*  Ptero- 
narcys*  stone  flies.  EPHEMERID^E,  fore  wings  large,  the  hinder  small  or 
absent ;  May  flies,  life  very  short  in  the  adult  state.  Ephemera,*  (fig.  508) 
Cleon*  Beetisca.*  ODONATA  (Libellulidse),  wings  nearly  equal,  the  hinder 
slightly  larger  ;  Dragon  flies,  veritable  insect  hawks  destroying  numberless 
mosquitos.  LibelLula*  JEsclma,*  Agrion*  Gfomphus.* 

Sub  Order  III.  PHYSOPODA  (Thysanoptera).  Wings  slender,  fringed 
with  hairs ;  tarsi  bladder-like  at  tip  ;  mouth  parts  bristle-like,  probably 
used  for  sucking.  The  position  of  this  group  is  very  uncertain.  Thrips,* 
Limothrips* 


480  ARTHROPODA. 


Order  III.  Orthoptera. 

Like  the  Archiptera  these  are  hemimetabolous  or  in  a  few 
•cases  ametabolous,  and  the  mouth  parts  (fig.  486)  are  fitted  for 
biting,  the  mentum  being  cleft.  On  the  other  hand  the  wings 
have  lost  the  delicate  membranous  character  and  have  become 
more  parchment-like,  the  fore  wings  being  smaller  and  serving  as 
•covers  for  the  larger,  softer,  and  folded  hind  wings,  which  are  the 
efficient  organs  of  flight;  the  condition  in  these  respects  recalling 
somewhat  the  Coleoptera.  The  abdomen  bears  cerci  and  fre- 
quently stylets.  In  internal  anatomy  the  large  number  of  Malpi- 
ghian  tubules  is  noticeable  (fig.  491). 

Sub  Order  I.  CURSORIA.  With  rather  long  legs  fitted  for  rapid  run- 
ning. Only  the  cockroaches  (BLATTID.E)  belong  here.  Wings  may  be 
absent,  according  to  the  species,  in  either  sex,  but  more  frequently  in 
females.  The  more  common  cockroach,  the  '  Croton  bug '  (Blatta  ger- 
manica  *),  is  a  well-known  pest  in  houses.  The  larger  Periplaneta  orien- 
talis*  is  common  in  ships  and  bakeries.  Other  species  in  our  woods. 

Sub  Order  II.  DEEMATOPTERA  (Euplexoptera).  Front  wings  short 
•elytra ;  the  hinder  wings  being  folded  crosswise  and  packed  beneath  them, 
or  rudimentary ;  cerci  developed  to  a  forceps-like  structure  terminating 
the  body,  whence  the  name  Forficula*  given  one  genus.  Labidura* 
These  forms  are  often  called  earwigs,  from  an  erroneous  belief  that  they 
enter  the  human  ear  and  injure  the  drum.  The  group  on  account  of  its 
wing  structure  is  often  made  a  distinct  order. 

Sub  Order  III.  GKESSORIA.  Legs  long,  slender,  adapted  to  a  slow 
•walking  motion.  In  the  MANTID.E  the  prothorax  is  very  long  and  bears  a 
pair  of  long  raptorial  feet  which  when  at  rest  are  held  in  a  position  which 
causes  these  insects  to  be  known  as  'praying  Mantes.'  Phasmomantis* 
warm  countries.  PHASMID^E,  with  short  prothorax,  almost  exclusively 
tropical,  represented  throughout  northeastern  United  States  by  Diaphero- 
mera  femorata*  the  walking  stick.  The  members  of  this  family  are 
noted  for  their  mimicry  of  twigs  and  leaves  (fig.  12.) 

Sub  Order  IV.  SALTATORIA.  Hinder  legs  long,  strong,  and  for 
jumping  ;  the  other  pairs  much  smaller.  Hinder  femora  large  and  muscu- 
lar, tibiae  elongate  and  spined.  Wings  usually  functional  and  in  the 
migrating  species  capable  of  sustained  flight.  Produce  sound  (stridulate) 
by  rubbing  the  anterior  wings  together  (Locustidae,  Gryllidae)  or  against 
the  legs  (Acridiidae).  Tympanal  apparatus  (p.  468)  on  the  anterior  tibiae 
(Locustidae,  fig.  493,  and  many  Gryllidae)  or  on  the  first  somite  of  the 
abdomen  (fig.  492).  Stridulation  occurs  only  in  males,  and  in  our  com- 
mon crickets  the  number  of  notes  is  directly  dependent  upon  tempera- 
ture, which,  on  the  Fahrenheit  scale,  may  be  determined  by  the  formula, 

T  =  50  +  ?LZ! —  ,  in  which  T  stands  for  temperature  and  n  for  number 


IV.   INSEGTA:  HEXAPODA,  NEUROPT£jRA. 


481 


of  chirps  per  minute.     The  females  may  readily  be  recognized  by  the  ovi- 
positor.    ACRIDIID.E  ;  antennae  and  ovipositor  short ;  tympani  abdomi- 


FIG.  509.— Locusta  caudata.    (After  von  Wattenwyll.)    I,  ovipositor. 

nal.  Acridium* ;  Melanoplus*  (M.  spretns,  the  'grasshopper7  which 
did  such  damage  in  the  Missouri  River  States  in  1873-75)  ;  (Edipoda* ; 
Tettix*  LOCUSTID.E  ;  antennae*  long  ;  tympani  on  first  tibiae  ;  ovipositor 
long,  flattened  ;  tarsi  four-jointed.  Haden&cus*  wingless,  blind,  in  caves; 
Conocephalus  *  ;  Cyrtoplnlus  *  and  Microcentrum,*  katydids  ;  Anabrus* 
wingless.  GRYLLID.E,  Crickets  :  antennae  long  ;  ovipositor  long,  cylindri- 
cal;  tarsi  three-jointed;  tympani  on  first  tibia.  Gryllus*;  (Ecanthus* 
tree  crickets ;  Gryllotalpa*  mole  crickets,  burrowing. 

OrderlV.  Weuroptera, 

The  Neuroptera  closely  parallel  the  Archiptera,  and  the  two 
were  formerly  united,  since  they  have  the  same  wing  structure  and 


FIG.  510. — Myrmeleo  formicarius.    (From  Schmarda.)    1,  imago;  2,  larva;  3,  pupa  in  its 

cocoon. 

show  in  general  appearance  great  similarities.  Thus  the  ant  lions 
(fig.  510)  recall  the  dragon  flies;  the  Chrysopinae,  the  Perlidae. 
The  Neuroptera,  however,  are  holometabolous  and  have  a  resting 
stage,  although  the  pupae  (pupae  liberae)  are  capable  of  some  mo- 
tion. The  mouth  parts  are  for  biting,  and  in  some  the  labium  has 
no  notch  in  the  middle. 


482 


ARTHROPODA. 


Sub  Order  I.  PLANIPENNIA.    Biting  mouth  parts.     SIALID.E,  wings 
well  developed,  mouth  not  rostrate,  larvae  aquatic,  commonly  called  dob- 


Fia.  511.— Corydalis  cornutus,*  hellgrammite,  male.    (From  Riley.) 


FIG.  512,—Phryganea  grandis.    (From  Schmarda.) 

sous.     Corydalis,*  hellgrammite ;    Stalls*     HEMEROBIID^,    lace  wings ; 
wings  well   developed,  mouth   not   rostrate  ;   larvae  with  sucking  mouth 


IV.   INSECTA  :  HEXAPODA,  COLEOPTEEA. 


483 


parts,  predaceous.  Chrysopa*  feeds  on  plant  lice  ;  Myrmeleo*  ant  lions; 
larvae  dig  funnel-like  pits  in  sand  and  capture  the  ants,  etc.,  which  fall 
into  them.  PANORPIDJE  (Mecoptera)  ;  mouth  prolonged  into  a  rostrum ; 
Panorpa,  *Bittacus* 

Sub  Order  II.  TRICHOPTERA  (caddis  flies).  Wings  usually  large ; 
mouth  parts  rudimentary,  forming  a  short  sucking  tube  which,  with  the 
wings  covered  with  hair-like  scales,  recalls  the  Lepidoptera  ;  larvae  aquatic 
with  tracheal  gills ;  build  cases  of  foreign  matter,  stones,  sticks,  etc.,  in 
which,  like  a  hermit  crab,  they  live ;  pupation  occurs  in  these  tubes. 
Phryganea  *  (fig.  512),  Hydropsyche* 

Order  V.  Strepsiptera. 

These  forms,  comprised  in  a  single  family,  STYLOPID.E,  are  parasitic  on 
the  Hymeiioptera.  The  six-legged  larvae  (fig.  513,  3}  press  in  between  the 
ventral  abdominal  plates  of  bees  or  wasps  and  pupate  there.  The  quickly 


FIG.  513.— Xenos  rossi.    (After  Boas.)    1,  female  :  «,  male  ;  3,  larva  ;   /-///»  thoracic 
somites ;  a1,  rudimentary  fore  wing  ;  a2,  hind  wing. 

flying  male  (2)  escapes  from  the  pupal  skin  ;  it  recalls  somewhat  a  beetle  ; 
has  small  fore  wings  and  large  hinder  ones.  The  wingless,  legless  female 
(1)  remains  in  the  pupal  skin  and  is  fertilized  there ;  she  is  viviparous. 
Insects  infested  with  these  parasites  are  '  stylopized.'  The  affinities  of 
the  order  are  doubtful.  The  forms  are  frequently  included  with  the 
beetles.  Stylops,*  Xenos.* 

Order  VI.  Coleoptera. 

The  beetles  are  the  highest  of  the  Hexapoda  with  biting  moutb 
parts.  They  are  closest  to  the  Orthoptera,  as  is  shown  by  the 
structure  of  mouth  parts  and  wings.  The  mandibles  are  strong; 
the  maxillae  (fig.  514)  have  lacinia  and  galea;  the  labium  consists, 
of  a  submentum  (often  called  mentum),  behind  which  the  rudi- 
mentary mentum  with  its  palpi,  paraglossae,  and  glossae  (the  latter 
frequently  fused  to  a  ligula)  are  retracted.  (In  the  genus 
Nemognatha  the  maxillary  galea  form  a  sucking  organ.)  The 
group  is  distinguished  from  the  Orthoptera  by  the  holometabolous 
development  with  pupae  liberae,  while  the  larvae  (fig.  500)  show 
many  modifications  corresponding  to  the  mode  of  lite.  Another 
character  is  afforded  by  the  wings.  The  anterior  pair,  separated 
at  the  base  by  a  scutellum,  are  hard  elytra  not  fitted  for  flight,  and 


484 


AETHROPODA. 


from  these  the  order  receives  its  name  Coleoptera,  sheath  wings. 
Tinder  the  elytra  are  protected  the  delicate  much  folded  hinder 
wings,  the  organs  of  flight  (lacking  in  insects  with  fused  elytra), 
the  second  and  third  thoracic  wings  and  those  of  the  abdo- 


FIG.  514.  FIG.  515. 

FIG.  514.— Maxilla  of  Procrustes  coriaceus.    c,  cardo  ;  le,  galea  ;  IL  lacinia  •  vm  nalrnis- 

st,  stipes. 
FIG.  515.— Calosoma  sycophanta.    (After  Ludwig-Leunis.) 

men  are  covered  by  the  elytra,  these  are  soft  above.  Externally 
the  relations  of  the  elytra  cause  a  regional  division  peculiar  to  the 
beetles  (fig.  515) :  head,  prothorax,  and  a  third  division  composed 
of  meso-  and  metathorax  plus  abdomen  covered  by  the  elytra. 

The  numerous  species  of  beetles— over  100,000  are  described— are  sub- 
divided into  normal  forms  and  Rhynchophora,  the  normal  forms  being 
subdivided  again  upon  characters  derived  from  the  tarsi  as  follows : 

Sub  Order  I.  PENTAMERA.  Tarsus  five-jointed,  the  last  club-shaped 
and  bearing  the  claws,  while  the  other  four  are  short  and  somewhat  heart- 
shaped  (fig.  516,  «).  This  is  the  largest  division  and 
contains  the  tiger  beetles  (CICINDELID^E)  and  the  pre- 
daceous  CARABIDJE  (fig.  515) ;  the  water  beetles,  HYDRO- 
PHILID.E  and  DYTISCIDJE  ;  the  LAMELLICORNIA  or  SCARA- 
BEID^,  represented  by  the  '  June  bugs,'  Melolontha*  and 
the  large  tropical  Dynastes ;  the  fire  flies,  LAMPYRIDTE  ; 
the  rove  beetles,  STAPHYLINIDJE,  etc. 

Sub  Order  II.  HETEROMERA.  First  and  second 
legs  pentamerous,  third  apparently  four-jointed  ;  few 
forms  belong  here,  among  them  the  '  oil  bottles '  (ME- 
LOID.E)  and  the  blister  beetles,  CANTHARID.E,  both  of 
them  containing  a  peculiar  substance,  cantharidin, 
which  renders  the  Spanish  flies,  Lytta  vesicatoria,  an 
important  ingedient  of  blistering  plasters.  Some  of  the 
TENEBRIONID^E  live  in  the  larval  stages  in  flour,  etc. 

Sub  Order   III.     TETRAMERA    (Cryptopentamera).     Tarsi   with   the 


FIG.  516.— a,  pen- 
tamerous tarsus 
of  Dytisciis ;  6, 
crypto  pentamer- 
ous tarsus  of 
Cocci  n  el  la  ;  t, 
tibia ;  *,  reduced 
tarsal  joint. 


IV.   INSECTA:  HEXAPODA,  HYMENOPTERA.  485 

penult  joint  rudimentary,   giving  the    impression   of  four  joints.     The 
families  included  here*-  very  numerous  in  species,  are  injurious  to  vegeta- 
tion.   The  larvae  of  the  long-horned  CERAMBYCID.E  bore  in  wood.     The 
CHRYSOMELID^E,  of  which  the  Colorado  potato  beetle  (Dory- 
phora    decemlineata)   is  the    most    notorious,    feed    on 
leaves. 

Sub  Order  IV.  TRIMERA  ;  tarsi  with  penult  and  anti- 
penult  joints  rudimentary,  so  that  they  appear  three- 
jointed.  Best  known  are  the  COCCINELLID^E,  or  lady  birds, 
whose  larvae,  because  of  their  destruction  of  plant  lice, 
etc.,  are  of  value  to  man. 

Sub  Order  V.  RHYNCHOPHORA,  snout  beetles  ;  head 
produced  into  a  long  snout,  at  the  apex  of  which  are  the 
mouth  parts.     Here  belong  several   families  of  weevils,    FIG  517.— 
some  of  which  do  damage  to  grain,    nuts,  timber,   etc.      U2Jel-nut**wee! 
Curculio*  Conotrachelus  *  Calandra*  Hylesinus*  Bala-      vil. 
ninus*  (fig.  517). 

Order  VII.  Hymenoptera. 

The  Hymenoptera,  of  which  bees,  wasps,  and  ants  are  well- 
known  representatives,  have  biting  jaws,  while  the  other  mouth 
parts  are  elongate  and  in  a  minority  of  the  group  converted  into  a 
sucking  organ.  In  the  bees  (fig.  48?)  the  glossa  unite,  producing 
a  nearly  closed  tube,  which  lies  in  a  sheath  formed  by  the  other 
mouth  parts,  the  mandibles  alone  retaining  the  primitive  form. 
Since  mouth  parts  vary,  the  structure  of  the  wings  and  body  seg- 
mentation have  great  value  in  defin- 
ing the  order.  The  wings  are  mem- 
branous and  are  supported  by  few 
nervures  (fig.  518),  and  in  flight  they 
act  as  one  pair,  since  the  two  are 
usually  connected  by  hooked  bristles 
on  the  hind  wing,  which  engage  in 
a  groove  on  the  hinder  margin  of 
the  front  wing.  The  fore  wings  are 
the  larger  and,  correspondingly,  the 
mesothorax  exceeds  the  other  tho- 
racic somites,  so  that  these,  especially 
FiG.5i8.-sir*r0/0a«,sawfiy.  (After  the  prothorax,  seem  but  parts  of  the 

strong  mesothorax.     Besides,  the  first 

abdominal  ring  unites  to  the  thorax  so  intimately  in  the  En- 
tophaga  and  Aculeata  as  to  seem  part  of  it.  The  constriction 
which  then  separates  thorax  and  abdomen  comes  between  the  first 
and  second  abdominal  somites,  and  when  the  second  (petiole)  is 
elongate  the  stalked  abdomen,  familiar  in  the  wasps,  results. 


486  AETHROPODA. 

The  sexes  are  readily  distinguished  by  the  genital  armature. 
The  female  is  provided  with  the  ovipositor  already  described,  which 
when  used  for  this  purpose  only,  projects  from  the  hinder  end  of 
the  body  (fig.  518),  but  when  used  as  a  sting  (aculeus),  can  be 
drawn  back  in  the  body  when  at  rest.  The  sting,  naturally  lacking 
in  the  male,  is  connected  with  a  poison  gland,  the  secretion  of 
which  owes  its  effect,  not,  as  once  believed,  to  formic  acid,  but  to 
a  little  known  basic  substance. 

The  distinction  between  ovipositor  (terebra)  and  aculeus  affords  chp.rac- 
tersof  systematicimportance;  othersare  furnished  by  the  development,  which 
is  always  holometabolous.  The  pupae,  in  all  important  points,  are  similar 
(pupae  liberae),  but  two  kinds  of  larvae  are  distinguished.  Some  have  larvae 
with  well-developed  legs ;  many  of  them  are  green  in  color  and  distin- 
guished from  the  larvae  of  Lepidoptera  by  the  greater  number  of  prolegs. 
Others  have  footless  larvae  (fig.  59).  The  first  occur  where  the  larva  must 
shift  for  itself,  the  second  where  it  is  surrounded  by  an  abundance  of 
food,  either  provided  by  the  parents  or  by  the  host  in  which  it  is  parasitic. 
Sub  Order  I.  TEREBRANTIA.  Terebra  present ;  larvae  with  feet  at 
least  on  the  thorax;  eggs  laid  on  leaves  or  in  wood,  usually  without  gall 
formation;  the  larvae  therefore  must  move  in  order  to  feed.  The  TEN- 
THREDINID^E,  saw  flies,  feed  on  leaves  and  have  caterpillar-like  larvae. 
Cimbex*  Nematusf  SIRICHLE  (Uroceridae),  horn  tails,  the  larvae  bore  in 
wood  and  are  whitish. 

Sub  Order  II.  ENTOPHAGA.  Terebra  present ;  larvae  legless,  para- 
sitic in  galls  or  in  animals.  Some  use  the  ovipositor  to  lay  their  eggs  in 
leaves,  roots,  or  stems  of  plants.  Galls  are  then  produced,  diseased  struc- 
tures by  which  the  larvae  are  nourished.  Others  use  the  ovipositor  to  lay 
their  eggs  on  or  in  other  insects  and  larvae.  The  young  feed  on  the  tissues 
of  the  host  and  at  last  cause  its  death,  often  before  the  completion  of  the 
metamorphosis.  The  gall-producing  forms  are  the  CYNIPHLE,  some  of  which 
afford  examples  of  heterogony  (p.  145),  in  which  the  alternating  generations 
are  distinguished  by  different  structure,  by  sexual 
and  parthenogenetic  reproduction,  and  by  different 
kinds  of  galls.  So  different  are  the  two  generations 
that  they  have  frequently  been  described  as  differ- 
ent genera.  The  inquilines  do  not  form  galls,  but  lay 
their  eggs  in  the  galls  of  other  species.  The  insect 
parasites  are  divided  among  several  families,  the  more 
prominent  being  the  ICHNEUMONID^,  BRACONID.E,  and 
CHALCIDID^E,  those  of  the  first  being  large,  the  others 

FIG  519  —  Chalets  flavi-  sma^  or  minute.  These  forms  are  of  immense  value 
pes*  (After  Howard.)  to  agriculture,  as  they  keep  down,  as  no  economic 
entomologists  or  insecticides  can,  injurious  forms. 

Sub  Order  III.  ACULEATA.  Females  with  stings  ;  larvae  footless, 
maggot-like.  The  digger  wasps  or  FOSSORES  excavate  tubes  in  the  earth  in 
which  they  lay  their  eggs  and  then  bring  into  the  holes  as  nourishment 


IV.   INSECTA:  HEXAPODA,  HTMENOPTERA.  487 

other  insects  which  they  have  paralyzed  by  a  sting  in  the  ventral  cord. 
Some  true  wasps  have  similar  habits.  Most  wasps  (VESPARLE)  and  bees 
(APIARLE)  have  different  habits.  They  bu/M  wonderful  homes  of  chewed 
wood  (the  first  pulp  paper)  or  skilfully  trimmed  leaves,  earth,  etc.,  or  of 
wax  which  the  animals  (bees)  secrete  between  the  joints  of  the  abdomen. 
The  nests,  which  are  to  contain  the  young,  are  either  small  tubes  or 
hexagonal  cells  which  are  united  to  horizontal  or  vertical  combs;  the  food 
is  either  honey,  pollen,  or  chewed  fruits.  The  fact  that  the  offspring  are 
better  protected  when  numerous  individuals  protect  them  has  apparently 
led  in  the  wasps  and  bees  to  different  grades  of  social  states.  The  honey 
bees  (Apis  mellifica*),  which  live  in  a  colony,  consist  of  three  kinds  of 
individuals  distinguished  by  structure  of  the  head  (fig.  520)  and  other 


FIG.  520.— Heads  of  Apis  mellifica.    (After  Boas.)    a,  queen;  b,  worker;  c,  drone  with 
the  compound  eyes  meeting  above. 

features  :  a  single  queen,  some  hundred  drones,  and  about  ten  thousand 
workers.  These  last  are  females  and  hence  have  stings,  but  have  rudi- 
mentary functionless  sexual  organs;  their  work  being  to  build  the  home, 
to  protect  the  young,  and  provide  food  for  the  winter.  The  business  of 
egg-laying  belongs  to  the  queen,  who  copulates  but  once,  at  the  beginning 
of  her  reign,  when  she  and  a  drone  take  a  wedding  flight.  For  the  four 
years  of  her  life  the  sperm  retains  its  vitality  in  the  receptaculum  seminis. 
In  laying  the  eggs  she  can  permit  entrance  or  not  of  the  spermatozoa  at 
will  and  thus  produce  males  or  females.  A  queen  who  has  not  been  fer- 
tilized or  who  has  exhausted  her  supply  can  only  lay  drone  eggs.  The 
further  fate  of  the  eggs  depends  upon  the  food  of  the  larvae;  with  a  small 
amount  of  bee  bread  (pollen)  workers  are  produced,  but  the  same  larva 
placed  in  a  larger  cell  and  fed  with  the  *  royal  jelly '  (much  like  blanc 
mange  in  appearance)  will  develop  into  a  sexually  mature  queen.  On 
escape  from  the  queen-cell  the  young  queen  with  a  part  of  the  colony 
swarm  and  start  a  new  colony.  This  operation  may  be  repeated  once  or 
twice,  but  if  there  be  danger  of  depleting  the  hive  the  remaining  queen 
Iarva3  are  killed.  Wasps  and  bumble-bee  colonies  last  but  a  year  and  are 
reformed  by  a  fertilized  female  which  has  lived  through  the  winter. 

The  ants  (FORMICARY)  have  gone  beyond  the  bees  in  the  social  organi- 
zation. They  have  also  departed  most  from  the  other  Hymenoptera  in 
that  the  workers,  sometimes  the  sexual  individuals,  are  wingless  and  the 
sting  is  rudimentary  or  entirely  lacking.  Only  the  Poneridee  sting  like 
bees  and  wasps  ;  the  others  bite  and  squirt  the  secretion  of  the  persistent 
poison  gland  (formic  acid)  into  the  wound.  The  homes  of  the  ants  are 


488 


ARTHROPODA. 


less  wonderful  than  those  of  the  bees,  but  their  social  organization  is  fre- 
quently more  complicated.  In  the  colony  occur  the  wingless  workers 
(rudimentary  females  with  wing  pads  in  larval  life  which  are  lost  in  pupa- 


FIG.  521. 


FIG.  522. 


FIG.  5Zl.—Myrmecocystus  melliger*  honey-sac  ant.    (Orig.) 

FIG.  522.— Plant  of  Hydnophyton-    (After  Forbes.)    Showing   the  bulb  occupied  by 
ants. 

tion),  and  of  these  frequently  there  are  different  kinds,  large-headed 
soldiers  and  small-headed  workers  ;  *  honey  sacs'  in  Myrmecocystus ;  and 
the  sexual  animals,  queens  and  drones,  which  copulate  in  a  marriage 
flight.  The  queen  after  the  flight  returns  to  the  colony.  Frequently 

other  insects,  like  the  Aphides,  are  con- 
nected with  the  colony,  these  being  kept 
for  the  honey  dew  they  produce.  Many 
ants  steal  the  pupae  of  others  and  keep 
the  adults  when  they  emerge  as  slaves. 
In  Polyergus  rufescens  this  has  gone  so 
far  that  the  masters  cannot  care  for 
themselves  and  must  be  fed  by  the  slaves  ; 
otherwise  they  die.  The  ants  possess 
extreme  interest  on  account  of  their  care- 
fully planned  wars  (Ecitons) ;  on  account 
of  their  relations  to  plants,  some  species 
making  nests  in  the  growing  plant  and 
protecting  it  by  their  bites ;  the  leaf- 
cutting  ants  take  leaves  from  trees  and 
carry  them  into  their  underground  nests 
for  the  cultivation  of  fungi  on  which 
they  feed  ;  the  agricultural  ants  from 
their  plantations  and  stores  of  grain,  and 
the  honey  ants  from  the  fact  that  certain 
FIG.  523.— Head  of  Cicada  septendedm,  workers  (fig.  521)  act  as  reservoirs  of 


the  mouth  parts  separated  (ori 
>nna;  e,  compound  eye;  /, 
bium  ;  wd,  mandible  ;  nix,  maxilla,  enormous  Size. 


a,  antenna;  e,  compound  eye;  S&  boney,  these  '  honey  sacs'  swelling  up  to 


IV.   INSECTA:   HEXAPODA,  RHYNCUOTA.  489 

Order  VIII.  Rhynchota. 

The  Khynchota,  or  bugs,  in  their  external  appearance  are 
nearest  to  the  Archiptera  and  Orthoptera.  The  head,  thorax,  and 
abdomen  are  joined  in  the  same  way;  the  development  is  hemi- 
metabolous,  and  in  the  wingless  species  ametabolous.  In  some 
cases,  as  the  Cicadas  with  their  membranous  wings,  the  confusion 
with  the  Orthoptera  has  led  to  these  being  called  locusts;  on  the 
other  hand  the  delicate-winged  Aphides  resemble  the  Archiptera. 
Yet  all  Rhynchota  may  be  recognized  by  the  sucking  proboscis 
(fig.  523),  consisting  of  the  grooved  labium  in  which  the  needle- 
like  mandibles  and  maxillae  play.  The  wing  structures  afford  the 
basis  of  division  into  three  sub  orders. 

Sub  Order  I.  HEMIPTERA  (Heteroptera).  Anterior  wings  hemelytra, 
i.e.,  leathery  at  the  base,  soft  and  elastic  at  the  tip  (fig.  524);  between  the 

6 


FIG.  524.— Pentatoma  rufipes.    (From  Hajek.)    s,  scutellum. 

hemelytra  is  a  conspicuous  triangular  scutellum  (s)  which  covers  more  or 
less  of  the  dorsal  surface.  Hemelytra  and  scutellum  occasionally  disap- 
pear. A  further  characteristic  is  the  presence  of  stink  glands,  producing 
a  most  disgusting  odor,  which  open  in  the  adults  ventrally  on  the  rneta- 
thorax;  in  the  larvae  dorsally  on  the  abdomen.  According  to  habits  the 
many  families  may  be  grouped  into  the  aquatic  HYDROCORES  and  the 
terrestrial  GEOCORES.  Of  the  first  the  BELOSTOMHXE  are  noticeable  from 
their  size,  Belostoma  americana*  being  nearly  2£  inches  long  and  capable 
of  inflicting  severe  wounds.  Other  families  are  NEPHXE  (Ranatra,  water 
scorpion),  NOTONECTID.E,  HYDROBATIDJS,  etc.  Of  the  Geocores  the  REDU- 
VIID.E,  which  feed  on  other  insects;  the  ACANTHIIDJE  (Acanthia  lectuaria* 
the  bed  bug);  the  LYGJEHXE,  containing  the  chinch  bug,  Blissus  leucop- 
terus*  so  injurious  to  grain;  and  the  PENTATOMID^E,  or  stink  bugs,  may  be 
mentioned. 

Sub  Order  II.  HOMOPTERA,  Wings,  when  not  degenerate,  similar  in 
texture  throughout,  although  often  differing  in  size.  They  are  either  parch- 
ment-like or  delicate  membranes.  The  CICADID^,  represented  by  Cicada 
septendecim*  the  seventeen-year  'locust,'  and  C.  tibicen*  or  dog-day  har- 
vest fly,  are  noticeable  from  their  shrill  notes,  produced  by  a  stridulating 
drum  on  the  abdomen.  C.  orni  of  the  Old  World  fig.  526)  punctures  ash 
trees,  causing  the  flow  of  manna.  The  CERCOPID.E  contains  the  spittle 
bug  (Apropliora  *)  which  causes  drops  of  foam  on  grass.  The  leaf  hoppers, 


490 


ARTHROPOD  A. 


FIG.  525.—  Cicada  septendecim*  seventeen-year  locust.    (From  Riley.)    a,  pupa;  b, 
case  from  which  the  imago,  c,  has  escaped;  d,  twig  bored  for  oviposition. 


pupa 


FIG.  526.— Cicada  orni.    (From  Schmarda.) 

or  JASSHXE,  contain  some  injurious  forms,  Erythronura  vitis*  damaging 
the  grape,  while  the  true  hoppers,  MEMBRACID.E  (fig.  527),  are  scarcely  less 
^  injurious.  None  of  these,  however,  are  such  serious 

pests  as  the  plant  lice  and  scale  insects.  In  the 
COCCID.E,  or  scale  insects,  the  wingless  female  dies 
after  laying  the  eggs  and  covers  them  with  her  dead 
scale-like  body.  Here  belong  the  cochineal  insects, 
Coccus  cacti,*  the  dried  bodies  of  which  furnish  the 
pigment  carmine,  and  the  lac  insects,  Coccus  lacca, 
as  well  as  a  host  of  injurious  forms,  like  the  orange 
scale,  Aspidotus  aurantii*  and  the  worse  San  Jose 
scale,  A.  pernieiosus*  which  has  recently  been  spread 
throughout  the  country.  The  APHID^E,  or  plant  lice, 
are  soft-skinned  and  with  their  honey-containing 
excrement  form  a  substratum  for  the  growth  of 

FIG.  "wt.-Ceresa  buba-  inJurious  fungi-     They  reproduce  largely  by  parthe- 
lus*  buffalo  leaf  hop-  nogenesis,  a  reason  for  their  rapid  multiplication ; 
Marlatt.)  . 


IV.  INSECTA:  HEXAPODA,  DIPTERA.  491 

.arous  females  are  wingless.    At  times  winged  females  appear  and  spread 

2 


FIG.     528. — Phylloxera   vastatrix.     (From   Ludwig  -  Leunis.)     1,  winged   generation; 
2,  grape  root,  with  nodosities  (a)  caused  by  Phylloxera ;  #,  wingless  root-generation. 

the  pests.  Winged  males  appear  in  the  autumn,  and  the  fertilized  eggs 
endure  the  winter.  Of  all  the  species  none 
is  more  injurious  than  the  Phylloxera  vasta- 
trix *  of  the  grape,  which  with  us  does  slight 
damage,  but  in  Europe  has  destroyed  whole 
vineyards.  This  is  one  of  our  returns  for  the 
many  pests  the  Old  World  has  sent  us. 

Sub  Order  III.  APTEKA.  Wingless  bugs 
with  direct  development,  commonly  known 
as  lice,  of  which  three  species  attack  man, 
one  living  in  the  hair  (Pediculus  capitis*), 
the  others  (P.  vestimentorum  *  and  Phthirius  FIG.  52d.—Phthirtus 


inguinalis*)  upon  the  body, 
live  on  other  mammals. 


Other  species 


crab  louse.    (After  Leuckart.) 


Order  IX.  Diptera. 

Like  the  Rhynchota,  the  Diptera,  or  flies,  are  sucking  insects, 
but  the  sucking  tube  or  haustellum  is  different,  here  consisting 
of  a  tube  formed  of  both  labium  and  labrum,  and  containing 
stylets  which  include,  besides  mandibles  and  maxillae  (often  rudi- 
mentary), the  hypopharynx  (fig.  488),  the  maxillary  palpi  being 
present.  Only  the  anterior  wings  (hence  Diptera)  are  well  de- 
veloped, the  hinder  wings  being  replaced  by  the  halteres  or  bal- 
ancers, small  drumstick-like  structures  richly  supplied  with  nerves 
and  functioning  as  organs  of  equilibration.  The  thorax  is,  as  in 


492 


ARTHROPODA. 


FIG.  530.—  Musca,  house  fly  (orig.). 

the  Hymenoptera,  sharply  marked  off  from  head  and  abdomen,  its 
somites  being  frequently  fused.  The  development  is  holometab- 
olous,  two  kinds  of  larvae  and  pupae  occurring 
in  its  course.  The  larvae  are  always  apodal,  but 
have  either  a  distinct  head  with  biting  mouth 
parts  or  they  are  headless  and  have  a  rudimen- 
tary sucking  apparatus  (fig.  531).  The  pupae 
are  correspondingly  either  free  with  powers  of 
motion,  or  are  pupae  coarctatae  (p.  474).  De- 
velopment thus  affords  characters  of  systematic 
importance,  and  these  are  supplemented  by  dif- 
ferences in  length  of  legs,  antennae,  haustellum, 
FIG.  531.  -Larva  of  and  in  body  form.  In  number  of  species  the 
:  Diptera  stand  next  to  the  Coleoptera;  in  num- 

ber of  individuals  they  far  exceed  them. 
Sub  Order  I.  NEMOCERA.  Elongate  with  long,  many-jointed  antenna', 
long  proboscis,  long  legs.  The  larvae  live  in  damp'places  or  in  water,  where, 
lacking  legs,  they  swim  by  movements  of  the  body.  The  pupae  can  also 
swim  well.  Best  known  are  the  innocuous  crane  flies  (TIPULID^E)  and  the 
mosquitos  (CuLiciD^E)  with  their  numerous  species  affecting  man,  among 
them  the  forms  which  carry  yellow  fever,  and  Anopheles,*  which  distribute 
malaria.  The  CECIDOMYID^E  include  the  injurious  Hessian  fly,  Cecidomyia 
destructor*  and  the  paedogenetic  Miastor  (fig.  498). 

Sub  Order  II.    TANYSTOMA.    Resemble  the  Muscariae  (with  which 


IV.   INSECTA:  HEXAPODA,  APHANIPTERA. 


493 


they  were  formerly  united)  in  the  short  stout  bodies,  short  antennae  and 
legs.  They  are  distinguished  from  them  and  resemble  the  Nemocera  in 
their  long  proboscis  and  in  development.  The  larvae  and  pupae  live  in 
damp  places  or  in  water  and  move  rapidly,  the  larvae  having  biting  mouth 
parts.  Here  belong  the  black  flies,  SIMULIID.E,  which  excel  the  mosquitos 
in  their  viciousness,  and  the  horse  flies,  TABANHLE,  the  females  of  which 
attack  cattle  and  men,  as  well  as  horses,  with  their  painful  bites. 

Sub  Order  III.  MUSCAKL/E  (Brachycera,  after  removal  of  Tanystoma). 
Body  short,  stout ;  antennae  three-jointed  with  a  bristle  (arista)  (fig. 
532) ;  legs  short,  ending  in  an  adhesive  organ  (pulvillus) ;  larvae  headless 


FIG.  532.  FIG.  533. 

FIG.  532.— Left,  Erax  bast  unit,  robber  fly  ;  right,  antenna  of  Muscid  showing  arista 

at  a. 
FIG.  533,—Gastrophilus  equi,*  bot  fly.    (From  Hajek.)   7i,  halteres. 

living  in  decaying  substances  or  parasitic  in  other  animals.  The  Mus- 
CID^E  include  the  house  ft\e&(Muscadomestica*  and  other  species),  the  blow 
fly  (Calliphora  vomitoria  *),  and  the  flesh  fly  (Sarcophaga  carnaria*),  which 
is  viviparous.  The  ASILID.E,  or  robber  flies,  prey  on  other  insects,  as  do 
some  of  the  SYRPHHLE  :  Eristfilis*  of  this  family  has  an  aquatic  'rat- 
tailed  larva,'  one  end  being  drawn  out  into  a  long  breathing  tube. 
<ESTRID^E,  bot  flies  ;  the  larvae  always  parasitic ;  those  of  the  sheep  bot 
((Estrus  ovis*)  in  the  frontal  sinuses  of  the  sheep,  causing  the  disease 
called  'staggers';  those  of  the  ox  warble  (Hypoderma  lineata*)  just 
beneath  the  skin  of  cattle  ;  those  of  the  horse  (GastropMlus  equi*  fig. 
533)  in  the  stomach  of  the  horse.  In  the  tropics  Dermatobia  noxidlis 
lives  MS  a  larva  in  the  human  skin. 

Sub  Order  IV.  PUPIPARA.  Very  active,  often  wingless  forms  living 
as  parasites  on  mammals  and  insects ;  larval  development  inside  the 
mother ;  pupation  occurring  soon  after  birth. 
Nelopliagus  ovinus,*  sheep  tick  ;  Braula  cceca* 
bee  louse. 

Order  X.  Aphaniptera  (Siphonaptera). 
In  spite  of  the  lack  of  wings  the  fleas  are 
closely  related  to   the    Diptera,  since  they 
have  doubtless  descended  from  winged  forms, 
as   is   shown  by  the  fact  that  they  have  a  Fm.  534.  —  Puiex  irritant* 

,     ,  ,11  -in  mi         i  flea-    (From  Blanchard.) 

holometabolous  development.      The   larvae, 

long  and  footless,  live  in  decaying  wood  or  dust  in  cracks  in  the 


494  ARTHROPODA. 

floor,  etc.,  and  give  rise  to  pupae,  both  without  traces  of  wings. 
Yet  fleas  and  flies  differ  in  that  the  fleas  have  similar  body  somites 
but  the  haustellum  is  lacking,  the  sucking  tube  being  formed 
of  labrum  and  mandibles,  while  the  sharp  maxillae  puncture  the 
skin.  Besides  Pulexirritans,*  the  flea  that  attacks  man,  many 
other  species  occur  on  other  animals.  In  warm  countries  the  jigger 
or  chigoe,  Sarcophsylla  penetrans,*  attacks  man,  the  female  boring 
into  the  skin,  usually  under  the  nails,  and  there  laying  the  eggs. 

Order  XI.  Lepidoptera. 

This  group  of  butterflies  and  moths  is  the  most  sharply  limited 
of  any  order  of  Hexapods.  The  wings,  both  pairs  of  which  are 
well  developed  (rarely  lacking,  as  in  many  female  Psych  idae  and 
some  Geometridaa),  are  covered  with  scales  (flattened  hairs),  and 
to  these  are  due  the  frequently  brilliant  color  patterns.  Frequently 
the  fore  and  hind  wings  are  united  by  hooks  (frenulum)  on  the 
latter,  engaging  in  a  retinacuhim  in  the  fore  wing.  The  mesothorax 
is  large  and  the  smaller  pro-  and  metathorax  are  closely  united  to 
it,  giving  the  region  a  distinctness  from  head  or  abdomen.  The 
mouth  parts  are  peculiar  (fig.  489),  although  foreshadowed  in  the 
Phryganids,  and  not  fully  developed  in  the  Microlepidoptera.  The 
mandibles  are  rudimentary  or  absent,  while  the  fused  maxillae, 
greatly  elongate,  form  the  proboscis.  Maxillary  and  labial  palpi 
are  present,  the  former  smaller  and  often  degenerate.  The  de- 
velopment is  holometabolous;  the  larvae,  frequently  called  cater- 
pillars (fig.  502),  have  biting  mouth  parts,  the  mandibles  very 
strong;  and  also  silk  glands  (sericteria),  a  pair  of  internal  organs 
which  open  together  on  the  labium  and  produce  a  secretion  hard- 
ening to  silk;  besides  the  thoracic  legs,  prolegs,  two  to  five  pairs, 
are  present.  The  pupae  are  usually  pupae  obtectae,  and  are  rarely 
free.  In  some  the  pupae  are  ornamented  with  golden  spots,  whence 
the  name  chrysalides  often  applied  to  them. 

Sub  Order  I.  MICROLEPIDOPTERA.  Small,  inconspicuous;  at  rest 
holding  the  wings  horizontally  over  the  back  ;  maxillary  palpi  very  large  ; 
proboscis  small.  TINEID.E  ;  the  larvae  form  a  tube  of  the  food  material 
which  they  carry  around  with  them.  Tinea  pellionella,*  the  clothes- 
moth.  ToRTRicnxE  ;  the  larvae  roll  leaves  into  a  tube.  Carpocapsa  pomo- 
nella*  the  codlin  moth,  the  larvae  infesting  apples. 

Sub  Order  II.  GEOMETRINA.  Moths  slender,  the  wings  in  pattern 
and  shape  recalling  those  of  butterflies,  but  held  horizontally  when  at 
rest ;  '  tongue '  (proboscis)  small ;  larvae  with  two,  rarely  three,  prolegs, 
known  as  span  or  measuring  worms  from  their  gait.  Species  numerous. 
Canker  worms  (Paleacrita  vernata,*  Alosophila  pometaria,*  females 
wingless),  Diastictis  ribearia*  currant  worm. 


IV.   INSEOTA:  HEXAPODA,  LEPIDOPTERA. 


495 


Sub  Order  III.  NOCTUINA.  Owlet  moths;   with  short  bodies;  fore 
wings  usually  gray  and  ornamented  by  two  spots  and  zigzag  lines  which 


FIG.  535. — Leucania  unipunctata^  army-worm  and  moth.    (From  Riley.) 

at  rest  cover  the  frequently  (as  in  Catocala  *)  brightly  colored  hind  wings. 
1800  species  in  United  States.  Hypenahumuli,*  hop  worm  ;  Aletia  argil- 
lacea*  cotton  worm  ;  Leucania  unipunctata*  army  worm  ;  cut  worms. 

Sub  Order  IV.  BOMBYCINA,  silk  worms.  Body  large,  wooly,  usually 
broad  dull-colored  wings ;  occasionally  lacking  in  the  females ;  proboscis 
frequently  rudimentary.  Antennae  long,  pectinate ;  larvae  hairy,  with, 
well-developed  spinning  powers.  Most  important  are  the  true  silk  worms 
(Bornbyx  mori*),  natives  of  China,  while  others,  like  Telea  polyphemus* 
furnish  silk  of  value.  Still  others  cause  great  damage  to  forest  trees, 
among  them  the  tent  caterpillars  (Clisiocampa*)  and  the  imported  gipsy 
moth  Ocneria  dispar  (fig.  72). 

Sub  Order  V.  SPHINGINA.  Body  long,  stout ;  fore  wings  long,  slender, 
hind  wings  shorter ;  proboscis  very  long ;  antennae  short ;  larvae  with  a 


FIG.  536. — Every  x  my  run.    (From  Riley.) 

caudal  spine.  Phlegetliontius  celeus,*  tomato  worm  ;  P.  Carolina*  tobacco 
worm.  Considerably  different  are  the  SESIID^E,  or  'clear  wings,'  which 
resemble  bees  and  wasps. 

Sub  Order  VI.    RHOPALOCERA,  butterflies.    Body  slender  ;   wings 
held  vertically  when  at  rest,  proboscis  long  ;  antennae  clubbed  at  the  tip  ; 


496 


ABTHROPODA. 


larvae  usually  spiny  ;  pupae  hung  by  a  thread,  never  a  cocoon.  Species 
numerous.  Vanessa  antiopa  *  lives  over  winter  ;  the  species  of  Pieris  * 
attack  cabbages,  etc.;  Papilio,*  swallow  tails. 

Class  V.  Diplopoda  (Chilognatha). 

The  Diplopoda  are  usually  united  with  the  Chilopoda  in  a 
group  of  Myriapoda;  but  while  they  agree  in  having  a  head  fol- 
lowed by  numerous  foot-bearing  segments,  they  differ  so  greatly 
that  no  union  is  possible.  The  body  is  nearly  cylindrical,  although 
in  the  Polydesmids  by  lateral  growth  it  may  be  flattened  above; 
the  legs  are  close  together  on  the  ventral  surface,  with  the  tracheal 
openings  near  them,  while  on  the  sides  of  the  body  are  other 
openings  of  defensive  glands,  the  foramina  repugnatoria. 


FIG.  537. 


FIG.  538. 


FIG.  537.— Schematic  section  of  Diplopod  (compare  with  fig.  481;.    d,  digestive  tract ; 

y,  gonad ;  /i,  heart ;  r,  repugnatorial  gland  ;  s,  spiracle  and  tracheae. 
FIG.  538.— Mouth  parts  of  lulus.    (After  Latzel.)    2,  mandibles  of  1.  molybdinus  ;  3, 

gnathochilarium  (fused  maxillae)  of  I.  luridus. 

A  more  marked  feature  is  that  each  segment  of  the  body  except 
the  first  four  or  five  bears  two  pairs  of  appendages,  which,  with  a 
similar  duplicity  in  chambers  of  the  heart,  tracheae,  ganglia,  etc., 
shows  that  a  fusion  has  occurred.  The  anterior  somites  bear  at 
most  but  a  single  pair  of  legs;  both  legs  and  antennae  are  short. 
The  head  bears,  besides  the  antennae,  but  two  pairs  of  appendages, 
a  pair  of  several- jointed  mandibles  (fig.  538),  and  a  pair  of  rudi- 
mentary maxillae  fused  to  a  single  plate,  the  gnathochilarium. 

The  gonads  lie  ventral  to  the  intestine  far  back  in  the  body,  those  of 
the  right  and  left  sides  enclosed  in  a  single  sac  ;  the  ducts  open  separately 
on  the  second  somite  of  the  trunk.  The  spermatozoa  are  not  motile.  The 
legs  of  the  seventh  segment  of  the  male  are  used  in  copulation.  The 


F.   DIPLOPODA.     SUMMAET  OF  IMPORTANT  FACTS.    497 

young  escape  from  the  egg  with  three  pairs  of  legs,  a  point  once  thought  to 
show  resemblances  to  the  Hexapoda,  but  which  does  not,  for  these  legs  are 


Fio.  539.— Hexapod  young  of  Strongylosoma.    (After  Metschnikoff.) 

on  the  fourth,  sixth,  and  seventh  somites  of  the  body.     The  IULID.E  have 
elongate  cylindrical  bodies  ;    Spirobolus*    GLOMERID^:  short,  capable  of 


FIG.  540.— lulus  maximus.     (After  Schmarda.) 

rolling  into  a  ball  ;  POLYDESMID.E,  with  wing-like  processes  to  the 
segments,  giving  them  a  flattened  appearance.  PAUROPODA  :  minute  ;  body 
with  twelve  segments,  tending  to  fuse  to  six.  Pauropus,*  Eurypauropus* 
More  uncertain  in  position  are  the  SYMPHYLA  (Scolopendrella  *),  but  from 
the  position  of  the  genital  opening  they  are  placed  here. 

Summary  of  Important  Facts. 

1.  The  ARTHROPODA  are  animals  with  evident  internal  and 
external  segmentation  (metamerism). 

2.  The  metamerism  is  expressed  internally  in  the  ladder-like 
nervous  system,  in  the  structure  of  the  heart,  and  in  the  arrange- 
ment of  segmental  organs  and  tracheae  so  far  as  these  are  present. 

3.  The  outer  segmentation  is  expressed  in  the  rings  of  the  chi- 
tinous  coat  of  the  body  as  well  as  in  the  metameric  arrangement 
of  the  appendages. 

4.  From  the  similarly  metameric  Annelida  the  Arthropoda  are 
distinguished  by  the  presence  of  jointed  appendages,  at  most  a 
pair  to  a  somite.     The  appendages  may  be  divided  according  to 
function  into  antennae,  jaws,  accessory  jaws,  feet,  and  swimmerets. 

5.  A  further  distinction  is  the  grouping  of  the  somites  into 
regions  of  which  usually  head,  thorax,  and  abdomen  are  recognized. 

6.  The  head  bears  the  tactile    and  eating  appendages;    the 


493  ARTHROPODA. 

thorax  those  used  in  locomotion  (pereiopoda),  the  abdomen  the 
swimmerets  (pleepoda),  or  it  lacks  appendages. 

7.  By  fusion  of  head  and  thorax  a  cephalothorax  is  produced; 
a  postabdomen  may  be  separated  from  the  abdomen. 

8.  The  eyes  are  either  ocelli  or  compound  eyes. 

9.  Hermaphroditism  is   rare;    reproduction  is  by  eggs;    fre- 
quently there  is  parthenogenesis,  rarely  paedogenesis.     The  eggs 
usually  have  a  superficial  segmentation. 

10.  The   Arthropoda    are    divided  into    Crustacea,   Acerata, 
Malacopoda,  Insecta,  and  Diplopoda. 

11.  The  CRUSTACEA  respire  by  gills;  they  usually  have  two 
pairs  of  antennae,  and  usually  biramous  feet;  the  reproductive  ducts 
open  near  the  middle  of  the  body. 

12.  The   Crustacea  are  divided  into    Trilobitae,    Phyllopoda, 
Copepoda,  Ostracoda,  Cirripedia,  and  Malacostraca. 

13.  Phyllopoda,  Copepoda,  Ostracoda,  and  Cirripedia  are  fre- 
quently called  Entomostraca;  they  have  a  shell  gland  and  the 
nauplius  as  a  larval  stage. 

14.  The  Trilobitm  are  extinct  forms  with  one  pair  of  antennae, 
and  the  body  divided  by  longitudinal  grooves  into  three  regions. 

15.  The  Phyllopoda  have  variable  segments  and  primitive  leaf- 
like  feet  recalling  the  parapodia  of  the  annelids. 

16.  The  Copepoda  are  without  shells  and  have  biramous  feet. 

17.  The  Ostracoda  have  reduced  bodies  enclosed  in  a  bivalve 
shell. 

18.  The  Cirripedia  are  usually  hermaphroditic  and  are  sessile. 

19.  The  Malacostraca  have  20  (21)  segments,  of  which  7  (8) 
are  abdominal;  the  male  sexual  openings  are  on  the  13th,  the 
female  on  the  llth,  segment;  the  excretory  organ  is  the  antennal 
gland;  the  larva  is  a  zoea,  rarely  a  nauplius. 

20.  The  Malacostraca  are  divided  into  Leptostraca,  Thoracos- 
traca,  and  Arthrostraca. 

21.  The  Leptostraca  have  twenty-one  somites;  they  are  closely 
related  to  the  Phyllopoda. 

22.  The  Thoracostraca  or  Podophthalmia  (Schizopoda,  Stoma- 
topoda,  Decapoda)  have  stalked  eyes  and  some  or  all  of  the  tho- 
racic somites  fused  with  the  head  to  a  cephalothorax. 

23.  The  Arthrostraca  or  Edriophthalmia  have  sessile  eyes  and 
have  seven  free  thoracic  segments.     They  are  divided  into  Am- 
phipoda  and  Isopoda. 

24.  The  ACERATA  lack  antennae;  the  body  is  divided  into  ceph 
alothorax  and    abdomen;  the   cephalothorax   bears   six   pairs  oi 


F.   ARTHROPODA,  SUMMARY  OF  IMPORTANT  FACTS.  499 

appendages;  the  genital  ducts  open  on  the  seventh  somite;  the 
respiratory  organs — gills,  lungs,  or  trachea — develop  from  the 
abdominal  appendages. 

25.  The  Acerata  are  divided  into  Gigantostraca  and  Arachnida. 

26.  The   Gigantostraca  are  large,  and  breathe  by  gills.     The 
only  living  forms  are  Xiphosures. 

27.  The  Arachnida  breathe  by  lungs  or  by  tracheae  derived 
from  lungs,  the  openings  to  which  are  on  the  abdomen ;  they  have 
a  pair  of  chelicene,  a  pair  of  pedipalpi,  and  four  pairs  of  legs ;  they 
have  in  addition  several  pairs  of  highly  developed  ocelli. 

28.  The  Arachnida  are  divided  into  nine  orders:   Scorpionida, 
Phrynoidea,   Microthelyphonida,   Solpugida,   Pseudoscorpii :  Pha- 
langida,  Araneina,  Acarina,  and  Linguatulida. 

29.  The  Scorpionida  have  chelate  pedipalpi  and  a  postabdomen. 
terminated  by  a  sting. 

30.  The  Phrynoidea  have  the  first  pair  of  legs  tactile  and  not 
used  in  walking,  and  a  continuous  cephalothorax. 

31.  The   Microthelyphonida   and   the    Solpugida    have    three 
'  thoracic '  segments  free.     The  Microthelyphonida  have  a  long, 
jointed  postabdomen,  lacking  in  the  Solpugida. 

32.  The  Pseudoscorpii  resemble  the  Scorpionida,  but  lack  the 
postabdomen  and  sting. 

33.  The  Phalangida  have  very  long  legs  and  spider-like  bodies. 

34.  The  Araneina  have  an  unsegmented  abdomen,  bearing  four 
or  six  spinnerets  and  numerous  spinning  glands.     They  are  divided 
into  Tetrapneumones,  with  four  lungs,  and  Dipneumones,  with 
two  lungs  and  two  tracheae. 

35.  The  Acarina  have  cephalothorax  and  abdomen  fused  and 
the  mouth  parts  for  sucking.     Several  species  are  parasitic  on  man. 

36.  The  Linguatulida  are  complete  parasites,  ribbon-like  and 
without  legs;  the  young  live  in  the  lungs  and  liver. 

37.  The  Tardigrada  and  Pycnogonida  agree  with  the  Arachnida 
in  the  number  of  walking  legs.     Their  position  is  very  uncertain. 

38.  The  MALA  co  POD  A  are  intermediate  between  Annelida  and 
Insecta.     They  have  indistinctly  segmented  bodies  with  parapodia- 
like  feet,  segmental  organs,  and  tracheae. 

39.  The  INSECTA  breathe  by  tracheae;  the  head  bears  four  pairs 
of  appendages:     antennae,     mandibles,     maxillae,    labium;    since 
tracheae  are  present  the  circulatory  system  is  reduced;  the  repro- 
ductive organs  open  at  the  hind  end  of  the  body. 

40.  The  Insecta  are  divided  into  Chilopoda  and  Hexapoda. 

41.  The  Chilopoda  have  numerous  body  segments  with  a  pair 


500  ARTHROPODA. 

of  long  legs  on  each;  close  behind  the  head  are  a  pair  of  poison 
feet. 

42.  The  Hexapoda  have  the  body  divided  into  head,  thorax, 
and  abdomen. 

43.  The  abdomen  consists  of  a  varying  number  of  somites  and 
lacks  appendages. 

44.  The  thorax  consists  of  three  segments,  pro-,  meso-,  and 
metathorax,  each  bearing  a  pair  of  legs,  and  meso-  and  metathorax 
usually  a  pair  of  wings  each. 

45.  The  head  bears,  besides  three  pairs  of  mouth  parts,  an  un- 
paired upper  lip  (labrum)  and  two  compound  eyes,  besides  usually 
one  to  three  ocelli. 

46.  The  structure  of  the  mouth  parts  varies  with  the  food; 
they  are  either  biting,  licking  and  sucking,  or  piercing  in  function. 

47.  Wingless  insects  usually  have  a  direct  (ametabolous)  de- 
velopment with  numerous  ecdyses. 

48.  Winged  insects  (and  many  without  wings  which  have  de- 
scended from  winged  forms)  have  a  metamorphosis  in  which  the 
larva  differs  more  or  less  from  the  imago  (metabolous  insects) ;  the 
larva  never  has  wings. 

49.  An  incomplete  metamorphosis  (hemimetabolous  develop- 
ment) occurs  when  the  larva  with  each  molt  becomes  more  like 
the  adult,  developing  wing  pads  which  with  each  ecdysis  become 
larger. 

50.  In  complete  metamorphosis  (holometabolous  development) 
the  changes  occur  in  the  last  molting  stage,  which  is  a  stage  of 
rest,  the  pupa. 

51.  Classification  of  Hexapoda  is  based  upon  structure  of  mouth 
parts  and  wings  as  well  as  upon  regional  relations  and  development. 

52.  The  Apterygota  are  wingless,  ametabolous  Hexapoda  with 
biting  mouth  parts. 

53.  The  Archiptera  have  biting  mouth  parts  with  incompletely 
fused  labium,  net-veined  wings,  and  incomplete  metamorphosis. 

54.  The  Orthoptera  resemble  the  Archiptera  in  mouth  parts 
and  development,  but  have  parchment -like  wings. 

55.  The  Neuroptcra  have  net-veined  wings  and  a  holometabolous 
development ;  'the  mouth  parts  are  modified. 

56.  The    Coleoptera   are   biting   insects  with  the   fore   wings 
changed  to  elytra;  they  differ  from  the  somewhat  similar  Orthop- 
tera  by  the  complete  metamorphosis. 

57.  The  Strepsiptera  are  parasitic  forms  allied  to  the  Coleoptera. 

58.  The  Hytnenoptera  have  partly  biting,  partly  licking  mouth 


CHORDATA.  501 

parts;  membranous  wings  with  few  nervures  and  holometabolous 
development. 

59.  The  Rhynchota  are  hemimetabolous  or  ametabolous,  with 
piercing  mouth  parts;  the  bed  bugs  and  the  Pediculina  are  parasitic. 

60.  The  Dipt  era  are  holometabolous,  with  piercing  mouth  parts 
and  not  more  than  one  pair  of  wings.     The  larvae  of  the  (Estridae 
are  parasitic. 

61.  The  Aphaniptera  are  holometabolous,  wingless,  parasitic, 
with  sucking  mouth  parts. 

62.  The  Lepidoptera  have  the  wings  covered  with  scales;  labium 
and  labrum  rudimentary,  the  maxillae  altered  to  a  sucking  tube; 
the  development  holometabolous. 

63.  The  DIPLOPODA  have  a  head  with  three  pairs  of  appendages; 
the  trunk  with  double  segments,  each  bearing  two  pairs  of  legs, 
the  genital  openings  anterior. 

64.  The  term  Myriapoda  is  frequently  used  to  include  Chilop- 
oda  and  Diplopoda. 

PHYLUM  VIII.    CHOKDATA. 

Within  recent  years  it  has  been  realized  that  a  number  of  ani- 
mals, formerly  distributed  among  various  groups,  possess  structural 
features  of  great  importance  which  ally  them  to  the  vertebrates. 
On  the  other  hand  they  lack  the  vertebrae  and  many  other  features 
characteristic  of  that  ^roup,  so  that  the  name  cannot  be  extended 
to  include  them.  Yet  since  all  these  forms  possess,  as  a  temporary 
or  a  permanent  feature,  a  structure  known  as  the  chorda  dorsalis 
or  notochord,  the  term  Chordata  has  been  adopted  to  include  them. 

The  notchord  is  a  smooth  elastic  rod  arising,  in  development, 
from  the  entoderm  and  coming  to  lie  between  the  digestive  tract 
and  the  nervous  system  (fig.  9).  In  all  Chordates  the  anterior 
(pharyngeal)  portion  of  the  alimentary  canal  develops  one  or 
more  pairs  of  pockets  which  grow  outwards  and  fuse  with  the 
ectoderm.  The  fused  portion  then  breaks  through,  and  the  pock- 
ets become  converted  into  gill  slits  (branchial  clefts),  which,  in 
the  lower  forms,  allow  the  passage  of  water  over  the  gills  which 
liue  the  slits. 

The  central  nervous  system  lies  on  one  side  of  the  alimentary 
canal,  there  being  no  ring  of  nervous  matter  (Enteropneusta  ex- 
cepted)  around  the  oesophagus,  such  as  is  so  common  in  the  in- 
vert ebrata.  It  arises  as  a  medullary  plate  on  the  dorsal  side  of  the 
body  around  the  blastopore.  The  edges  of  this  plate  are  rolled 


502 


CHORD  ATA. 


inwards,  converting  the  plate  into  a  tube  with  nervous  walls  and 
a  central  canal.  From  this,  as  will  readily  be  seen,  it  happens, 
when  the  blastopore  remains  open  behind  (fig.  547,  ne),  that  a 
temporary  communication,  the  neurenteric  canal,  exists  between 
the  neural  and  alimentary  canals. 

On  the  other  hand  the  chordates  share  with  the  annelids  and 
arthropods  a  segmentation  of  the  body  which,  however,  is  internal 
and  only  exceptionally  is  visible  from  the  surface. 

The  Chordates  include  the  Leptocardii,  the  Tunicata,  doubt- 
fully a  group  of  Enteropneusta,  and  the  Vertebrata. 

SUB  PHYLUM  I.  LEPTOCARDII  (CEPHALOCHORDIA,  ACRANIA). 

Until  recently  but  a  single  genus  (Amphioxus)  was  recognized 
as  belonging  to  this  group,  and  this  form,  known  for  over  a  hun- 
dred years,  was  at  first  described  as  a  mollusc  (Limax  lanceolatus). 
Its  chordate  nature  was  first  recognized  by  Johannes  Muller, 
while  the  embryological  researches  of  Kowalewsky  showed  its  close 
relations  to  the  Tunicata. 

In  structure  it  is  comparatively  simple.  The  fish-like  body, 
pointed  at  either  end  (whence  Amphioxus),  lacks  paired  appendages, 
but  has  a  median  fold  or  fin  best  developed  at  the  caudal  end.  The 
epithelium  covering  the  body  is  but  a  single  cell  in  thickness  and 


FIG.  541.— Amphioxus  lanceolatus.  (Diagram  after  Boveri.)  a,  anus  ;  nu,  eye  ;  />,  peri- 
branchial  space;  c,  notochord  ;  0,  gonads ;  /,  liver;  m,"  muscles ;  ?i,  nephridia  ;  o, 
mouth  ;  p,  atrial  opening  ;  r,  spinal  cord  ;  sp,  gill  slits. 

allows  the  underlying  muscle  segments  to  show  through.  It  differs 
from  the  fishes  in  lack  of  skull  (Acrania),  vertebrae,  brain,  heart, 
and  kidneys,  although  the  rudiments  of  brain  and  excretory  organs 
are  present.  Connective  tissue  is  almost  entirely  absent,  the  body 
consisting  of  much-folded  epithelia  separated  by  thin  gelatinous 
layers. 

An  axial  skeleton  is  present  in  the  notochord,  which  extends  the 
whole  length  of  the  body  (fig.  541,  c).  Above  it  lies  the  spinal 
cord,  with  a  central  canal,  which  expands  in  front  into  a  rudi- 
mentary cerebral  vesicle.  A  pigment  spot  in  this  brain  is  the 


/.   LEPTOCARDIL 


503 


primitive  eye,  but  other  places  are  sensitive  to  light.  The  olfac- 
tory organ  is  an  unpaired  pit  on  the  anterior  end  of  the  body; 
and  at  its  bottom,  in  the  young,  is  an  opening,  the  anterior  neu- 
ropore,  which  leads  into  the  anterior  end  of  the  neural  canal.  It 
is  a  point  of  delayed  closure  of  the  embryonic  medullary  folds. 

Of  the  alimentary  tract  more  than  a  third  is  occupied  by  the 
pharynx  with  the  gill  slits.  It  begins  with  an  oval  mouth,  sur- 
rounded by  cirri,  and  is  perforated  by  numerous  gill  slits,  be- 
tween which  elastic  gill  arches  form  a  support  for  the  walls  (fig. 
542,  kb).  In  the  young  the  gill  slits  open  directly  to  the  anterior, 
but  later,  somewhat  as  in  Tunicata,  into  a  peribranchial  chamber 


FIG.  542.— Section  of  the  gill  region  of  Amphioms.  (After  Lankester  and  Boveri.) 
a,  aorta  descendens;  6,  peribranchial  space  ;  r,  notochord  ;  co,  ccelom  (branchial 
body  cavity) ;  e,  hypobranchial  groove,  beneath  it  the  aorta  ascendens ;  g,  gonad; 
kb,  gill  arches:  fcd,  pharynx;  I,  liver ;  m,  muscles;  n,  nephridia,  on  the  left  with 
an  arrow;  ?•,  spinal  cord  ;  *n,  spinal  nerve  ;  sp,  gill  slit. 

{#)  which  allows  the  escape  of  the  water  through  a  porus  branchialis 
(fig.  541,  JP),  behind  the  middle  of  the  body.  On  the  ventral  floor 
of  the  pharynx  is  a  ciliated  hypobranchial  groove  (fig.  542,  e),  the 
homologue  of  the  tunicate  endostyle  and  of  part  of  the  vertebrate 
thyroid.  It  leads  back  to  the  straight  digestive  tract  which  opens 
on  the  left  side  near  the  end  of  the  body,  and  bears  in  front  a 
blind  liver  sac  which  extends  forward  into  the  gill  region  (figs. 


504  CHOEDATA. 

541,  542,  I).  The  vascular  system,  with  colorless  blood,  consists 
of  a  dorsal  arterial  (a)  and  a  ventral  venous  trunk  connected  by 
]ateral  loops  or  arches.  The  ventral  trunk  begins  as  a  subintestinal 
vein  under  the  intestine,  branches  as  a  portal  vein  over  the  liver, 
and,  reuniting  again  in  a  ventral  vessel,  continues  forward,  as  the 
aorta  ascendens,  below  the  gills.  From  this  the  vascular  arches — 
gill  arteries — pass  up  between  the  gill  slits  and  form  the  dorsal 
vessel,  the  aorta  descendens.  A  true  heart  is  lacking,  but  various 
parts  of  the  vessels — a  part  of  the  ventral  trunk  and  the  bases 'of 
the  gill  arteries — are  contractile,  whence  the  name  Leptocardii. 

As  the  pharynx  lies  in  the  peribranchial  chamber,  the  digestive 
portion  of  the  tract  lies  in  a  true  body  cavity  or  ccelom,  which  ex- 
tends forward  (fig.  542,  co)  into  the  branchial  region  as  well  as 
into  the  gill-walls  (branchial  coalom)  and  into  the  outer  walls  of 
the  peribranchial  chamber  (peribranchial  coelom).  In  the  peri- 
branchial  ccelornare  the  gonads  (g),  a  series  of  pouch-like  cell  fol- 
licles which,  by  dehiscence,  allow  their  products  to  escape  into  the 
peribranchial  chamber.  Into  this  chamber  also  empty  the  excre- 
tory organs  which  were  long  sought  for  in  vain.  These  are  (n)  a 
series,  on  right  and  left  sides,  of  ciliated  canals  apparently  cor- 
responding to  the  pronephros  of  the  vertebrates.  Each  canal 
begins  with  at  least  one  ciliated  nephrostome  in  the  coelom  and 
opens  separately  like  an  annelid  nephridium. 

Like  the  structure,  the  development  is  comparatively  simple.  The 
following  points  deserve  special  mention  :  (1)  The  eggs  have  a  nearly 
equal  segmentation  (fig.  96).  (2)  A  typical  invaginate  gastrula  (fig.  105) 
occurs.  (3)  The  mesoderm  arises  as  a  series  of  pouches,  right  and  left, 
from  the  mesenteron,  which  later  separate  and  represent  the  primitive 
segments.  Hence  these  are  clearly  mesothelial  in  nature.  From  the  cavi- 
ties of  these  arises  the  body  cavity,  which  is  consequently  an  enteroccele. 
(4)  The  dorsal  surface  of  the  entoderm  between  these  coelomic  pouches 
becomes  folded  off  from  the  rest  and  forms  the  notochord,  which  lies 
between  the  digestive  tract  and  the  nervous  system.  (5)  The  nervous 
system  arises  from  a  longitudinal  groove  which  becomes  folded  into  a 
tube  and  is  connected  for  a  while  with  the  digestive  tract  by  a  neuren- 
teric  canal. 

Amphioxus  *  contains  a  few  closely  related  species  which  occur  on  our 
southeastern  coast,  in  Europe,  Indian  Ocean.  Recently  other  genera  have 
been  described — Asymmetron  *  in  America,  Heteropleuron  in  the  South 
Seas.  The  animals  live  in  quiet  bays  and  bury  themselves  in  the  sand, 
with  only  the  mouth  above  the  surface.  Like  all  animals  with  rudimen- 
tary eyes,  they  shun  the  light  and  are  greatly  excited  by  strong  illumi- 
nation. 


I/.    TUNIC  AT  A. 


505 


SUB  PHYLUM  II.   TUNICATA  (UROCHORDA). 

In  their  adult  condition  the  Tunicata,  or  sea-squirts,  bear  some 
resemblance  to  the  siphonate  mollusca,  especially  in  the  posses- 
sion of  incurrent  and  excurrent  orifices,  usually  close  together, 
and  a  mantle.  Hence  these  forms  were  long  associated  with  the 


FIG.  543.— Diagram  of  a  tunicate  (orig.).  a,  atrium ;  6,  nervous  ganglion ;  e,  endo- 
style ;  i,  intestine;  m,  mouth;  n,  subneural  gland;  s,  stomach;  /,  tunic.  In  the 
centre  the  branchial  basket  with  the  gill  slits  communicating  with  the  peri- 
branchial  space,  and  this  in  turn  with  the  atrium. 

molluscs ;  later  they  were  associated  with  the  worms,  but  their  de- 
velopment shows  them  to  be  more  nearly  related  to  the  vertebrates. 
The  group  owes  its  name  to  the  tunic  or  mantle — lacking  in  the 
Copelatse — an  envelope  (fig.  543,  t)  which  is  formed  like  a  cuticle 
by  the  epithelium  of  the  skin,  but  which  is  distinguished  from 
ordinary  cuticula  by  its  structure.  It  much  resembles  connective 
tissue  in  that  cells  from  the  mesoderm  wander  into  the  ground 
substance,  which  is  sometimes  fibrous,  sometimes  homogeneous, 
and  has  an  interesting  chemical  nature.  It  consists  of  the  same 
proportions  of  carbon,  oxygen,  and  hydrogen  (C6H1006)  as  cellulose 
and  agrees  with  this  substance,  so  characteristic  of  plants,  in  its 
reactions  (blue  color  with  iodine-iodide  of  potassium  and  sulphuric 


506  CHORDATA. 

acid,  violet  with  chloriodide  of  zinc).  Nowhere  else  among  ani- 
mals is  there  such  a  rich  formation  of  cellulose. 

The  anterior  part  of  the  digestive  tract  is  modified  into  a 
pharynx  or  branchial  chamber,  the  walls  of  which  become  per- 
forated with  a  varying  number  of  gill  slits,  these  leading  either 
directly  to  the  exterior  or,  more  usually,  into  a  peribranchial 
chamber,  and  from  this  to  a  cloaca  or  atrium  (a),  before  reaching 
the  outside  world.  While  the  respiratory  water  passes  through 
the  gill  slits  the  food  particles  which  it  contains  are  received  by  a 
ring-shaped  ciliated  band  (peripharyngeal  band)  and,  enveloped 
by  mucus,  are  ]ed  to  the  oesophagus.  This  mucus  is  formed  by  a 
ciliated  glandular  groove,  the  endostyle  (e),  on  the  ventral  surface 
of  the  pharynx. 

Between  the  gill  region  (end  of  the  endostyle)  and  the  stomach 
lies  the  ventral  heart  enclosed  in  a  pericardium.  It  has  the 
peculiarity,  met  nowhere  else,  of  changing  the  direction  of  its 
contractions  at  frequent  intervals;  after  the  heart  has  driven  the 
blood  for  a  time  to  the  gills  it  rests  a  while  and  then  begins  to 
force  the  blood  in  the  opposite  direction,  pumping  it  from  the 
gills  and  sending  it  towards  the  stomach. 

If  we  add  to  the  foregoing  that  a  dorsal  ganglion  and  a  her- 
maphroditic gonad  are  present,  the  striking  features  of  the  group 
are  enumerated.  The  extreme  forms,  the  Copelatae  and  the 
Thaliacea,  are  rather  remote,  but  they  are  connected  by  interme- 
diate forms,  the  Ascidiae  and  Pyrosomas. 

Order  I.  Copelatae. 

These  small  forms — one  or  a  few  centimeters  in  length — are 
pelagic;  they  have  the  anterior  end  inserted  in  a  gelatinous  envelope 
or  'house'  which  replaces  the  lacking  tunic.  They  swim  like  a 
tadpole  by  means  of  a  tail  which  arises  from  the  hinder  end  of 
the  trunk.  The  alimentary  canal  (fig.  544)  is  bent  on  itself,  and 
both  it  and  the  two  large  gill  slits,  in  contrast  to  all  other  tuni- 
cates,  open  directly  to  the  exterior.  The  heart  (lacking  only  in 
the  Kowalewskidae)  is  ventral  and  the  hermaphroditic  gonads  and 
the  nervous  system  dorsal.  The  latter  consists  of  a  cerebral 
ganglion,  with  beside  it  an  extremely  simple  auditory  organ  and  a 
ciliated  groove,  and  farther  a  chain  of  ganglia  extending  into  the 
tail.  The  notochord,  a  gelatinous  structure  enclosed  by  a  sheath 
of  cells,  forms  the  skeletal  axis  of  the  tail  ventral  to  the  nerve  cord 
and  gives  attachment  to  muscles.  Oikopleura,*  Appendicularia,* 
Fritillaria;  Kowalewskia. 


II.    TUNIC  ATA:    COPELAT^. 


507 


FIG.  544.— Oikopleura  cophocerca.  (After  Fol.).  A,  the  whole  animal,  removed  from  its 
'house,'  dorsal  view;  B,  body,  side  view  with  base  of  tail.  «,  anus  ;  c,  notochord; 
a',  branchial  region  ;  d",  stomach  ;  en,  endostyle ;  /,  ciliated  peripharyngeal 
bands;  0,  g',  brain  and  first  ganglion  of  tail;  7i,  testis;  m,  mouth;  o,  ov,  ovary; 
s,  gill  slits. 


B 


FIG.  545.— Ciona  intestinalis.  A,  from  the  left  side,  the  cellulose  tunic  and  dermal 
muscular  sac  removed ;  B,  from  the  right  side,  the  tunic  entirely  removed,  pharynx 
opened  from  the  mouth,  a,  anus ;  c,  cellulose  tunic  below  with  adhesive  processes ; 
d,  cloaca;  d,  rectum;  e,  atrial  opening;  en,  endostyle  ending  above  in  the  peri- 
pharyngeal band;  g,  ganglion;  7i,  mouth  of  the  'hypophysis';  hr,  heart,  with  peri- 
cardium ;  ho,  branched  testes ;  i.  oral  opening;  /c,  gill  sac  ;  m,  muscular  sac ;  oe, 
oesophagus ;  orf,  oviduct,  the  black  line  beside  it  the  vas  deferens ;  ov,  ovary ;  s, 
partition  between  atrium  and  body  cavity  ;  st,  stomach  ;  t,  crown  of  tentacles. 


508  CHORDATA. 

Order  II.  Tethyoidea  (Ascidiaeformes). 

With  the  exception  of  the  pelagic  Pyrosomidae  all  of  the  ascidi- 
ans  are  attached  to  rocks,  etc.,  in  the  sea.  The  greater  necessity 
for  protection  caused  by  this  sedentary  life  has  resulted  in  a  great 
development  of  the  cellulose  tunic,  which,  enveloping  the  internal 
organs,  gives  these  animals  a  swollen,  somewhat  shapeless  appear- 
ance. Two  openings,  mouth  and  atrial  opening,  lead  into  the 
interior,  and  the  water  which  issues  from  these,  when  the  animals 
are  taken  from  the  ocean,  has  given  them  the  common  name  of 
*  sea-squirts/ 

On  removing  the  tunic,  which  is  but  slightly  attached  to  the 
other  parts  except  at  mouth  and  atrial  opening,  a  muscular  sac  is 
seen  (fig.  545),  the  fibres  running  circularly  and  longitudinally.  In- 
side this  sac  are  the  viscera,  the  pharyngeal  region  by  far  the  most 
conspicuous.  The  mouth  leads  to  a  short  tube  with  tentacles  (/), 
and  then  to  the  pharynx,  a  wide  sac  which 
lies  in  a  large  cavity,  the  peribranchial 

fl  ^^f^WW^      "~\   chamDer>  the  walls  of  the   pharynx  and 
!  '  vliiff]  M  n  fl  )     ^^  ^e  enclosing  space  uniting  on  the  ventral 
U  liMJlMMJI  I          side  (fig.  543).     The  pharyngeal  walls  are 
perforated  like   a   net   by  small   ciliated 
gill   slits,  arranged   in   longitudinal   and 
transverse  rows  (fig.  546),  through  which 
the  water  received  from  the  mouth  passes 
into  the  peribranchial  chamber  and  thence 

FlG.  546.— dona  intestinalis,  a 

bit  of  the  wall  of  the  gill  sac  to  tiie  atrium,  and  so  out  to  the  external 

enlarged  to   show  the   gill  ,  , 

slits.  world. 

While  the  respiratory  water  thus  passes  out  in  a  nearly  direct 
course,  the  food  particles  which  it  contains  pass  into  the  digestive 
tract.  By  means  of  a  ciliated  tract  (peripharyngeal  band)  just 
inside  of  the  tentacles  and  surrounded  by  mucus  secreted  by  the 
endostyle  (or  hypobranchial  groove),  the  food  is  carried  back  to 
the  oesophagus  (oe)  at  the  base  of  the  gill  chamber,  and  thence  to 
the  stomach  (usually  provided  with  liver  glands),  and  on  to  the 
intestine.  The  anus  is  at  the  base  of  the  special  portion  of  the 
peribranchial  chamber,  which  also  receives  the  genital  ducts  and 
hence  is  known  as  the  cloaca  or  atrium. 

In  the  body  cavity,  which  is  greatly  reduced  in  the  species 
with  concentrated  bodies,  occur  the  digestive  tract,  the  sexual 
organs,  and  the  heart;  the  latter,  frequently  S-shaped,  extends  be- 
tween the  stomach  and  the  endostyle.  Opposite  to  the  endostyle 


//.    TUNIC  AT  A:    TETHTOIDEA. 


509 


is  the  ganglion  in  the  dorsal  wall  between  oral  and  atrial  open- 
ings. Below  it  (rarely  above  it)  is  a  branched  subneural  gland 
which,  from  its  relations  and  its  opening  into  the  prebranchial 
part  of  the  alimentary  tract,  has  been  compared  to  the  vertebrate 
hypophysis.  In  many  there  exist  special  excretory  organs,  numer- 
ous blind  vesicles  filled  with  excreta. 

From  the  eggs  are  hatched  small  swimming  tadpole-like  larvae 
(fig.   547),  resembling  Appendicularia  and,  like  it,  consisting  of 


FIG.  547.— Development  of  an  Ascidian.  (After  Kupffer  and  Kowalevsky.)  1,  larva, 
just  hatched  ;  2,  cross-section  through  the  tail  of  a  slightly  younger  larva;  3,  much 
younger  stage,  formation  of  notochord  and  nervous  system ;  U,  anterior  end  of  a 
larva  just  before  attachment.  (2,  Phallusia  mentula ;  8,  A,  Ph.  mammillata.)  cm, 
eye;  c,  notochord  ;  cZ,  tunic:  d,  digestive  tract;  d',  its  nutritive,  d",  its  respira- 
tory division;  e,  atrial  vesicle;  ek,  ectoderm;  en,  entoderm;  h,  brain;  t,  oral  in- 
vagination  ;  ni,  muscles  of  tail;  n,  neural  tube  ;  ne,  neurenteric  canal ;  o,  otocyst. 

trunk  and  tail,  in  which  the  chordate  features  are  strongly  marked. 
The  digestive  tract  is  confined  to  the  trunk;  dorsal  to  it  lies  the 
tubular  nervous  system  in  which  three  parts  are  recognizable:  in 
front  a  vesicular  brain  with  a  simple  eye  and  an  otocyst  imbedded 
in  its  walls;  farther  back  a  narrower  portion  ('  medulla  oblongata  ') ; 
lastly,  a  spinal  cord  extending  into  the  tail.  In  the  axis  of  the 
tail  is  a  notochord  which  extends  forward  a  short  distance  into  the 
trunk  between  digestive  tract  and  nervous  system. 

In  the  metamorphosis  of  the  free  larvae  into  the  sessile  ascid- 
ians  four  processes  are  important:  (1)  The  larvae  attach  themselves 
by  means  of  three  ventral  anterior  papillae ;  (2)  The  tail  is  retracted 
and,  after  preliminary  fatty  degeneration,  is  absorbed;  (3)  The 
body  becomes  more  or  less  spherical  by  development  of  the  tunic; 


510 


CHORD  AT  A. 


(4)  Two  dorsal  invaginations  are  formed,  these  grow  together,  en- 
velop the  pharyngeal  region,  and  form  the  atrium  and  peribranchial 
chamber.  It  is  to  be  noted  that  this  arises  from  the  dorsal  sur- 
face and  extends  ventrally,  while  the  peribranchial  chamber  of 
Amphioxus  arises  by  folds  which  grow  ventrally  over  the  pharynx. 
Besides  sexual  reproduction  many  ascidians  reproduce  by  bud- 
ding. Where  this  occurs  it  results  in  the  formation  of  colonies,  a 
matter  of  systematic  importance. 

Sub  Order  I.  MONASCIDI^E.  Simple  ascidians  of  considerable  size  ; 
sometimes  with  transparent,  sometimes  with  thick  opaque  tunic.  The 
CLAVELLINID.<E  produce  small  colonies  by  basal  budding,  each  individual 


FIG.  548. 


Fia.  549. 


FIG.  548.— .4.  Molguln  manhattensis*  ;  B,  Eugyra  pillularis*    (From  Verrill.) 
FIG.  549. — Botryllus  violaceus.     (After  Carpenter.)    ^4,  small  colony  of  eighteen  indi- 
vidual groups;  BI  two  individual  groups  somewhat  enlarged. 

with  its  own  test;  Perophora*  CYNTHIID^E,  test  leathery,  oral  and  atrial 
openings  four-lobed;  Cynthia*  MOLGULID^E,  oral  opening,  six-lobed, 
atrial  four-lobed.  Molgula*  Eugyra* 

Sub  Order  II.  SYNASCIDL55.  Compound  ascidians  consisting  of 
numerous  small  individuals  imbedded  in  a  common  cellulose  tunic  and 
forming  considerable  crusts  on  stones,  plants,  etc.  Usually  (fig.  549)  the 
individuals  of  a  colony  are  divided  into  small  groups,  the  oral  openings 
(6-20  in  number)  forming  a  rosette  around  a  common  central  atrium. 
Distaplia*  Leptodinum*  Polyclinum*  Amaroucium*  Botryllus.* 

Sub  Order  III.  LUCLE.  Free-swimming  pelagic  synascidians,  having 
the  form  of  a  hollow  cylinder  closed  at  one  end.  The  animals  imbedded  in 
the  tunic  vertically  to  the  axis  of  the  cylinder,  the  oral  apertures  on  the 
outside,  the  atrial  in  the  central  cavity.  Pyrosoma,  very  phosphorescent, 
tropical,  some  species  four  feet  long. 

Order  III.  Thaliacea  (Salpaeformes). 

These,  tike  the  Luciae  and  Copelatae,  are  pelagic,  and  play  an 
important  part  in  the  plankton,  either  by  the  vast  numbers  of 
small  individuals  or  by  the  formation  of  colonies  of  considerable 
size.  In  form  a  Salpa  may  be  compared  to  a  barrel  formed  out- 
side of  a  cellulose  tunic,  lined  internally  with  a  muscular  wall. 
The  muscles  run  circularly  (fig.  550),  are  six  or  eight,  not  always 


//.    TUNICATA:   THALIACEA. 


511 


closed  rings,  like  hoops.  By  their  contraction  the  water  is  expelled 
through  the  posterior  or  atrial  end  of  the  body,  while  fresh  water 
on  their  relaxation  enters  the  other  or  oral  aperture.  By  the 
reaction  the  animals  swim  through  the  water  with  the  oral  end  in 
front.  The  cavity  of  the  barrel  corresponds  to  pharyngeal  and 
peribranchial  chambers  of  the  ascidian.  In  the  Dolioliidae  the  two 


FIG.  550.— A,  B,  Salpa  democratica  with  stolon,  ventral  and  lateral  views;  C,  Salpa 
mucronata,  part  of  a  young  chain  not  yet  separated,  a,  anus;  c,  tunic;  d,  diges- 
tive tract;  e,  atrial  opening;  en,  endostyle;  /,  peripharyngeal  groove;  g,  gan- 
glion with  horseshoe-shaped  eye,  and  near  it  the  tentacle  and  hypophysial 
groove;  /t,  testis;  t,  mouth;  /c,  gill;  m,  muscle  hoops;  st,  stolo  prolifer. 

chambers  are  separated  by  a  partition  perforated  by  gill  slits  (fig. 
551) ;  in  the  common  Salpce  the  partition  is  reduced  to  a  bar  with 
transverse  rows  of  cilia  so  that  branchial  and  peribranchial  cham- 
bers are  not  distinct;  yet  the  endostyle  and  the  peripharyngeal 
band  are  retained. 

The  viscera  lie  in  the  muscular  sac,  where  the  branchial  bar  and 
the  endostyle  meet  and  are  usually  compacted  into  a  mass,  the 
i  nucleus  '  (intestine,  liver,  gonads,  heart).  The  ganglion  is  dis- 
tinct and  lies  dorsally  opposite  the  endostyle,  just  in  front  of  the 
branchial  bar.  Associated  with  it  is  a  horseshoe-shaped  eye. 

For  a  long  time  two  kinds  of  Salpce  have  been  known,  one 
solitary,  the  other  consisting  of  numerous  individuals  connected 
together  like  a  chain  or  a  rosette  (fig.  550,  C).  At  the  beginning 
of  the  last  century  the  poet  Chamisso  discovered  that  the  chain 


512  CHORD  AT  A. 

salps  were  produced  by  the  solitary  individuals,  and  that  these  in 
turn  came  from  the  chain  form,  a  peculiar  type  of  reproduction 
to  which  Steenstrup  later  gave  the  name  alternation  of  generations. 
The  solitary  salp  is  asexual;  gonads  are  lacking,  but  near  the 
hinder  end  is  a  budding  cone  or  stolo  prolifer  from  which  one 
after  another  bud  colonies  of  salps.  When 
the  first  is  separated  a  second  matures  and 
en  a  third  begins.  These  colonial  forms,  the 
chain  salps,  are  sexual,  and  each  produces  a 
single  egg  from  which  a  solitary  individual  is 
formed. 


Since  both  the  solitary  and  the  chain  forms  have 
received  names,  the  species  of   Salpa*  now  have 
double   names  like  Salpa  democratica-mucronata, 
democratica  being  the  asexual,  mucronata  the  sex- 
ual, individual,  etc.     From  the  true  Salpce  Dolio- 
orex  lana"  ^um*  is  distinguished  by  the  better  developed  gills, 
tion  of  letters  see  fig.  the  complete  muscular  bands,  and  a  more  compli- 
cated alternation  of  generations. 

SUB  PHYLUM  III.  ENTEROPNEUSTA  (HEMICHORDIA). 

The  few  marine  forms  here  included  are  decidedly  worm-like, 
and,  like  many  worms,  they  burrow  in  the  mud.  The  body  con- 
sists of  three  parts — proboscis,  collar,  and  body  (fig.  552).  The 
proboscis  contains  a  cavity  opening  to  the  exterior  by  a  dorsal 
pore,  while  two  similar  cavities  in  the  collar  open  separately. 
These  can  be  filled  with  water,  and  by  alternately  enlarging  and 
contracting  these  parts  the  animal  is  able  to  burrow  like  a  razor 
clam  (Ensis).  The  mouth  lies  ventral  and  in  front  of  the  collar 
and  leads  into  a  digestive  tract,  which  in  its  anterior  part  is  per- 
forated by  numerous  paired  gill  slits,  while  the  part  behind  it  is 
covered  with  hepatic  caeca.  The  intestine  is  supported  in  the 
ccelom  by  dorsal  and  ventral  mesenteries,  and  is  accompanied  by  a 
dorsal  and  a  ventral  blood-vessel,  to  which  are  added  lateral  canals 
and  numerous  anastomoses.  A  vesicle  on  the  dorsal  vessel  in  the 
proboscis  is  called  the  heart.  The  nervous  system  is  very  peculiar. 
There  is  a  dorsal  portion  lying  in  the  collar  region,  which  is  pro- 
duced by  inrolling,  as  is  the  central  nervous  system  in  the  Chor- 
dates,  and  a  ventral  part,  as  yet  lying  in  the  ectoderm,  the  two 
being  connected  by  nerves  in  the  collar.  The  gonads  are  numerous 
follicles  lying  between  gill  and  liver  region  and  opening  to  the 
exterior. 


///.   ENTEROPNEU8TA. 


513 


FIG.  652.— Balanoglassus  koimlewskii*  (From  Korschelt-Heider,  after  A.  Agassiz.) 
db,  dorsal  blood-vessel;  e^*psab£>scis;  g,  sexual  region;  k,  gill  region;  kr,  collar; 
vb,  ventral  blood-vessel. 

The  systematic  position  of  the  Enteropneusta  is  not  settled  beyond  a 
doubt.  In  the  possession  of  gill  slits  and  in  the  formation  of  the  dorsal 
nervous  system  it  closely  resembles  the 
other  chordates,  and  the  resemblance 
is  strengthened  by  similarities  in  de- 
tails of  structure  of  the  gills.  The 
advocates  of  this  view  recognize  the 
notochord  in  a  blind  tube,  sur- 
rounded by  tough  membrane  and 
thickened  beneath,  which  extends 
from  the  pharynx  into  the  proboscis. 
Embryology  throws  but  little  light  on 
the  problem.  Some  species  have  a 
direct  development  (fig.  553,  B,  C), 
while  others  have  a  larva  (Tornaria, 
fig.  553,  A)  which  so  resembles  the 
larvae  of  certain  echinoderms  that 
it  was  long  held  to  belong  to  that 
phylum.  The  chief  resemblances  are  in  the  relations  of  the  ciliated 
bands  to  the  alimentary  tract  and  in  the  presence  of  the  proboscis  cavity, 


FIG.  553.— A,  Tornaria  larva  of  Balano- 
qlossus.  (After  Morgan.)  n,  apical 
plate ;  ac,  preoral  part  of  ciliated 
band ;  be1,  fee2,  btf,  coelomic  pouches  ; 
m,  mouth  ;  p,  postpral  part  of  ciliated 
band  5,  C,  twqF  stages  of  Balano- 
ylossus  with  direct  development.  (Af- 
ter Bateson.)  a+  anus  ;  be,  branchial 
clefts  ;  c,  collar  ;  dc,  digestive  part  of 
alimentary  «anal ;  in,  intestine ;  ?ic, 
fc  notochord  ';  p,  proboscis. 


514:  CHORD  ATA. 

which,  like  the  ambulacral  system,  opens  to  the  exterior.  Two  deep-sea 
forms,  Cephalodiscus  and  Rhabdopleura,  have  the  same  type  of  '  noto- 
chord,'  and  the  first  has  a  pair  of  gill  slits.  In  other  respects  these  are 
strikingly  Polyzoau  in  appearance. 

SUB   PHYLUM   IV.    VERTEBRATA. 

In  the  vertebrates  only  the  internal  segmentation  occurs.  This- 
is  shown,  and  most  clearly,  in  the  lower  Vertebrata,  in  the  muscles 
(myotomes,  myomeres),  the  myocommata  or  myosepta  which  sep- 
arate them,  and  the  protovertebrae  from  which  they  arise;  in  the 
nerves  (neurotomes),  the  skeleton  (sclerotomes),  the  blood-vessels,, 
and  in  the  excretory  organs  (nephrotomes).  In  the  higher  verte- 
brates this  metamerism  is  visible  only  in  the  embryonic  stages. 
In  part  this  absence  of  external  segmentation  has  its  cause  in  the 
heteronomy  (p.  399)  of  the  body  and  the  obliteration  of  segmental 
boundaries,  consequent  upon  the  union  of  somites  into  body  re- 
gions, of  which  at  least  three — head,  trunk,  and  tail — at  most  six 
— head,  neck,  (cervical)  thorax,  lumbar,  pelvic  (sacral),  and  tail 
(caudal) — occur.  Not  less  important  in  this  respect  is  the  charac- 
ter of  the  skeleton.  The  cuticular  skeleton,  which  is  the  cause  of 
the  annulation  of  the  arthropod,  is  entirely  lacking.  The  skin 
remains  soft,  or  contributes  to  a  subordinate  degree,  more  for  pro- 
tection than  for  support,  to  the  formation  of  a  skeleton  (dermal 
skeleton  of  fishes,  alligators,  turtles).  On  this  account  firmer 
tissue  is  formed  in  the  axis  of  the  body,  which,  in  the  lowest  ver- 
tebrates and  the  embryos  of  the  higher,  appears  as  the  notochord 
already  mentioned,  but  in  the  higher  is  supplemented  by  the  verte- 
bral column  and  skull. 

The  skin  of  the  vertebrates  is  distinguished  from  that  of  all 
invertebrates  by  two  characters  (figs.  26,  27):  the  many-layered 
condition  of  the  epidermis,  and  the  considerable  thickness  of  the 
derma.  The  epidermis  is  but  rarely  covered  by  a  delicate  cuticle; 
usually  such  a  protection  is  unnecessary  since — and  especially  in 
the  land  forms — the  superficial  layers  become  cornified  and  hence 
furnish  the  necessary  resistance  without  a  cuticle.  There  are  two 
layers  to  be  distinguished,  the  deeper  stratum  Malpighii  and  the 
superficial  stratum  corneum  (sM and  sc\  see  p.  76). 

The  second  constituent  of  the  integument,  the  derma  (cutis, 
corium),  arises  from  the  mesoderm  (mesenchyme).  It  consists  of 
many  layers,  often  stratified,  of  close  connective  tissue,  and  is 
usually  separated  from  the  underlying  structures,  especially  the 
muscles,  by  a  loose  tissue  rich  in  lymph  spaces,  the  subcutaneous 


IV.    VERTEBRATA. 


515 


tissue.  Both  of  these  constituents  of  the  skin,  aside  from  their 
own  firmness,  can  give  rise  to  protective  structures.  The  horny 
layer  of  the  epidermis  in  places  becomes  greatly  developed  and 
thus  forms  the  tortoise  shell  of  the  turtles,  the  scales,  shields,  and 
scutes  of  the  snakes  and  lizards,  the  feathers  of  the  birds,  the  hair 
and  horns  of  the  mammals.  Other  epidermal  products  are  the 
claws,  nails,  and  hoofs  of  the  terrestrial  vertebrates.  The  derma 
is  often  the  seat  of  ossifications  which,  in  contrast  to  the  deeper 
bones,  are  called  the  dermal  skeleton. 

First  of  the  dermal  skeletal  parts  are  the  scales  of  the  fishes,, 
which,  in  spite  of  similarity  of  name, 
are  different  from  the  epidermal  scales 
of  the  reptiles.  They  may  be  traced 
back  to  the  primitive  form,  the  pla- 
coid  scales  of  the  Elasmobranchs. 
These  are  rhombic  plates,  bearing  in 
the  middle  pointed  spines,  which  are 
called  dermal  teeth  from  similarity  in 
structure  and  development  to  the  teeth 
of  the  mouth  cavity  (fig.  554).  They 
consist  of  dentine  (d)  and  have  a  large 
pulp  cavity  (p),  with  numerous  blood- 
vessels in  the  interior.  Whether  the 
thin  layer  (.sr/<)  covering  the  tip  can 
be  called  enamel  is  disputed.  Der- 
mal teeth  and  true  teeth  are  identical 
structures  which,  in  consequence  of 
different  position  and  consequent  dif- 
1'erence  of  function,  have  developed  differently. 

The  scales  of  fishes  have  a  wider  anatomical  interest,  since  from 
them  have  arisen,  besides  the  bony  plates  which  form  the  resistant 
armor  of  the  turtles,  alligators,  and  many  mammals  (Armadillos), 
important  parts  of  the  axial  skeleton,  the  secondary  or  membrane 
bones.  A  membrane  bone  is  a  bony  plate  which  has  arisen  from 
a  fusion  of  dermal  ossifications,  becomes  transferred  to  a  deeper 
position,  and  contributes  to  the  completion  of  the  axial  skeleton. 
After  what  was  said  above  about  the  relations  of  dermal  and  true 
teeth  it  is  readily  seen  that  a  further  source  of  formation  of  mem- 
brane bones  lies  in  the  lining  of  the  mouth  cavity. 

In  describing  the  axial  skeleton,  the  notochord  comes  first. 
This  has  already  been  mentioned  in  connexion  with  the  lower 
Chordates.  It  persists  in  the  cyclostomes,  but  from  them  upwards 


516 


C HOED  AT  A. 


it  is  gradually  replaced  by  the  vertebrae  arising  around  it.  It  is  of 
'cntodermal  origin  (fig.  9),  arising  as  a  longitudinal  band  of  the 
epithelium  of  the  archenteron  (/,  di),  and,  becoming  cut  off, 
•comes  to  lie  in  the  long  axis  of  the  body  between  digestive  tract 
and  nervous  system  (//,  ///)  ;  here  it  forms  a  cylindrical  rod  con- 
sisting of  a  connective  tissue  which,  as  already  said,  resembles 
plant  tissues  because  of  the  vesicular  nature  of  its  cells  (fig.  38). 
In  transverse  section  (fig.  555)  the  chorda  is  surrounded  by 
three  layers,  internally  by  a  fibrous  noto- 
chorda]  sheath,  then  an  elastic  layer  (not 
always  present),  the  elastica  externa,  so 
called  because  an  elastica  interna  is  some- 
times present  inside  the  notochordal 
sheath;  and  lastly  a  skeletogenous  layer 
(SS),  also  called  the  outer  notochordal 
sheath.  This  last  is  a  mesodermal  con- 
nective-tissue layer  and  is  therefore  con- 
nected with  the  other  connective-tissue 
sheaths  which  surround  muscles,  nerves, 
etc.,  and  deserves  special  mention  because 
in  it  the  cartilages  and  bones  arise  from 
which  the  vertebrae  and  skull  are  formed. 
Cells  from  it  can  penetrate  the  notochor- 
dal  sheath,  converting  it  into  fibrous  car- 

,  ._ 

tilage,  thus  enabling  it  to  participate  in 

,      to  ' 

the  formation  of  the  vertebras. 

0.  ,n  ,       n         -,  -.     .,         1,1 

Since  the  notochord  and  its  sheaths 
are  elastic  and  give  under  the  strain  of  the 
muscles,  they  are  unsegmented.  The  seg- 
mentation of  the  axial  skeleton  begins  with 
the  appearance  of  firmer  tissue  in  cartilage  and  bone.  Then  there 
is  a  separation  of  successive  parts,  and  with  this  the  gradual  forma- 
tion of  vertebral  column  and  skull.  For  both  there  is  a  con- 
nected series  of  developments,  if  studied  with  reference  to  the 
ontogenetic  processes  or  in  the  comparative  manner  from  the 
lower  to  the  higher  forms. 

The  first  parts  of  the  vertebral  column  to  appear  are  the  upper 
and  lower  (figs.  555,  556),  or  neural  and  hcemal  arches.  These 
consist  of  paired  parts  in  the  skeletogenous  layer  which  abut 
against  the  notochord,  and  which  are  usually  a  pair  to  the  somite, 
although  occasionally  two  or  more  pairs,  the  arches  proper  and 
the  intercalaria^  may  occur.  The  neural  arches  (arcus  vertebras 


/z 


555.-Transverse  section 

of  axial    skeleton    of    Pe- 

tromyzou.  (From  wieders- 

heim.)    C,  notochord;    Cs, 

notochordal  sheath  ;   Ee, 

elastica  externa  :  f,  fatty 

tissue   M,  spinal  cord  ;  P, 


tissue  ;  -SVS,  skeletogenous 
tissue  ;  Ul>,  lower  process 
of  skeletogenous  tissue. 


IV.    VERTEBRATA. 


517 


of  human  anatomy)  enclose  a  spinal  canal  surrounding  the  spinal 
cord,  the  parts  of  the  arch,  neurapophyses,  uniting  above  the  cord 
to  form  the  spinous  process  (frequently  an  independent  part  of  the 
skeletal  axis).  In  the  caudal  region,  in  the  same  way,  hcemal 
arches  may  be  formed  of  licemapopliyses  and  hcemal  spine,  the  arches 
surrounding  the  blood-vessels  of  the  tail  (fig.  557).  In  the  trunk 
region  the  ventral  arch  behaves  differently.  Since  the  large  body 


n 


FIG.  556.— Vertebrae  of  sturgeon,  ch,  notochord;  /,  exit  of  nerve  ;  j,  dorsal  and  ventral 
intercalaria ;  n,  neural  canal:  o7>,  neural  arch;  s,  chordal  sheath;  r,  rib;  ubf 
haemal  arch.  Bone  white,  cartilage  dotted. 


A. 


FIG.  557.  FIG.  558. 

FIG.  557.— Caudal  vertebrae  of  a  carp,  section  (.4)  and  nearly  side  view  (B).  cli,  space 
filled  by  notochord;  /t,  haemal  arch;  ?i,  neural  arch;  06,  neural  spine ;  ub,  haemal 

FIG.  558. — Thoracic  vertebra,  ribs,  and  sternum  of  a  mammal.  (From  Wiedersheim.)' 
Co,  capitular  head  of  rib  ;  Co,  neck  of  rib;  Cp,  bony  rib;  Kn,  cartilaginous  rib :  Ps, 
spinous  process ;  Pt.  transverse  process  (diapophysis) ;  St,  sternum ;  T6,  tuber- 
cular head  of  rib;  WK,  vertebral  centre. 

cavity  with  its  viscera,  varying  in  size  (digestive  and  reproductive 
organs),  is  here,  the  haemapophyses  extend  outwards  and  down- 
wards and  are  divided  into  two  parts,  a  basal  apopliysis  and  a, 
lower  movable  portion,  the  rib  (fig.  556).  Also  the  lower  union 
of  haemapophyses  with  haemal  spine  does  not  occur;  the  ribs  ar& 
either  free  (fishes)  or  are  (at  least  in  part)  connected  ventrally  by 


518  CHORD  ATA. 

a  breast  bone  or  sternum  (Amniotes,  fig.  558).  The  sternum  is 
a  derivative  of  the  ribs.  In  development  the  ventral  ends  of  the 
ribs  of  a  side  fuse  and  then  these  fused  tracts  of  the  two  sides 
unite  to  form  the  sternum. 

The  haemal  arches  lie  internal  to  the  longitudinal  muscles  of  the  body, 
and  in  the  trunk  region  they  lie  in  the  same  position  just  beneath  the 
peritoneum.  These  are  hcemal  ribs  and  are  found  only  in  teleosts  and 
ganoids.  The  ribs  of  all  other  vertebrates  (elasmobranchs,  amphibia, 
amniotes)  are  morphologically  different  and  are  called  lateral  or pleural 
ribs.  They  develop  independently  of  the  vertebral  column  in  a  horizontal 
connective-tissue  septum  which  extends  out  through  the  longitudinal  mus- 
cles from  the  axial  skeleton  to  the  skin,  dividing  the  musculature  into 
dorsal  (epaxial)  and  ventral  (hypaxial)  portions  (fig.  89).  In  the  elasmo- 
branchs these  pleural  ribs  are  attached  to  the  haemapophyses,  in  the 
others  to  the  transverse  processes  (diapophyses),  which  arise  from  the 
neurapophyses,  and  parapophyses,  which  arise  from  the  vertebral  centres. 
In  the  caudal  region,  often  also  in  the  cervical,  lumbar,  and  sacral  regions, 
the  pleural  ribs  and  dia-  and  parapophyses  fuse  to  form  lateral  processes. 
These  occur  concurrently  with  haemal  arches  in  the  tails  of  many  Am- 
phibia and  reptiles  and  some  mammals,  forming  the  chevron  bones  which, 
as  in  fishes,  enclose  the  caudal  blood-vessels.  The  presence  of  intercalaria 
in  cyclostomes,  sharks,  and  ganoids  indicates  that  primitively  a  double 
vertebra  arose  in  each  somite.  Paleontological  and  embryological  re- 
searches on  reptiles  support  this  view. 

In  most  vertebrates  either  the  basal  ends  of  the  arches  broaden 
•out  around  the  notochord  and  fuse  with  one  another,  or  perichordal 
cartilages  arise  independently,  furnishing  in  either  case  firm  sup- 
ports, the  vertebral  bodies,  or  centra,  for  the  system  of  arches. 
These  increase  in  size  at  the  expense  of  the  notochord  on  the  in- 
side, sometimes  leading  to  its  almost  complete  obliteration,  as  in 
the  mammals;  in  others,  as  the  fishes,  the  reduction  is  less  com- 
plete. The  fishes  have  awphicoele  vertebra}  (fig.  557),  that  is,  the 
centra  are  hollow  at  either  end.  In  these  cups  the  notochord 
exists  even  in  the  adult,  and  when  small  connecting  portions  ex- 
tend through  the  centra  the  notochord  takes  the  form  of  a  rosary 
with  alternating  enlargements  and  contractions. 

Histologically  the  vertebral  column  may  be  either  cartilage  or 
bone;  usually  it  is  first  formed  in  cartilage,  which  is  later  replaced 
by  bone.  If  the  ossification  does  not  occur,  the  column  remains 
cartilaginous;  if  incomplete,  cartilage  and  bone  appear  side  by  side. 
iSince  these  histological  differences  are  combined  with  varying  de- 
grees of  persistence  of  the  notochord  and  with  modifications  in  the 
form  of  the  vertebrae  and  their  processes,  there  results  an  extraor- 
dinary variety  in  the  appearance  of  the  vertebral  column. 


/  V.    VER  TESSA  TA.  519 

In  order  to  allow  for  bending  where  complete  centra  are  present  vari- 
ous conditions  occur,  (a)  Opisthocode  vertebrae  have  a  socket  on  the 
hinder  surface  which  receives  the  convex  anterior  end  of  the  succeeding 
centrum,  forming  a  ball-and-socket  joint.  (6)  Precocious  vertebrae  have 
these  relations  reversed,  the  socket  being  in  front,  (c)  The  vertebrae  may 
articulate  with  a  'saddle  joint'  (birds),  (d)  Between  two  successive 
vertebrae  an  elastic  intervertebral  ligament  may  occur  (mammals).  The 
neurapophyses  may  bear,  in  addition  to  the  transverse  processes,  anterior 
and  posterior  articulating  processes  (zygapophyses)  connecting  the  sepa- 
rate vertebrae. 

The  skull,  the  anterior  continuation  of  the  axial  skeleton, 
occurs  in  all  vertebrates;  it  appears  before  the  vertebrae,  for  it  is 
found  in  the  cyclostomes,  which  lack  these.  It  surrounds  the  brain 
as  the  vertebrae  do  the  spinal  cord:  and,  like  them,  its  first  stages 
are  formed  in  the  skeletogenous  layer  surrounding  the  anterior  end 
of  the  notochord.  It  is  so  related  to  the  surrounding  parts  that 
it  may  in  general  be  said  to  be  equivalent  or  homodynamous  with 
the  vertebrae,  although  we  cannot  agree  with  Oken  and  Goethe,  the 
founders  of  the  vertebrate  theory  of  the  skull,  that  it  has  arisen 
by  the  fusion  of  vertebrae.  On  the  other  hand  skull  and  vertebrae 
are  parts  arising  in  the  common  basis  of  the  skeletogenous  layer, 
but  which  have  developed  in  different  directions. 

Three  stages  are  recognized  in  the  development  of  the  skull: 
the  membranous,  the  cartilaginous  cranium,  and  the  bony  skull. 
The  first,  which  consists  of  connective  tissue,  occurs  only  in  the 
early  embryonic  stages,  scarcely  a  trace  of  it  persisting  in  the 
adults.  It  is  early  replaced  by  the  cartilaginous  skull,  which  may 
persist  unaltered  throughout  life  in  the  lower  fishes  (elasmo- 
branchs,  sturgeon).  In  most  vertebrates,  however,  ossification  sets 
in,  embracing  a  part  (fishes,  amphibians)  or  the  whole  of  the  carti- 
lage (birds,  mammals),  converting  it  in  the  latter  case  into  a  bony 
capsule.  In  the  bony  skull  two  kinds  of  bone,  primary  and  sec- 
ondary, are  recognized,  these  varying  in  their  origin.  The  pri- 
mary or  cartilage  bones  develop  from  the  cartilage  itself,  either  in 
its  interior  (entochondrostoses)  or  in  its  enveloping  perichondium 
(ectochondrostoses).  The  secondary  or  membrane  bones  are,  in 
their  origin,  foreign  to  the  axial  skeleton  and  arise  from  the  ossifi- 
cations in  the  skin  (scales)  or  in  the  mouth  (teeth),  already  re- 
ferred to  (p.  515  ).  They  sink  into  the  deeper  portions  and  apply 
themselves  to  the  axial  skeleton,  especially  to  those  parts  where, 
from  lack  of  cartilage,  no  primary  bones  can  be  formed  (parostoses). 
Still  it  is  not  settled  how  far  these  distinctions  may  be  carried. 
According  to  Gegenbanr  all  ossifications  arose  primarily  in  the  skin 


520 


CHORD  AT  A. 


or  mucous  membranes,  and  primary  bones  are  merely  membrane- 
bones  which  have  entered  the  cartilages  and  replaced  them.  Ac- 
cording to  this  view  it  is  conceivable  that  the  same  bone  in  one 
animal  may  arise  as  a  membrane  bone  and  in  another  as  a  primary 
bone,  a  view  which  is  of  importance  in  the  homologies  and  no- 
menclature of  many  bones.  It  is  but  just  to  say  that  this  view  is 
not  universally  accepted. 

The  cartilaginous  cranium  (chondrocranium)  is  most  complete 
beneath  the  brain.     This  basal  portion  is  a  direct  continuation  of 


FIG.  559.— Chondrocranium  of  Amplduma.  o?ip,  antorbital  process;  r/p,  ascending 
process  of  quadrate;  c,  cornu  trabeculee;  e,  ethmoid  plate;  ef,  endolymph  fora- 
men; j,  jugular  foramen;  I,  lamina  cribrosa;  m.  Mockers  cartilage ;  N,  notocnorcr 
oc,  oculomotor  foramen;  ocp,  occipital  process;  o/,  optic  foramen ;  p,  parachor- 
dal;  pal,  palatine  foramen  ;  p/,  perilymphatic  foramen;  q  quadrate;  s,  stapes; 
sp,  stapedial  process  ;  f,  trabecula;  trc,  crest  of  trabecula;  V,  VII,  VIII,  foramina 
for  V,  VII,  VIII  nerves. 

the  vertebral  column,  and  a  part  of  it  (the  paracliordals)  embraces 
the  anterior  end  of  the  notochord,  while  part  (the  trdbeculce)  ex- 
tends in  front  of  the  end  of  the  notochord.  The  side  walls  of  the 
skull  are  increased  by  the  cartilaginous  envelopes  of  the  two  sense 
organs,  the  nasal  and  otic  capsules,  around  the  nose  and  ear.  Be- 
tween these  is  a  hollow  for  the  eye  which  contributes  nothing  to 
the  skull.  In  only  a  few  forms  is  the  chondrocranium  completely 


IV.    VERTEBEATA. 


521 


closed;  usually  gaps  (fontanelles)  occur  in  its  roof,  and  frequently 
in  its  floor.  The  higher  the  animal  intellectually  and  the  larger 
its  brain  the  more  the  connective  tissue  (primordial  cranium)  is 
called  upon  to  roof  in  the  chondrocranium.  Hence  it  is  that  in 
the  reptiles,  birds,  and  mammals,  where  it  is  also  confined  to 
embryonic  life,  the  chondrocranium  is  relatively  the  smallest. 
Since  it  only  closes  above  in  the  occipital  (hinder)  region,  while 
it  gaps  widely  in  front,  it  follows  that  the  secondary  bones  play  an 
important  part  in  the  completion  of  the  skull. 

The  bony  skull  presents  great  difficulties  from  the  standpoint 
of  comparative  anatomy,  in  part  from  its  varying  appearance  in 
the  different  groups,  in  part  on  account  of  the  number  and  com- 
plicated arrangement  of  the  constituent  bones.  It  may  be  said  in 
beginning  that  as  a  rule  the  same  bone  reappears  in  the  separate 
classes,  and  that  the  difficulties  are  connected  with  the  fact  that 
certain  bones  may  fail  to  develop  (Amphibia),  or  they  may  fuse 
to  larger  elements  (mammals).  A  further  complication  results 
from  the  intimate  union  with  the  cranium  of  bones  of  the  visceral 
arches,  which,  strictly  speaking,  do  not  belong  to  it. 


me. 


fis  firo    as 


FIG.  560.— Skull  of  carp,  the  visceral  skeleton  removed.  (A)  Cartilage  bones:  ocb 
ocl,  ocs,  basi-,  ex-,  and  supraoccipitals ;  ego,  epiotic ;  pto,  pterotic  ;  sp/io,  sphe- 
notic;  pro,  prootic  ;  as,  alisphenoid  ;  o.s,  orbitosphenoid;  me,  mesethmoid  ;  ee,  ect- 
ethmoid.  (B)  Ventral  membrane  bones  :  p.s,  parasphenoid  ;  vo,  vbmer.  (C)  Dorsal 
membrane  bones  :  p,  parietal ;  /?-,  frontal ;  l-U,  exits  of  nerves. 

The  primary  bones  (preformed  in  cartilage)  can  be  divided  ac- 
cording to  the  cranial  regions  into  four  groups:  (1)  bones  of  the 
hinder  part  of  the  head — occipitalia;  (2)  bones  of  the  ear  region 
— otica;  (3)  bones  near  the  eye — splienoidalia;  and  (4)  of  the 
nasal  capsule — ethmoidalia.  The  occipitalia — four  in  number 
(figs.  560-562) — united  in  the  higher  mammals  to  a  single  occipital 


522 


CHORD  AT  A. 


bone,  surround  the  foramen  magnum,  the  opening  through 
which  the  spinal  cord  passes  to  connect  with  the  brain.  These  are 
&  pair  of  exoccipitals,  right  and  left,  a  supraoccipital  above  and 
.a  basioccipital  below.  The  otica  depend  in  their  development 
upon  the  extent  of  the  otic  region.  In  the  fishes,  where  this  part 
is  large,  several  bones  may  be  present :  epiotic,  pterotic,  sphenotic, 
prootic,  and  often  opisthotic.  In  the  mammals,  on  the  other  hand, 
these  are  fused  to  a  single  petrosal  bone  (figs.  561,  562)  of  small 
size. 

Since  the  otic  bones  usually  do  not  reach  the  middle  line  below, 
the  sphenoidalia  rest  direct  upon  the  basioccipital  behind  and  in 
front  upon  a  presphenoid  bone,  both  unpaired  but  arising  from 


P<* 


No, 


Jmt 


01 


FIG.  561. -Skull  of  goat.  (From  Clans.)  Als,  alisphenoid;  Bs,  basisphenoid;  (7,  occip- 
ital condyle;  Eth,  mesethmoid,  covering  the  ectethmoid;  Fo,  optic  foramen  in 
orbitosphenoid;  Fr,  frontal;  I»ix,  premaxillary;  7p,  interparietal ;  Ju,  jugal 
(malar);  La,  lachrymal  ;  MX,  maxillary;  JV«,  nasal;  O/>,  basioccipital;  Ol<  exoc- 
cipital  ;  Ors,  orbitosphenoid;  Pa,  parietal;  Pal,  palatine;  Pe,  petrosal;  Pm. 
paramastoid  process;  Ps,  presphenoid  ;  Pt,  pterygoid  ;  S/,  frontal  sinus  in  frontal 
bone  ;  £>pb,  basisphenoid  ;  <S'g,  squamosal ;  2'j/,  tympanic;  F6,  vomer. 

paired  centres.  With  each  is  connected,  right  and  left,  a  pair  of 
bones ;  with  the  basisphenoid  the  alisphenoids,  with  the  presphenoid 
the  orbitosphenoids,  just  as  the  exoccipitals  flank  the  basioccipital. 
In  the  region  of  the  nasal  capsule  there  is  an  unpaired  mesethmoid 
with  a  pair  of  ectethmoids.  Hence  the  cranium  of  primary  bones 
may  be  described  as  a  chain  of  four  median  basal  bones,  basioccipi- 
tal, basisphenoid,  presphenoid,  and  mesethmoid;  right  and  left  of 
this  a  row  of  exoccipital,  alisphenoid,  orbitosphenoid,  and  ecteth- 
moid. The  position  of  the  otic  capsule  results  in  the  sum  of  the 
otic  bones,  the  petrosal,  being  wedged  in  between  the  exoccipitals 


IV.    VERTEBRATA.  523 

and  the  alisphenoid.     Only  behind  is  there  a  dorsal  element,  the 
supraoceipital. 

The  skull  must  be  roofed  in  by  membrane  bones,  and  of  these 
three  pairs  are  almost  constantly  present.  These  are,  from  behind 
forwards,  a  pair  of  parietals,  a  pair  of  f rentals,  and  a  pair  of  nasals, 


FIG.  562.— Sagittal  section  of  hinder  part  of  goat  skull.    (From  Gegenbaur.) 
For  lettering  see  fig.  561. 

the  latter  covering  the  nasal  capsules.  Confined  to  the  lower 
vertebrates  is  a  large  membrane  bone  on  the  floor  of  the  skull,  the 
parasphenoid,  which  reaches  from  the  basioccipital  to  the  meseth- 
moid. 

The  scheme  of  the  cranium  thus  outlined  undergoes  the  most  modifi- 
cations in  the  sphenoidal  region.  Parasphenoid,  on  the  one  hand,  and 
basi-and  presphenoid,  on  the  other,  may  be  substituted  for  one  another, 
so  that  when  the  parasphenoid  is  present  (fishes,  Amphibia)  the  others 
are  small  or  absent  and  vice  versa  (mammals).  In  the  mammals,  besides, 
the  alisphenoids  fuse  with  the  basisphenoid  (greater  wings),  the  orbito- 
sphenoids  with  the  presphenoid  (lesser  wings),  so  there  arise  here  an  an- 
terior and  a  posterior  sphenoid,  fused  in  man  to  a  single  sphenoid  bone. 
Mesethmoid  and  ectethmoids  likewise  fuse  in  the  mammals  to  an  eth- 
moid bone. 

The  brain  case,  or  cranium,  is  developed  into  the  complete  skull 
by  the  addition  of  the  visceral  skeleton,  a  series  of  arches  which, 
like  ribs,  embrace  the  beginning  of  the  alimentary  tract  and  are 
related  to  the  cranium,  much  as  are  ribs  to  the  vertebra.  These 
must  be  considered  as  parts  of  the  skull,  although  in  part  they  are 
shoved  backwards  and  lie  under  the  anterior  end  of  the  vertebral 
column.  As  the  ribs  arise  in  alternation  with  the  musculature 
(myomeric),  so  the  visceral  arches  are  similarly  related  to  the  gill 
formation  (branchiomeric).  Analogous  to  the  cranium  the  visceral 
skeleton  has  a  cartilaginous  and  a  bony  stage.  The  visceral  skeleton 
is  entirely  cartilaginous  only  in  Elasmobranchs,  and  here  it  is  so 


524  CHOIWATA. 

loosely  connected  with  the  cranium  as  to  be  easily  separated  from 
it.  It  consists  in  these  forms  usually  of  eight  (rarely  eleven) 
arches  (fig.  588);  these  are,  from  in  front  backwards,  the  rudi- 
mentary labial  cartilages,  then  the  large  mandibular  arch,  the 
Jiyoid  arch,  and  five  (rarely  seven)  gill  or  branchial  arches.  The 
mandibular  arch  consists,  on  either  side,  of  two  pieces  which  bear 
teeth  and  oppose  each  other  in  biting ;  the  upper  half,  attached  to 
the  skull  in  front  and  behind,  is  the  pterygoquadrate  (is  not  the 
upper  jaw  of  higher  forms).  The  lower  part,  which  is  hinged  to 
the  other,  is  the  mandibular  or  MeckePs  cartilage.  In  the  same 
way  the  hyoid  a«rch  is  divided  into  an  upper,  or  hyomandibular, 
and  a  lower  hyoid  proper  on  either  side,  the  hyomandibular  being 
fastened  to  the  otic  capsule.  The  hyoids  are  united  below  by  an 
unpaired  piece,  the  copula.  A  copula  also  exists  between  the 
halves  of  the  branchial  arches,  each  of  which  consists  of  four  parts 
on  either  side.  Hyoid  and  gill  arches  bear  gills.  Certain  features 
(existence  of  rudimentary  gills  and  a  rudimentary  gill  cleft,  the 
spiracle)  indicate  also  that  the  mandibular  arch  was  once  gill- 
bearing  and  that  it  lost  its  original  function  upon  being  converted 
into  an  organ  of  mastication.  Recently  the  labial  cartilages  have 
been  regarded  as  remnants  of  a  support  for  tentacles  around  the 
mouth  like  those  of  Amphioxus  and  Myxine,  and  which  reappear 
anew  in  the  barbels  of  bony  fishes.  Hence  they  are  not  compar- 
able to  the  other  arches. 

By  ossification  the  visceral  arches  of  the  higher  fishes  and  all 
higher  vertebrates  produce  a  great  modification  of  the  skull,  this 
being  increased  by  a  progressive  change  of  function  of  the  arches, 
which  depart  more  and  more  from  their  relations  to  the  respiratory 
apparatus.  From  this  standpoint  they  may  be  divided  into  two- 
groups,  an  anterior,  consisting  of  labial  cartilages,  maudibular  arch,, 
and  the  hyomandibular;  and  a  posterior,  of  the  hyoid  and  the  gill 
arches.  The  hinder  arches  are  well  developed  as  long  as  branchial 
respiration  persists.  With  the  loss  of  gills  they  largely  disappear, 
but  what  remains  forms  the  hyoid  or  tongue  bone  (not  to  be  con- 
fused with  the  hyoid  proper),  its  body  being  composed  of  the 
copula,  its  anterior  horns  of  the  hyoid,  and  its  posterior  horns  of 
the  remnants  of  a  gill  arch.  Other  gill  arches  contribute  to 
laryngeal  cartilages,  the  epiglottis  and  the  cartilages  of  the  audi- 
tory meatus. 

The  anterior  members  of  the  visceral  skeleton  (labials,  pterygo- 
quadrate, Meckelian,  and  hyomandibular)  become  developed 
further,  but  lose  more  and  more  their  individuality  and  unite  with 


IV.    VERTEBRATA.  525 

the  cranium;  in  the  mammals  forming  the  '  bones  of  the  face/  It 
is  therefore  a  source  of  additional  bones  which  are  difficult  to  fol- 
low from  class  to  class,  since  they  change  in  their  functions  and 
•consequently  in  shape  and  relative  size. 

All  vertebrates  with  bony  visceral  skeleton  (figs.  561,  589) 
have  two  pairs  of  membrane  bones,  right  and  left,  in  front  of  the 
pterygoquadrates,  the  premaxillaries  (intermaxillaries)  and  max- 
illaries.  They  bear,  in  toothed  vertebrates,  the  marginal  row  of 
teeth,  which  are  distinguished  from  the  palatopterygoid  teeth  in 
that  they  are  opposed  by  the  teeth  of  the  lower  jaw.  The  ptery go- 
quadrates  are  thus  forced  backwards  and  form  a  second  series  of 
bones,  parallel  to  the  maxillary  series,  which  likewise  may  bear 
teeth.  This  row  of  bones  consists  of  an  anterior  palatine  portion 
.and  a  posterior  quadrate  part.  The  cartilages  of  the  palatine  part 
largely  disappear  and  are  replaced,  in  front,  by  a  pair  of  vomers 
followed  by  a  pair  of  palatines,  while  farther  back  are  a  pair  of 
pterygoids.  The  quadrate  portion  ossifies  into  the  quadrate  bone, 
which  affords  the  articulation  for  the  lower  jaw,  The  ossifications 
lor  the  lower  jaw  occur  in  a  similar  way;  in  front  a  series  of  mem- 
brane bones,  of  which  the  dentary  is  most  important,  surrounding 
Meckel's  cartilage,  while  the  hinder  part  of  the  Meckelian  ossifies 
into  the  articulare,  so  called  because  it  articulates  with  the  quad- 
rate. The  hyomandibular  forms  only  one  constantly  present  bone 
known  by  the  same  name. 

If  all  vertebrates  with  bony  skeletons  be  compared,  it  is  found 
that  those  with  terrestrial  habits  have  a  sound-conducting  apparatus 
in  connexion  with  the  ear.  This  is  composed  of  elements  which, 
in  the  fishes,  lie  in  the  neighborhood  of  the  otic  capsule,  the 
hyomandibular,  the  quadrate,  and  the  articulare,  to  which  is 
added  another  element,  the  stapes,  which  occupies  the  fenestra 
ovalis  (p.  544)  and  is  derived  from  the  otic  capsule  itself.  In 
Anura,  reptiles,  and  birds  the  hyomandibular  apparently  gives 
rise  to  an  element,  the  columella,  which  abuts  against  the  stapes. 
In  the  mammals  stapes  and  columella  are  possibly  fused,  while 
quadrate  and  articulare  undergo  a  change  of  function,  losing 
their  position  in  connexion  with  the  articulation  of  the  jaws  and 
being  COD  verted  into  part  of  the  sound-conducting  apparatus,  the 
quadrate  furnishing  the  incus,  the  articulare  the  malleus  (figs. 
576,  577).1  Since  the  lower  jaw  in  this  way  loses  its  articulation, 
a  new  one  is  formed  by  a  process  from  the  membrane  bones. 

According  to  this  view  the  lower  jaw  of  a  mammal  is  not  equivalent 
to  the  lower  jaw  of  a  bird,  since  in  the  latter  the  hinge  is  furnished  by  the 


52ti  CHORDATA. 

quadrate-articulare  joint.  It  should  be  said  that  another  view  obtainsr 
though  not  so  well  supported,  which  considers  the  ear  bones  as  exactly 
homologous  throughout  terrestrial  vertebrates,  and  which  recognizes  incus, 
and  malleus  in  the  coluraella  and  maintains  that  quadrate  and  articulare 
form  the  hinge  of  the  mammalian  jaw. 

In  conclusion  three  other  bones,  widely  distributed,  must  be 
mentioned — the  squamosal,  the  tympanic,  and  the  jugai.  The 
squamosal  is  a  membrane  bone  arising  at  the  boundary  of  quadrate 
and  otic  capsule  (petrosal),  and  hence  with  relations  to  both  these 
bones.  It  increases  in  size  as  the  quadrate  diminishes  in  changing 
to  the  incus,  and  in  the  mammals  fuses  with  the  petrosal  to  form 
the  temporal  bone.  In  common  with  the  tympanic,  which  in 
mammals  also  fuses  with  the  petrosal,  it  forms  a  frame  for  the 
attachment  of  the  tympanic  membrane  of  the  ear.  The  jugal 
(malar,  zygomatic)  belongs  to  the  maxillary  series.  In  many  ver- 
tebrates this  series  is  articulated  only  in  front,  its  posterior  end 
terminating  freely  in  the  soft  parts,  but  when  the  jugal  occurs  it 
forms  a  jugal  or  zygomatic  arch  which  bridges  the  gap  between 
the  maxillary  and  the  quadrate  region  of  the  skull.  When  the 
quadrate  becomes  modified  to  the  incus,  the  jugal  articulates  with 
its  companion,  the  squamosal,  which  extends  a  zygomatic  process. 
forward  for  this  purpose. 

Difficulties  in  ascertaining  the  morphological  relations  of  bones  arise- 
where  the  visceral  and  cranial  parts  join  and  where  primary  and  second- 
ary bones  touch,  especially  since  in  the  latter  no  general  criteria  of 
distinction  can  be  drawn.  Thus  the  pterotic,  sphenotic,  and  ectethmoid  of 
the  fishes  are  often  replaced  by  other  secondary  bones  in  the  Amniotes ; 
the  primary  pterotic  by  the  secondary  squamosal ;  the  primary  sphenotic 
and  ectethmoid  by  two  membrane  bones  in  front  of  and  behind  the  frontals, 
the  pref  rentals  and  post  frontals  of  reptiles  and  other  forms. 

Just  as  skull  and  vertebral  column  form  a  firm  axis  for  the  body, 
the  appendages  are  supported  by  axial  skeletal  structures.  Two 
kinds  of  appendages  are  recognized,  paired  and  unpaired,  which 
generally  occur  together  only  in  fishes  (figs.  598-608).  The  un- 
paired consist  of  a  fold  of  the  skin  beginning  in  the  sagittal  plane 
behind  the  head,  running  back  around  the  tail  and  forward  on 
the  ventral  surface  to  the  anal  region.  This  continuous  fold  is 
nearly  always  divided  into  three  parts,  a  dorsal  fin  (often  sub- 
divided into  smaller  fins),  a  caudal  fin,  and  an  anal  fin.  In  a 
similar  way,  apparently,  the  paired  appendages — an  anterior  or 
thoracic  and  a  posterior  or  pelvic  pair — have  arisen  from  a  pair  of 
continuous  folds,  by  development  of  the  appendages  themselves. 


IV.    VERTEBRATA. 


and  suppression  of  the  intermediate  regions.  Of  these  the  un- 
paired are  possibly  the  oldest,  since  they  occur  not  only  in  the 
cyclostomes,  but  in  Ampliioxus  and  the  tunicates  as  well,  where 
paired  appendages  are  lacking;  on  the  other  hand  they  disappear 
in  the  higher  forms.  Since  they  are  of  service  only  in  an  aquatic 
life,  they  are  lost  in  the  Amphibia,  in  which  a  continuous  fin,  un- 
supported by  skeletal  elements,  occurs  only  in  larval  life.  On  the 
other  hand  the  paired  appendages  gain  in  importance  with  terres- 
trial habits. 

In  the  fins  of  fishes  two  kinds  of  skeletal  elements  occur  which, 
in  the  Elasmobranchs,  are  distinguished  by  their  histological 
structure,  since  the  one,  the  fin  supports  (basalia  and  radialia), 
consist  of  cartilage,  the  others  (actinotrichia,  dermal  skeleton) 


FIG.  563.— Pectoral  girdle  and  left  fin  of  Heptanchus.  (After  Wiedersheim.)  a,  prin- 
cipal row  of  the  cartilaginous  fin  supports ;  /i,  horny  threads  or  fin  rays  cut 
across  at  /*';  n.Z,  foramen  for  nerve;  ?•,  accessory  cartilaginous  fin  supports  ; 
s,  s',  scapula  ;  w,  ventral  portion  of  girdle. 

are  of  horny  consistency  (fig.  563).  Since  in  the  teleosts  both 
kinds  of  supports  may  ossify,  the  distinction  is  here  less  striking, 
yet  the  basalia  and  radialia  arise  from  cartilage  and  lie  in  the  basal 
part  of  the  fin,  while  the  others  are  never  cartilaginous  and  occur 
in  the  distal  portion.  These  distinctions  are  of  importance,  since 
the  actinotrichial  portions  play  no  part  in  the  development  of  the 
extremities  of  the  higher  groups.  These  arise  from  the  basal  sup- 
ports of  pectoral  and  pelvic  fins,  which  therefore  alone  need  further 
mention. 

The  skeleton  of  the  paired  appendages,  preformed  in  cartilage, 
consists  of  two  parts,  the  girdles  lying  in  the  lateral  walls  of  the 
trunk,  and  the  skeleton  of  the  limbs  themselves.  A  girdle — a 
shoulder  or  pectoral  girdle  in  front,  a  pelvic  girdle  for  the  hind 


528 


CHORD  ATA. 


limbs — is  in  its  simplest  form  an  arch  with  right  and  left  halves, 

each  half  with  an  articular  surface  for 
the  limb,  dividing  it  into  dorsal  and 
ventral  portions  (fig.  563).  The  dorsal 
portion  is  the  scapula  (shoulder  blade)  in 
the  pectoral,  ilium  in  the  pelvic  girdle. 
The  lower  portion  is  usually  split  into 
anterior  and  posterior  parts  (fig.  564). 
The  anterior  of  these  is  the  clavicle  in 
the  pectoral  girdle,  pubic  bone  in  the 
pelvis;  the  hinder  part  is  the  coracoid 
or  the  ischium  in  the  two  girdles  re- 
spectively. These  parts  are  most  con- 
stant in  the  pelvic  girdle.  In  the  pec- 
toral girdle  either  coracoid  or  clavicle 
may  be  lacking,  at  times  both  are  absent ; 
but  no  vertebrate  with  fore  limbs  lacks 
a  scapula.  In  the  clavicle  there  is  fre- 
quently an  element,  preformed  in  carti- 
lage, the  procoracoid,to  be  distinguished 
from  a  membrane  bone,  the  clavicle  in 
the  strict  sense. 

In  the  fishes  the  girdles  are  largely 
or  entirely  held  in  position  by  muscles; 
in  most  terrestrial  vertebrates  there  is  a 
more  intimate  connexion  with  the  axial  skeleton  and  especially  with 
the  vertebral  column.  In  the  case  of  the  pelvic  girdle  the  connexion 
is  direct,  since  the  ilium  is  articulated  with  one  or  more  sacral 
vertebrae  (in  reality  not  with  the  vertebrae  themselves,  but  by  the  in- 
tervention of  sacral  ribs).  The  connexion  of  the  pectoral  girdle  is 
less  direct  and  is  looser.  This  is  effected  by  clavicle  and  coracoid. 
The  latter  connects  with  the  sternum,  which  in  turn  is  connected 
to  the  vertebral  column  by  the  ribs;  the  clavicle  articulates  with 
a  bone,  the  episternum,  which  rests  upon  the  breast  bone,  the 
morphological  relation  of  which  is  doubtful,  since  under  this  term 
have  been  included  different  structures  (the  membrane  bone  of 
Reptiles,  episternum  in  the  strict  sense,  the  cartilage  bone,  the 
prosternum  of  the  monotremes  and  the  praeclavia  of  the  mammals). 
Since  only  the  free  portions  of  the  appendages  are  concerned 
directly  in  locomotion,  and  since  the  various  modes  of  motion — 
swimming,  flight,  running,  leaping,  climbing — demand  special 
modifications,  the  skeleton  of  the  limbs  shows  great  variety.  It 


Fio.  564.— Right  half  of  shoulder 
girdles  of  (A)  frog,  (B)  turtle, 
(C)  lizard.  (After  Gegenbaur, 
slightly  modified.)  cf,  clavicle; 
co,  coracoid;  e,  episternum; 
•s,  scapula;  s',  suprascapula;  sf, 
sternum,  in  C  with  bases  of 
ribs. 


IV.    VERTEBRATA. 


529 


is  usually  believed  that  all  these  forms  are  to  be  traced  back  to 
an  ancestral  type,  the  archipterygium.  In  this  (fig.  563)  are 
numerous  skeletal  parts  which  vary  little  in  size  and  form  and  are 
arranged  in  many  closely  appressed  rows.  One  of  the  rows  has 
acquired  prominence  and  is  called  the  principal  row;  it  begins 
with  a  larger  piece,  the  metapterygmm,  which  articulates  with  the 
girdle  and  bears  either  on  both  sides  (archipterygium  biseriale)  or 
only  on  one  (archipterygium  uniseriale)  the  lateral  rows  of  skeletal 
elements.  Usually  most  of  the  lateral  rows  are  not  attached  to 
the  principal  row,  but  arise  independently  from  the  girdle,  and 
may  begin  with  larger  parts,  the  propterygium  and  mesopterygium. 

From  this  archipterygium  can  be  derived  a  primary  form  which 
serves  for  all  terrestrial  vertebrates  from  the 
Amphibia  onwards;  it  is  the  pentadactyle  ap- 
pendage (fig.  565).  In  tracing  this  from  the 
archipterygium  (of  either  uniserial  or  biserial 
type)  the  following  modifications  must  be  sup- 
posed. First  a  reduction  in  the  number  of 
rows  to  five,  a  principal  row  and  four  acces- 
sory rows.  The  terminal  portions  of  the  prin- 
cipal row  produce  the  bones  of  the  fifth,  the 
accessory  rows  of  the  other  fingers.  Then 
there  is  an  unequal  growth  of  parts;  the  meta- 
pterygium,  already  in  Elasmobranchs  a  con- 
siderable element,  increases  in  size  and  forms 

the  fore  limb  the  humerus,  in   the  hind 


in 


limb  the  femur.  In  like  manner  the  second 
•element  of  the  principal  row  and  the  first  of 
the  first  accessory  row  increase  and  form  re- 
spectively ulna  and  radius  in  front,  fibula  and 
tibia  behind.  Then  follow  parts  which  remain 
small  and  somewhat  cubical,  carpal  bones  in 
the  fore  limb,  tarsals  in  the  hinder  extremity; 
they  bear  in  turn  slender  bones,  the  meta- 
•carpals  or  metatarsals,  and  these  at  last  the 
phalanges.  (For  the  nomenclature  of  carpals 
and  tarsals  see  the  explanation  of  fig.  565.) 

The  third  and  most  important  modification  is  brought  about 
by  the  development  of  joints.  So  long  as  the  appendage  served 
;as  an  oar  it  must  act  as  a  single  plate  with  its  parts  firmly  held. 
On  the  other  hand,  when  it  must  act  as  a  system  of  levers  to  sup- 
port and  move  the  body,  as  is  necessary  in  a  terrestrial  animal,  it 


FIG.  565.— Schema  of  a 
pentadactyle  appen- 
dage. (After  Gegen- 
baur.)  The  dotted 
lines  indicate  the  lat- 
eral rays ;  the  names 
for  the  hinder  ex- 
tremities in  parenthe- 
ses. H,  humerus  (fe- 
mur); U,  ulna  (fibula); 
#,  radius  (tibia).  Car- 

Eus  (tarsus)  consist- 
ig  of  two  rows  and 
two  central  portions: 
Row  I:  ?%  radiale  (tibi- 
ale);  i,  intermedium; 
u,  ulnare  (fibulare);  c, 
centralia.  Row  II : 
l-5,carpalia  (tarsalia); 
the  metacarpals  (met- 
atarsals) and  pha- 
langes not  lettered. 


530 


CHORDATA. 


must  be  divided  into  sections,  jointed  to  each  other.  By  this 
there  are  developed  two  joints  of  importance  in  both  fore  and  hind 
limbs;  the  elbow  (knee)  joint  between  humerus  (femur)  on  the 
one  hand  and  radius  and  ulna  (tibia  and  fibula)  on  the  other;  and 
the  wrist  joint  (ankle)  between  the  bones  of  the  fore  arm  (shank) 
and  the  carpals  (tarsals).  Less  important  are  the  joints  of  the 
fingers  and  toes. 

If  the  limbs  of  terrestrial  vertebrates  be  compared  with  this 
primary  form,  variations  are  seen  in  two  directions.  Rarely  are 
there  more  bones  than  in  the  schema;  then  there  occur  remnants 
of  a  sixth  or  even  a  seventh  row  or  finger.  More  frequently  there 
is  a  reduction  in  the  number  of  parts,  either  by  fusion  or  by  abso- 
lute loss.  Fusion  accounts  for  the  fact  that  with  complete  pen- 
tadactyly  the  number  of  carpalia  is  usually  less  than  ten,  as  would 
be  expected  from  the  schema.  Degeneration  and  loss  explain  the 
existence  of  animals  with  four,  three,  two,  or  even  one  digit,  and 
one  can  say  with  certainty  that  the  missing  parts  are  in  most  cases 
lost,  though  a  fusion  of  digits  is  not  unknown.  Paleontology,  for 
example,  teaches  that  the  one-toed  horse  has  descended,  by  gradual 
reduction,  from  five-toed  ancestors. 

The  completeness  and  character  of  the  skeleton  thus  sketched 


FIG.  566.— Horizontal  section  through  the  anterior  trunk  region  of  a  young  Rhodeus 
amarus  at  the  level  of  the  ventral  arches,  c,  notochord  ;  /i,  skin  ;  //,  interrnuscu- 
lar  ligament;  w,  longitudinal  muscles ;  r,  rib  end  of  the  cartilaginous  ventral 
arch  ;  v,  osseous  centrum. 

in  outline  has  a  great  influence  on  the  rest  of  the  organism.  It 
has  already  been  pointed  out  that  the  external  appearance  of  ver- 
tebrates has  been  influenced  by  it,  since  the  skin  is  no  longer,  as  in 
arthropods,  a  supporting  structure  and  has  consequently  lost  its 
segmentation.  More  immediate  is  its  influence  upon  the  arrange- 
ment of  the  musculature.  The  development  of  an  internal  skele- 
ton renders  it  necessary  that  the  point  of  resistance  of  the  muscles. 


IV.    VERTEBRATA.  531 

must  be  transferred  from  the  skin,  where  it  is  found  in  annelids, 
molluscs,  and  arthropods,  to  the  interior.  A  dermal  musculature 
occurs  only  as  an  inconspicuous  remnant  in  vertebrates;  it  is  re- 
placed by  a  body  musculature.  This  latter  consists  primarily  of  a, 
longitudinal  system  of  muscle  fibres  on  either  side  of  the  vertebral 
column  (fig.  566),  which  are  divided  by  connective-tissue  partitions,, 
the  myosepta  or  myocommata,  into  successive  segments,  the  myo- 
tomes.  Thus  when  the  connective  tissue  of  a  fish  is  dissolved  by 
cooking  the  muscles  fall  into  disk-like  parts.  The  myosepta  ex- 
tend from  skin  to  axial  skeleton.  Since  they  run  obliquely  back- 
wards from  the  skeleton  to  the  skin,  they  serve  to  render  the 
skeleton  a  point  of  resistance  for  the  action  of  the  muscles. 

A  segmented  trunk  musculature  occurs  in  the  Myxinoids  (and 
in  Amphioxus),  in  which  the  axial  skeleton  consists  only  of  noto- 
chord  and  is  consequently  un  jointed.  The  segmentation  of  the 
muscles  is  therefore  older  than  that  of  the  skeleton  and,  as  we  can 
further  say,  is  the  cause  of  it.  The  action  of  the  muscles  prevents, 
the  formation  of  a  cartilaginous  or  bony  vertebral  continuum  such 
as  the  notochord  and  skeletaginous  layer  are.  It  produces  at  in- 
tervals joints  or  flexible  parts  separating  the  cartilaginous  or  bony 
column  into  vertebrae.  Naturally  these  flexible  portions  cannot, 
coincide  with  the  boundaries  of  the  muscles,  but  must  lie  between 
them;  in  other  words,  muscle  segments  and  skeletal  segments — 
myotomes  and  sclerotomes — must  alternate.  Segmentation  is  lack- 
ing in  the  cranium,  since  the  myotomes  here  have  no  locomotor 
significance,  are  reduced,  and  only  small  remnants  of  them  persist. 

In  the  mammals  only  a  little  of  this  segrnental  arrangement  of 
muscles  is  recognizable,  a  result  of  the 
development  of  the  appendages;  and  the 
more  these  gain  in  importance  as  the 
locomotor  structures,  the  more  the  mus- 
cles are  modified  and  grouped  for  the  ser- 
vice of  the  limbs,  so  that  only  the  inter- 
costals  and  a  part  of  the  muscular  system 
to  the  sides  of  the  vertebral  column  show 
clearly  the  primitive  metamerism.  Yet 
in  all  vertebrate  embryos  the  muscles  ap- 
pear  at  first  strictly  segmental,  in  the 
form  of  the  primitive  somites  (fig.  567), 
formerly  called  protovertebrse.  tome). 

Another  important  point  in  the  musculature  lies  in  the  fact 
that  it  is  dorsal  in  origin  and  therefore  in  fishes  is  largely  dorsal 


532  CHOEDATA. 

in  position  throughout  life.  The  muscles  which  are  ventral  have 
largely  been  transferred  from  the  back,  and  the  cause  of  the  migra- 
tion is  to  be  recognized  to  a  large  extent  in  the  progressive  devel- 
opment of  the  appendages.  The  dorsal  position  of  the  muscles  is 
only  a  part  of  a  general  fact,  that  the  skeletal  axis  divides  the  body 
into  a  dorsal  zone,  containing  only  animal  organs,  and  a  ventral 
zone,  chiefly  vegetal  in  character.  Besides,  the  muscles,  the  cen- 
tral nervous  system,  and  the  most  important  sense  organs — eyes, 
nose,  ears — belong  to  the  dorsal  zone. 

The  central  nervous  system  of  vertebrates  consists  of  brain  and 
spinal  cord.  Like  that  of  all  chordates  it  is  distinguished  from 
that  of  other  segmented  animals — annelids,  arthropods,  in  which 
there  is  a  dorsal  brain  and  a  ventral  nerve  chain — in  its  purely 
dorsal  position.  It  is  further  distinguished  from  that  of  all  non- 
chordates  by  its  tubular  character,  that  is,  by  the  presence  of  a 
central  canal  in  the  axis  of  the  elongate  central  system  (fig.  76), 
lined  by  a  special  epithelium,  the  ependyma,  and  containing  a 
fluid,  the  liquor  cerebrospinalis.  This  central  canal  is  the  result 
of  the  mode  of  development,  the  nervous  system  arising  by  an  in- 
rolling  of  the  ectoderm  and  not  by  a  splitting  from  it  as  in  the 
invertebrates  (fig.  9).  Besides  the  neurenteric  canal  already 
referred  to  (p.  502),  there  long  persists  at  the  anterior  end  an 
opening  to  the  exterior,  the  neuropore.  In  all  vertebrates,  in  con- 
tradistinction to  the  lower  chordates,  the  brain  is  large  and  sharply 
marked  off  from  the  spinal  cord. 

The  spinal  cord  is  a  cylindrical  structure  (flattened  in  Cyclo- 
stomes,  fig.  555)  which,  in  the  middle  line  above  and  below,  is 
marked  by  two  longitudinal  grooves,  the  dorsal  and  ventral  fissures 
of  the  cord  (sp,  sa,  fig.  76).  The  central  canal  (Cc)  has  its  lumen 
greatly  narrowed  by  the  growth  of  the  nervous  tissue,  in  which, 
.as  in  the  ganglia  of  the  invertebrates,  two  layers  are  distinguished, 
one  containing  almost  solely  nerve  fibres,  the  other  both  fibres  and. 
nerve  or  ganglion  cells.  The  arrangement  of  these  layers  is  con- 
trasted with  that  of  the  invertebrates  in  that  the  ganglion-cell 
layer — the  gray  matter — lies  in  the  centre,  the  fibrous  layer — white 
matter  ( W) — on  the  periphery,  a  reversed  position  consequent 
upon  the  development  TDJ  infolding.  The  distinction  in  color 
indicated  in  the  names  depends  upon  the  fact  that  white  medullated 
fibres  run  in  the  cortex,  while  in  the  gray  matter  gray  non-medullated 
fibres  are  present  between  the  nerve  cells.  The  color  distinctions 
fail  in  the  cyclostomes  (and  AmpJiioxus),  which  have  no  medullated 
fibres,  although  the  same  general  structure  occurs. 


IV.    VE1UEBRATA. 


533 


The  gray  matter  surrounds  the  central  canal,  but  extends  on 
either  side  dorsally  and  ventrally  into  the  white  matter,  so  that 
in  section  it  resembles  somewhat  the  letter  H,  with  its  dorsal 
(fig.  76,  HH)  and  ventral  horns  (VH).  By  means  of  these  horns 
and  the  dorsal  and  ventral  nerve  roots  arising  from  them,  the  white 
matter  on  either  side  is  divided  into  three  tracts,  the  dorsal  (H)> 
ventral  (s),  and  lateral  (S)  columns  of  the  cord. 

Corresponding  to  each  muscle  segment  two  nerve  roots  arise- 
from  the  cord,  a  dorsal  root,  with  a  ganglion  (spinal  ganglion)  at 
some  distance  from  the  cord,  and  a  ventral  root,  without  a  ganglion. 
The  dorsal  root  contains  only  sensory  fibres — i.e.,  those  carrying; 
nervous  impulses  to  the  cord — and  is  afferent,  while  the  ventral 
roots  are  efferent  and  contain  only  motor  elements  (Bell's  Law). 
These  roots  unite  into  a  mixed  root,  which  then  divides  into  dorsal 
and  ventral  branches. 

The  brain  of  vertebrates  in  general  corresponds  in  its  funda- 
mental plan  (fig.  568),  best  seen  in  development,  with  the  brain  of 
man.  At  an  early  stage  it  consists  of  three 
vesicles,  one  after  the  other,  a  fore  brain 
(prosencephalon),  a  mid  brain  (mesencepha- 
lon),  and  a  hind  brain  (metencephalon). 
Usually  this  stage  is  reached  before  the 
closure  of  the  medullary  folds.  Formerly  it 
was  stated  that  a  condition  with  five  vesicles 


-ir 


FIG.  568.  FIG.  569. 

FIG.  568.— Diagram  of  a  vertebrate  brain.     (From  Wiedersheim.)    Aq,  aqueduct ;  O, 

central  canal ;  FM,  foramen  of  Monro  (connexion  of  lateral  ventricles  with  each 

other  and  with  the  third) ;  HH:  cerebellum  ;  MH,  corpora  bigemina  (optic  lobes) ; 

NHt  medulla  oblongata;  J?,  spinal  cord;  <SF,  lateral  ventricles;    VH,  cerebrum; 

ZH,  optic  thalami  ('twixt  brain) ;  ///.  IF,  third  and  fourth  ventricles. 
FIG.  569.— Scheme  of  brain  in  sagittal  section,    c,  cerebrum  ;  c-6,  cerebellum;  rr,  canal 

of  spinal  cord  ;  c/i,  notochord ;  c-s  corpus  striatum  ;  /i,  hypophysis  ;  *',  infundibu- 

lum  ;  m,  medullary  region  ;  o,  optic  chiasma  ;  t>/,  olfactory  lobe ;  oZ,  optic  lobes  ; 

p,  pinealis. 

followed  upon  this  with  three,  the  mid  brain  remaining  undivided, 
while  the  hind  brain  divides  into  cerebellum  (cb)  and  medulla 
oblongata  (m) ;  the  fore  brain  into  cerebrum  and  'twixt  brain. 
This  is  unnatural  so  far  as  the  hind  brain  is  concerned,  for  cere- 


534  CHORD  AT  A. 

bellum  and  medulla  are  related  to  one  another  as  roof  and  floor  of 
one  and  the  same  cavity  (fig.  569).  The  distinction  between  the 
first  and  second  vesicles  is  problematical.  The  fore  brain  becomes 
divided  into  three  parts  by  an  inpushing  at  its  anterior  end:  an 
impaired  middle  portion,  and  in  front  a  right  and  a  left  diver- 
ticulum.  These  paired  portions,  increasing  in  size,  form  the  cere- 
bral hemispheres,  and  together  with  a  small  connecting  part 
represent  the  first  cerebral  vesicle,  while  the  unpaired  portion 
forms  a  second  vesicle,  the  'twixt  brain. 

Introducing  the  terms  of  human  anatomy  for  the  separate  parts 
of  the  brain,  the  first  vesicle  consists  of  the  two  cerebral  hemi- 
spheres whose  dorsal  and  lateral  walls  are  usually  thick  and  are 
called  the  pallium,  while  in  the  floor  of  each  hemisphere  is  an 
enlargement,  the  corpus  striatum  (cs).  The  spaces  in  the  hemi- 
spheres are  the  first  and  second  ventricles  (sv).  From  the  front 
portion  of  each  hemisphere  arises  a  distinct  region,  the  olfactory 
lobe  (o/),  which  gives  origin  to  the  olfactory  nerve.  Since  the 
organ  of  smell  is  frequently  at  some  distance  from  the  brain,  the 
olfactory  nerve  must  be  elongate,  as  in  the  Amphibia  (fig.  614),  or 
the  olfactory  lobe  must  lengthen,  as  in  many  Elasmobranchs  (fig, 
592).  In  the  latter  case  the  swollen  end  of  the  lobe  is  close  to 
the  olfactory  epithelium  and  is  connected  with  the  brain  by  a  long 
stalk,  the  tractus,  while  the  swelling  is  called  the  bulbus  olfacto- 
rius.  Both,  as  parts  of  the  brain,  must  be  distinguished  from  the 
olfactory  nerve. 

In  the  region  of  the  second  vesicle  only  the  lateral  walls  become 
thickened,  producing  the  optic  thalami,  directly  adjoining  the 
corpora  striata;  the  roof  of  this  vesicle  develops  no  nervous  sub- 
stance, but  remains  a  thin  layer  of  epithelium  closing  in  the  third 
ventricle  above  (///)•  The  floor  is  also  thin-walled  between  the 
thalami  and  is  pushed  downwards,  forming  a  funnel-like  pocket, 
the  infundibulum  (i).  The  third  vesicle,  as  a  rule,  is  divided  by 
a  deep  longitudinal  dorsal  groove,  dividing  the  cavity  into  a  right 
and  left  ventricle,  while  the  two  halves  of  the  roof  are  known  as 
the  optic  lobes  or  corpora  bigemini.  In  the  mammals  alone  (in 
^vhich  there  is  also  a  transverse  groove  dividing  the  optic  lobes 
into  the  corpora  quadrigemini)  the  cavity  of  this  mid  brain  is  re- 
duced, by  thickening  of  the  walls,  to  a  narrow  canal,  the  iter  or 
aqueduct  of  Sylvius,  with  the  result  that  the  term  fourth  ventricle 
is  transferred  to  the  cavity  of  the  hind  brain. 

This  last  region  is  called  the  medulla  oblongata ;  it  is  a  prolonga- 
tion of  the  spinal  cord,  and  in  many  respects  shows  a  similar  struc- 


IV.    VEBTEBRATA.  535 

ture.  It  is  distinguished  from  the  cord  externally  in  that  it 
gradually  increases  in  size  in  front,  while  its  roof  is  reduced  to  a 
thin  epithelium,  often  torn  away  in  dissection,  leaving  an  opening, 
the  fossa  rhomboidalis,  into  the  ventricle.  In  front  of  this  fossa  is 
the  cerebellum,  often  a  thin  transverse  nervous  lamella,  but  usually 
is  a  considerable  part  of  the  brain,  composed  of  a  median  '  vermis ' 
and  two  lateral  cerebellar  hemispheres. 

Although  these  five  parts  are  present  in  all  vertebrates,  the 
appearance  of  the  brain  in  the  various  classes  is  very  different, 
because  the  relative  size  and  form  of  the  parts  undergo  great 
variations.  In  the  lower  vertebrates  optic  lobes  and  medulla 
oblongata  are  disproportionately  large,  while  the  cerebrum,  and 
often  the  cerebellum,  are  insignificant  in  size;  in  the  cerebrum, 
again,  the  hemispheres  may  be  smaller  than  the  corpora  striata 
and  the  olfactory  lobes.  In  the  higher  vertebrates,  on  the  other 
hand,  the  cerebrum  and  cerebellum  far  surpass  the  other  parts, 
the  increase  in  size  of  the  cerebrum  being  proportional  to  the  in- 
crease in  intelligence.  The  cerebral  hemispheres  grow  backwards, 
in  man  and  the  apes  covering  the  other  parts,  while  in  front  the 
olfactory  lobes  are  carried  by  a  similar  overgrowth  to  the  lower 
surface.  Since  the  capacity  of  the  skull  is  limited,  the  cortex  of 
the  cerebrum,  the  seat  of  intelligence,  is  increased  in  amount  by 
the  development  of  folds,  gyri,  separated  by  sulci.  Somewhat 
similar  conditions  exist  in  the  cerebellum,  which  in  mammals  and 
birds  is,  next  to  the  cerebrum,  the  largest  part  of  the  brain. 

Connected  with  the  'twixb  brain  are  two  problematical  organs,  one,  the 
epiphysis  (pinealis),  being  dorsal;  the  other,  the  hypophysis  (pituitary 
body),  ventral.  The  hypophysis  arises  like  a  gland  by  an  outgrowth  from 
the  embryonic  mouth.  This  hypophysial  pocket  cuts  off  from  its  source, 
increases  by  budding,  and  fuses  with  parts  derived  from  the  end  of  the 
infundibulum  to  a  single  two-lobed  body.  It  has  been  compared  with  the 
subneural  gland  of  the  Tunicata  (p.  509).  The  epiphysis  is  an  outgrowth 
from  the  roof  of  the  brain,  from  which  develops  in  many  vertebrates  the 
parietal  organ.  In  many  reptiles  this  has  the  structure  of  an  eye  (pineal 
eye),  and  in  these,  separated  from  the  brain,  but  connected  with  it  by  a 
nerve,  it  lies  in  a  special  cavity  in  the  parietal  bone,  which  occurs  not  only 
in  recent  but  in  fossil  forms.  Above  the  eye  the  skin  may  be  transparent. 

The  nerves  which  come  from  the  brain  mostly  arise  from  the 
region  between  the  mid  brain  and  the  spinal  cord,  especially  from 
the  medulla  oblongata.  The  olfactory  and  optic  nerves  are  an 
exception,  the  one  arising  from  the  cerebrum,  the  other  from  the 
'twixt  brain,  but  both,  and  especially  the  optic,  differ  so  much  from 
the  peripheral  nerves  that  they  can  hardly  be  classed  with  them. 


536 


CHORD  AT  A. 


Development  shows  that  the  optic  nerve  is  a  part  of  the  brain. 
Following  custom,  however,  and  including  these  two,  the  pairs  of 
cranial  nerves  may  be  enumerated  in  the  terms  of  human  anatomy 
as  follows:  I,  N.  olfactorius;  II,  N.  opticus;  III,  N.  oculomotoiius; 
IV,  N.  trochlearis  (patheticus);  V,  N.  trigeminus;  VI,  N.  abducens; 
VII,  N.  facialis;  VIII,  N.  acusticus;  IX,  N.  glossopharyngeus; 


FIG.  570. — Diagram  of  cranial  nerves  (shark),  a,  alveolaris  ;  ft,  buccalis  ;  c,  cere- 
brum ;  cb,  cerebellum;  ct,  chorda  tympani ;  e,  ear ;  er,  external  rectus  muscle  ; 
/,  inferior  rectus  muscle ;  g,  Gasserian  ganglion  ;  h,  hyoid  cartilage;  hm,  hyoman- 
dibular ;  i,  internal  rectus  muscle ;  10,  inferior  oblique  muscle  ;  j,  Jacobson's 
commissure  ;  I,  lateralis  of  vagus  ;  m,  mouth  ;  me,  Meckel's  cartilage:  md,  mandi- 
bularis  ;  mar,  maxillaris  superior;  n,  nose  ;  o,  optic  lobes;  op,  ophthalmicus  profun- 
dus;  os,  ophthalmicus  superficialis;  p,  pinealis;  pi,  palatine ;  po,  posttrematic 
branches;  j?»\  pretrematic  branches;  pn,  pneumogastric  (intestinal)  of  vagus; 
ptg,  pterygoquadrate;  s,  spiracle;  so,  superior  oblique  muscle;  sr,  superior  rectus 
muscle;  f,  'twixt  brain;  I-X,  cranial  nerves:  1-5,  gill  clefts. 

X,  N.  vagus  (pneumogastricus),  XI,  N.  accessorius;  XII,  N. 
hypoglossus.  The  accessorius  in  fishes  and  amphibia  is  a  part  of 
the  vagus;  the  hypoglossus,  strictly  speaking,  belongs  to  the  spinal 
nerves  and  only  secondarily  is  associated  with  the  cranial  nerves, 
which  explains  its  course,  outside  the  skull,  in  cyclostomes  and 
amphibia. 

Since  the  head  undoubtedly  consists  of  several  coalesced  body  seg- 
ments (at  least  as  many  as  there  are  visceral  arches,  and  apparently 
more),  the  question  arises  whether  the  cranial  nerves  are  as  evidently  seg- 
mental  as  are  those  of  the  trunk.  To  this  is  allied  the  further  question 
whether  Bell's  Law  that  a  mixed  nerve  consists  of  dorsal  sensory,  and 
ventral  motor  components  is  applicable  here.  Both  problems  have  been 
much  discussed  in  recent  years,  but  as  yet  the  final  answers  have  not  been 
given.  It  is  probable  that  the  present  cranial  nerves,  the  optic  and  olfac- 
tory excepted,  have  arisen  by  manifold  rearrangements  of  segmental 
nerves.  On  the  other  hand  it  seems  impossible  to  accept  Bell's  Law  here 
without  considerable  modification,  since  many  cranial  nerves  (facialis, 
trigemenus,  etc.)  contain  motor  fibres,  although  they  are  formed  like 
dorsal  roots. 


IV.    VERTEBBATA.  53T 

Besides  the  nervous  system  of  the  body  already  outlined,  the  vertebrates 
have  a  special  nervous  system  supplying  the  viscera, — the  sympathetic 
system, — and  in  this  a  special  central  organ  consisting  of  right  and  left 
cords  beneath  the  vertebral  column,  in  which  ganglia  are  incorporated. 
The  last  of  these  ganglia  lies  at  the  base  of  the  caudal  vertebrae,  the  most 
anterior  at  the  beginning  of  the  neck.  From  the  latter  nerve  corda 
extend  into  the  head  and  are  connected  with  ganglia  (otic,  sphenopalatine). 
This  system  sends  out  nerves  in  the  form  of  delicate  networks  (plexus 
sympathetici)  which  usually  accompany  the  blood-vessels  to  the  vegeta- 
tive organs  (intestine,  sexual  apparatus,  etc.).  It  is  also  connected  with 
the  spinal  nerves. 

Regarding  the  sense  organs  of  the  vertebrates  we  stand  on 
firmer  ground  than  with  the  invertebrates,  since  their  great  simi- 
larity to  those  of  man  supports  the  ideas  of  their  functions  derived 
from  studies  of  their  structure.  The  tactile  organs  make  an  ex- 
ception, since  only  in  land  animals,  and  not  in  fishes,  do  they 
resemble  those  of  man.  These  organs,  in  all  forms  above  fishes, 
have  the  peculiarity  that  the  nerves  do  not  end  in  epithelial  cells, 
but  in  special  tactile  cells  of  the  derma,  which  either  lie  isolated  in 
the  connective  tissue  (Amphibia,  reptiles),  or,  grouped  together,, 
produce  tactile  corpuscles  (birds,  mammals, 
fig.  571).  These  are  oval  bodies  and  are  im- 
bedded in  special  papillae  of  the  derma.  In 
form  and  position  they  are  much  like  the 
Vater-Pacinian  corpuscles,  which  are  distin- 
guished by  their  histological  structure  (fig.  78) 
and,  since  they  also  occur  in  internal  organs 
(mesentery  of  cat),  are  of  problematic  function.  v— >JV^T 

Besides  these  mesodermal  nerve  endings  there  FIG.  571.— Tactile  cor- 
are  present    in  all  vertebrates   intraepithelial      tongife. 
nerve  branchings  which  are  best  seen  in   the      cfeff6) 
cornea  of  the  eye  and  in  animals,  like  pigs  and      Petitions, 
moles,  with  sensitive  snouts.     Even  here  the  finest  nerve  twigs  do 
not  end  in  epithelial  cells,  but  in  small  knobs  between  them. 

Fishes  lack  tactile  cells,  tactile  corpuscles,  and  end  bulbs; 
hence  the  skin  is  provided  with  sense  organs  in  which  a  sensory 
epithelium  occurs.  The  dermal  nerves  pass  into  the  epidermis 
and  end  in  oval  corpuscles,  which,  while  imbedded  in  a  stratified 
epithelium,  consist  of  a  single  layer  of  sense  cells.  According  to 
structure,  nerve  hillocks  and  nerve-end  buds  are  distinguished. 
The  first  are  the  specific  organs  of  the  lateral  line,  to  be  men- 
tioned later,  of  fishes  and  branchiate  amphibians  and  amphibian 
larvae,  and  therefore  appear  to  subserve  special  and  important  sensa- 


538 


CHORD  AT  A. 


tions  connected  with  aquatic  life;  hence  the  idea  of  a  'sixth 
sense/  lacking  to  man  (cf.  p.  125).  The  end  buds  are  especially 
collected  in  the  neighborhood  of  the  mouth,  on  the  lips  and  bar- 
bels. Since  they  also  occur  in  the  mucous  membrane  of  the  mouth, 
especially  in  the  palatal  regions,  they  connect  with  the  taste  organs. 
The  taste  buds  have  the  same  structure  as  the  end  buds  of  fishes. 
They  occur  in  all  classes  of  vertebrates,  and  are  most  abundant  in 
man  in  the  walls  of  the  circumvallate  papillae  at  the  base  of  the 
tongue;  in  rodents  on  the  large  foliate  papillae,  etc. 

The  end  buds  also  lead  to  the  olfactory  organs.  The  olfactory 
epithelium  of  many  fishes  and  amphibia  is  a  stratified  epithelium 
with  closely  arranged  end  buds  (fig.  572),  By  disappearance  of 


FIG.  572.— Section  of  olfactory  epithelium  of  a  fish  (Belone).    (From  O.  Hertwig,  after 
Blaue.)    e,  epithelium  •  fc,  olfactory  buds ;  n,  nerves. 

the  isolating  parts  of  the  ordinary  epithelium  the  end  buds  form 
a  continuous  sensory  epithelium,  which  is  the  rule  in  most  ver- 
tebrates. 

The  olfactory  organ,  the  nose,  lined  with  its  sensory  epithelium, 
.acquires  a  special  interest  both  from  its  grade  of  development  and 
from  the  important  systematic  distinctions  it  affords.  Except  the 
cyclostomes,  which  have  an  unpaired  nasal  sac,  all  vertebrates  have 
paired  olfactory  organs.  In  adult  fishes  and  in  the  embryos  of 
higher  forms  are  two  pits  which  lie  in  front  of  or  dorsal  to  the 
mouth;  they  are  either  distinct  from  it  or  only  connected  with 
it  by  an  oronasal  groove  in  the  skin  (fig.  599).  If  the  animal 
be  terrestrial  and  replace  branchial  by  pulmonary  respiration,  a 
respiratory  canal  is  developed  in  connexion  with  the  nose.  The 
oronasal  groove  closes  to  a  tube  which  begins  with  an  opening 
(nostril)  on  the  surface  and  ends  with  a  second  opening  (choana) 
in  the  mouth  cavity.  The  olfactory  sac  proper  is  included  in  the 
wall  of  this  tube,  usually  on  its  dorsal  surface  (fig.  573).  In  Am- 


IV.    VERTEBRATA.  539 

phibia,  lizards,  snakes,  and  birds  the  choanais  far  forward,  behind 
the  upper  jaw;  in  alligators,  turtles,  and  mammals  it  is  carried  far 
back,  in  crocodiles  and  some  mammals 
(edentates)  nearly  to  the  vertebral  col- 
umn. This  position  is  brought  about  by 
the  development  of  the  hard  palate,  a 
parting  wall  which  divides  the  primitive 
mouth  cavity  into  two  portions,  a  lower, 
the  persistent  or  secondary  mouth  cavity, 
and  an  upper,  which,  as  secondary  nasal  Fl<?-  573.— Diagram  of  nose  of 

lizard.  (After  Wiedersheim.) 

•cavity,    contributes  to  the  air   passages.     AN,  outer  nasal  cavity;  c, 

.m,  ^    ,1  -11  -i         i    f«  olfactory  sac ;  O,  canal  from 

The  bones  of  the  maxillary  and  palatine     Jacobson's  organ  to  mouth : 

,    .,  ,-,       i         -,         i    ,          .  C7i,  choana;  IN,  inner  nasal 

Aeries  contribute  to  the  hard  palate,  since    cavity;  MS,  roof  of  mouth; 

•  11      •  -n      •  j  i      j.i          -P,  Jacobson's  organ ;  t,  con- 

premaxillanes,  maxillanes,  and  rarely  the  nexion  between  nasal  cavi- 
pterygoids  send  out  horizontal  processes 

"which  meet  in  the  middle  line.  In  the  mammals  this  partition 
is  continued  back  by  the  muscular  soft  palate.  In  crocodiles  there 
is  a  fibrous  palate. 

In  the  olfactory  organ  of  the  chordates  two  constituents  must  be 
recognized,  an  unpaired  and  two  paired  portions.  The  unpaired  portion 
alone  occurs  in  Amplrioxus,  this  being  supplied  by  the  lobus  olfactorius 
impar  ;  in  all  vertebrates  there  are  paired  sacs  with  paired  olfactory  lobes. 
The  unpaired  sac  of  the  cyclostomes  has  apparently  arisen  from  a  union  of 
paired  and  unpaired  parts,  hence  the  double  olfactorius. 

A  further  increase  in  the  nasal  cavity  is  brought  about  by  complicated 
folds  in  the  walls  supported  by  special  skeletal  parts,  the  turbinal  bones, 
.and  also  by  the  outgrowth  of  chambers,  lined  with  mucous  membrane 
which  extends  into  the  neighboring  bones.  Thus  are  formed  the  sinus 
frontalis  in  the  frontal  bone  ;  behind,  the  sphenoid  sinus  in  the  sphenoid, 
and  the  antrum  of  Highmore  in  the  maxillary.  Again,  a  part  of  the  primi- 
tive chamber  lined  with  olfactory  epithelium  can  be  cut  off  from  the  rest 
and  form  an  accessory  nose,  Jacobson's  organ,  which  opens  into  the 
mouth  behind  the  premaxillaries  by  'Stenson's  duct'  (fig.  573,  P).  This 
organ  is  best  developed  in  lizards,  mbnotremes  and  ungulates,  but  often 
occurs  in  a  reduced  condition  in  other  terrestrial  vertebrates. 

In  all  vertebrates  with  the  exception  of  Myxine  and  a  few  forms 
living  in  the  dark  the  eyes  are  composed  of  all  the  principal  con- 
stituents which  occur  in  the  human  eye  and  which  have  already 
been  briefly  described  (p.  131,  fig.  83).  In  most  vertebrates  it  is  a 
nearly  spherical  body  with,  the  optic  nerve  entering  it  from  behind, 
with  its  interior  occupied  by  transparent,  refractive  substances 
(lens,  vitreous  body),  and  its  walls  of  three  concentric  layers. 
The  outer  of  these  is  the  tough  protecting  sclera  (sclerotic),  a 


540  CHORD  ATA. 

usually  fibrous,  but  in  many  fishes  a  cartilaginous,  layer,  which  in 
front  becomes  transparent  and  strongly  curved,  forming  the  cornea. 
The  second  layer,  the  choroid  coat,  is  richly  vascular  and  pig- 
mented;  at  the  boundary  between  sclerotic  and  cornea  it  is  changed 
to  the  iris.  The  inner  layer  is  the  retina,  the  structure  and 
arrangement  of  which  are  characteristic  of  the  vertebrates. 

From  the  developmental  standpoint  the  retina  (fig.  82)  con- 
sists of  two  parts,  the  retina  proper  and  the  tapetum  iiigrum 
(pigmented  epithelium),  formerly  regarded  as  part  of  the  choroid. 
In  the  retina  the  following  layers  are  distinguished:  (1)  the  limi- 
tans  interna;  (2)  nerve-fibre  layer;  (3)  ganglionic  layer;  (4)  inner 
molecular  layer;  (5)  inner  granular  layer;  (6)  outer  molecular 
layer;  (7)  outer  granular  layer;  (8)  limitans  externa;  and  (9)  layer 
of  rods  and  cones.  The  limitans  externa  is  the  bounding  mem- 
brane of  the  embryonic  retina,  which  is  later  penetrated  by  the 
rods  and  cones.  Between  the  two  limiting  membranes  Miiller's 
fibres  (m)  extend,  large  supporting  cells  occurring  in  other  sensory 
epithelia,  the  nuclei  of  which  lie  in  the  inner  granular  layer,  and 
which  are  aided  in  their  supporting  function  by  the  fine  horny ' 
framework  of  both  molecular  layers.  The  nervous  elements 
which  are  imbedded  in  this  support  are  best  understood  by  begin- 
ning with  the  optic  nerve.  This  spreads  out  in  the  nerve-fibre 
layer,  and  on  its  way  to  the  end  apparatus  comes  twice  into  relation 
with  ganglion  cells;  first  in  the  ganglionic  layer,  second  in  the 
inner  granular  layer.  Thus  a  great  part  of  the  retina  (layers  1  to 
6)  are  to  be  considered  as  an  optic  ganglion,  such  as  occurs  in 
molluscs  and  arthropods,  but  which  there  lies  outside  the  sensory 
apparatus.  The  sensory  epithelium  (the  retina  in  the  sense  this 
term  is  used  in  invertebrates)  consists  of  but  two  layers,  the- 
outer  granular  layer  and  the  rods  and  cones.  The  outer  granules 
are  the  nuclei  of  the  extremely  slender  epithelial  cells  which  bear 
the  rhabdomes  (rods  and  cones)  on  their  peripheral  ends.  Pigment 
cells  are  lacking  between  these  visual  cells,  but  the  pigment  so 
necessary  for  the  visual  function  is  supplied  by  the  tapetum 
nigrum  already  mentioned.  This  is  a  layer  of  hexagonal  epithelial 
cells  which  lies  on  the  tips  of  the  rhabdomes  and  sends  pseud  opodia- 
like  processes  between  them,  and  since  the  tapetum  is  rich  in 
black  pigment  granules,  the  rods  and  cones  are  enveloped  in  a 
close  pigment  mantle. 

Although  in  this  relation  of  pigment  and  in  the  union  of  the 
optic  ganglion  with  the  sensory  cells  important  differences  are  to 
be  noted  from  the  eyes  of  the  invertebrates,  even  of  the  closely 


IV.    VERTEBRATA. 


.similar  cephalopod  eye  (p.  385),  the  most  striking  difference  re- 
mains to  be  mentioned.  The  retina,  with  its  limitans  interna  and 
nerve-fibre  layer,  abuts  against  the  vitreous  body;  with  its  rhab- 
domes and  tapetum  against  the  choroid.  Hence  the  incoming 
light  must  traverse  the  optic  ganglion  and  pass  through  the  layer  of 
.sense  cells  before  reaching  the  end  organs,  the  rhabdomes.  In 
nearly  all  invertebrates,  for  example  the  Cephalopoda  (fig.  383), 
tha  light  falls  directly  on  the  peripheral  end  of  the  rhabdome. 
The  rhabdomes  in  cephalopods,  as  in  most  invertebrates,  are 
turned  towards  the  light,  in  the  vertebrates  away  from  it. 

This  peculiar  and  functionally  purposeless  inversion  of  the  vertebrate 
retina  is  explained  by  the  development  of  the  eye.  This  can  be  divided, 
according  to  origin,  into  two  parts,  a  cerebral  (optic  nerve,  retina,  tape- 
tum) and  a  peripheral  (all  other  parts).  As  the  eye  in  tunicates  and  Am- 
phioxns  is  permanently  a  part  of  the  brain,  so  is  the  retina  of  vertebrates 
genetically,  and  of  the  first  cerebral  vesicle.  An  outgrowth  occurs  on 
•either  side  (fig.  574,  B)  of  the  'twixt  brain  and  becomes  expanded  distally 


FIG.  574.— Diagram  showing  the  inversion  of  layers  in  the  formation  of  the  retina 
(orig.).  The  nuclei  are  placed  in  the  (morphologically)  deeper  ends  of  the  cells. 
In  A  the  brain  (t>)  has  been  closed  in  ;  in  B  the  optic  vesicle  (v)  has  reached  the 
lens  (?)  and  on  the  right  is  being  converted  into  the  double-walled  optic  cup 
with,  as  shown  in  C\  an  outer  tapetal  (e)  and  an  inner  retinal  layer  (/). 

to  an  optic  vesicle  which  is  connected  with  the  brain  by  an  optic  stalk. 
The  vesicle  extends  out  to  the  periphery  and,  coincidently  with  the  de- 
velopment of  the  lens,  is  folded  into  a  double-walled  optic  cup  with  outer 
or  tapetal,  inner  or  retinal  layers.  If  the  position  of  the  epithelial  cells 
be  followed,  it  will  be  seen  that  the  peripheral  ends  rest  upon  the  tapetum, 
and  when  these  ends  develop  the  rhabdomes,  these  must  grow  into  the 
tapetal  layer. 

In  contrast  to  the  retina,  the  lens  develops  as  an  invagination  from 
the  epithelium  of  the  body  (fig.  574) ;  sclera,  cornea  and  vitreous  body  from 
connective  tissue.  Thus  the  important  part  of  the  eye  arises  from  the 
brain  and  is  later  provided  with  accessory  apparatus  which  arise  from 
peripheral  parts.  The  invertebrate  eye,  on  the  other  hand,  with  all  its 
parts  arises  from  the  skin. 

The  vertebrate  eye  is  furnished  with  secondary  structures :  with  mus- 
cles which  move  it,  with  lids  which  protect  the  cornea  from  injury  and 
drying.  The  lids  are  dermal  folds  which  extend  over  the  eyeball  from 
above  and  below.  To  these  a  third  lid,  the  nictitating  membrane,  may 


542 


CHORDATA. 


be  added.  It  arises  from  the  inner  angle  of  the  eye,  and  can  extend  over 
the  cornea  beneath  the  upper  and  lower  lids.  A  special  lachrymal  gland, 
which  occurs  at  the  outer  angle  of  the  eye,  provides  the  fluid  to  moisten, 
the  cornea,  while  a  second  or  Hurder's  gland  occurs  at  the  inner  angle^ 
when  a  nictitating  membrane  is  present.  Both  are  lacking  in  the  An- 
ainnia. 

The  ear,  at  the  level  of  the  medulla  oblongata,  rivals  the  eye  in 
its  complication  of  structure.     In  development  it  has  one  point  in 

common  with  the   invertebrate 
ofcocyst — it 
dermal    pit 


ass 


ecu 


arises  as  an  ecto- 
which  is  usually 
completely  cut  off  from  its  par- 
ent layer,  and  only  in  elasmo- 
branchs  remains  connected  with 
the  exterior  by  a  tube,  the 
elsewhere  closed  endolymphatic 
duct.  In  the  cyclostomes  it  con- 
sists of  a  single  vesicle  with  a 
single  macula  acustica;  from 
the  fishes  upwards  the  vesicle 
becomes  divided  by  a  constric- 
tion into  an  upper  utriculus 
and  a  lower  sacculus  (fig.  575), 
the  connecting  utriculosaccular 

FIG.  575.— Diagram  of  membranous  laby-   duct  being  narrow  ill   the   mam- 


rinth  of  a  fish.  (From  Wiederaheim.) 
aa,  ae,  ap,  anterior,  external,  and  poste-  maiS. 
rior  ampullae ;  ass,  superior  utricular 
sinus ;  ca,  ce,  cp,  anterior,  external,  and 
posterior  semicircular  canals ;  cus,  utri- 
culosaccular canal;  de,  ductus  en- 
dolymphaticus  ;  7,  lagena ;  rec,  recessus 
utriculi ;  se,  sacculus  utriculi ;  ss,  supe- 
rior utricular  sinus ;  sp,  posterior  utri- 
cular sinus;  u,  utriculus;  t,  origin  of  en- 
dolymph  duct. 


culus 
macula 
from  t 


Both  utriculus  and  sac- 
receive    a    part    of    the 


a 

-i. 

acustica.  Diverticula 
occur,  giving 
name  of  labyrinth. 
From  the  utriculus  arise  three 
semicircular  canals,  connected  at  either  end  with  this  cavity,  each 
swollen  at  one  end  to  an  ampulla,  containing  a  special  nerve 
termination,  the  crista  acustica.  These  canals  stand  at  right 
angles  to  each  other  in  the  three  dimensions  of  space  and  with- 
out doubt  subserve  the  sensation  of  equilibration  (p.  128).  They 
are  an  outer  horizontal,  an  anterior  vertical  (nearly  sagittal),  and 
a  posterior  vertical  (nearly  transverse).  The  non-ampullar  ends 
of  the  two  vertical  canals  unite,  a  condition  which  is  understood 
when  it  is  recalled  that  in  cyclostomes  these  canals  alone  are 
present,  and  in  Myxine  form  a  single  canal  with  two  ampullae. 
A  later  formation  is  a  diverticulum  from  the  sacculus,  which 


IV.    VERTEBRATA. 


543- 


appears  even  in  the  fishes  as  a  small  pocket,  the  lagena,  containing 
a  part  of  the  macula  acustica;  in  the  reptiles  and  birds  the  lagena 
becomes  much  larger,  and  in  the  mammals  is  spirally  coiled  and  is 
known  as  the  cochlea.  A  part  of  the  macula  acustica  of  the  lagena 
develops  into  a  special  nerve-end  apparatus,  the  organ  of  Corti. 

The  membranous  labyrinth  described  above  is  partially  or  en- 
tirely enclosed  in  the  side  wall  of  the  skull  in  the  otic  capsule, 
which  may  ossify  to  the  otic  or  petrosal  bones.  In  the  birds  and 
mammals  the  enclosure  is  such  that  the  structure  is  duplicated  in 
bone,  so  that  the  membranous  labyrinth  lies  in  a  bony  labyrinth,. 


FIG.  576.— Diagram  of  human  ear.  (From  Wiedersheim.)  a,  7),  vertical  semicircular 
canals  ;  c,  their  upper  connexion  ;  Co,  the  connexion  in  bony  labyrinth  ;  Cow, 
ductus  cochlearis;  Con',  cochlea;  Cr,  canalis  reunions;  Ct,  tympanic  cavity 
(left),  cupula  terminates  (right);  d,  perilymph;  De  ductus  endolymphaticus ;  Dp, 
Dp',  ductus  perilymphaticus  ;  Kl,  Kl\  bony  labyrinth  surrounding  the  mem- 
branous labyrinth,  the  perilymph  space  black ;  M,  conch  of  ear  (left),  membrane 
closing  fenestra  rotunda  (right);  Mae,  external  auditory  meatus;  Jffc,  tympanic 
membrane;  S,  sacculus;  SAp,  ear  bones  (represented  as  a  rod) ;  Se,  sacculus  en- 
dolymphaticiis ;  St,  Sv,  scalae  tympani  and  vestibuli ;  Tb,  Tb\  Eustachian  tube  and 
its  entrance  into  pharynx ;  *,  connexion  between  scalae  tympani  and  vestibuli; 
t,  insertion  of  ear  bones  in  fenestra  ovalis ;  2,  utriculus. 

the  two  being  separated  by  lymph  spaces  (fig.  576).  These  spaces- 
are  developed  in  the  cochlea  into  two  tubes,  the  scala  tympani  and 
scala  vestibuli,  the  two  connecting  only  at  the  tip,  being  separated 
elsewhere  in  part  by  the  membranous  cochlea  (the  ductus  cochlearis 
or  scala  media).  The  spaces  of  the  bony  labyrinth  are  filled  by 
two  different  fluids:  inside  the  membranous  labyrinth  an  en- 
dolvmph,  and  between  this  and  the  walls  of  the  bony  labyrinth  a 
perilymph. 


54:4  CHORD  AT  A. 

Accessory  structures  may  be  added  to  this  auditory  apparatus 
proper,  their  purpose  being  to  bring  sound  waves  to  it.  Such 
.structures  are  but  occasionally  present  in  fishes  (it  is  not  certain 
that  they  hear),  since  the  sound  waves  are  easily  carried  by  the 
water  to  the  tissues  and  thence  directly  to  the  ears.  On  the  other 
hand,  with  the  change  to  terrestrial  life  such  a  sound-conducting 
apparatus  is  necessary  on  account  of  the  differing  densities  of  the 
air  and  the  tissues.  So  we  find  from  Amphibia  onwards  a  vibrat- 
ing membrane  —  the  tympanic  membrane  —  which  receives  the  sound 
vibrations  from  the  air  and  carries  them  to  a  chain  of  ear  bones 
(ossicula  auditus),  which  in  turn  transmits  them  to  the  inner  ear 
or  labyrinth.  These  structures  are  not  always  functional  (cetacea), 
and  they  may  be  wholly  or  in  part  rudimentary  (urodeles,  snakes, 
Amphisbaenids). 

To  understand  this  apparatus  it  must  be  recalled  that  the  ear 
lies  between  the  hyoidand  mandibular  arches  in  the  neighborhood 
of  a  canal  which  leads  from  the  surface  to  the  pharynx.  In 
the  fishes  this  canal  is  the  spiracle,  a  reduced  gill  cleft.  In  the 
Anura  and  amniotes  it  consists  of  an  air  chamber  closed  exter- 
nally by  the  tympanic  membrane,  stretched  on  a  tympanic  an- 
nulus,  while  the  opening  to  the  pharynx  is  retained.  The 
part  next  the  membrane  becomes  expanded  into  the  tympanic 
cavity,  this  with  the  membrane  forming  the  tympanum  or  drum. 
The  part  connecting  with  the  pharynx  is  usually  narrowed  and  is 
called  the  Eustachian  tube.  The  membranous  labyrinth  lies  in 
the  wall  of  the  tympanic  cavity  and  touches  it  at  one  or  two  points 
where  the  bony  auditory  capsule  is  interrupted,  the  always  present 
fenestra  ovalis,  and  the  fenestra  rotunda,  lacking  in  Amphibia. 

When  it  is  recalled  that  the  mandibular  arch  lies  just  in  front 
•of  the  spiracle,  and  the  hyoid  close  behind  it,  it  is  readily  under- 

stood how  parts  of  these  arches  can 
enter  the  tympanum  and  produce  the 
ear  bones.  In  Anura,  reptiles,  and  birds 
a  columella  has  one  end  attached  to 
the  stapedial  plate,  which  lies  in  the 
fenestra  ovalis,  while  the  other  is  in- 
serted in  the  drum  membrane,  the 
whole  conveying  the  waves  across  the 
tympanum  to  the  labyrinth.  In  the 
'Fio.  577.—  Ear  bones  of  man.  mammals  the  structure  is  different,  since 


in™?;  ^SSffiSSf;    1;     the  columella  is  replaced  by  two  bones, 
stapes>  the   malleus,  which   is   attached   to  the 

<drum    membrane,    and    the   incus,    which    articulates    with    the 


IV.    VERTEBRATA. 


545 


stapes.  Most  students  believe  incus  and  malleus  to  be  parts 
(quadrate  and  articulare)  of  the  mandibular  arch — a  view  which 
has  its  opponents,  who  believe  these  to  be  a  divided  columella 
(fig.  577). 

The  tympanic  membrane  is  usually  flush  with  the  surrounding 
skin  or  only  slightly  below  its  level.  In  the  mammals  it  is  pro- 
tected by  being  placed  at  the  bottom  of  a  deep  tube,  the  external 
auditory  meatus.  The  ear  conch,  a  fold  of  skin  supported  by 
cartilage,  is  also  confined  to  the  mammals. 

The  more  important  vegetative  organs  of  the  body  are  enclosed 
in  a  large  body  cavity  or  ccelom  beneath  the  vertebral  column. 
This  is,  as  development  shows,  an  outgrowth  from  the  primi- 
tive digestive  tract,  an  enteroccele  (pp.  109  and  158),  lined 
with  epithelium.  Since  it  arises,  as  in  other  coel  ornate  animals, 
by  paired  outgrowths  from  the  archenteron,  it  follows  that 
at  first  the  two  cavities  must  be  separated  by  a  partition 


PIG.  578.— Section  of  vertebrate  in  abdominal  region.  (From  Kingsley.)  a,  dorsal 
aorta;  c,  coelom;  g,  gonad;  gl,  glomerulus;  i,  digestive  tract;  I,  liver;  w,  mesen- 
tery; rrm,  muscular  part  of  myotomes;  my,  its  crelom  (myocoele);  o,  omen  turn; 
s,  spinal  cord;  so,  sp,  somatic  and  splanchnic  epithelia;  t,  nephridial  tubule;  urn, 
ventral  mesentery  ;  u\  Wolfflan  duct. 

which    also    encloses    the    intestinal   tract    (fig.    578).        These 
walls  furnish  the  mesentery  which  supports  the  intestine  in  its 


i 


546  CHORD  AT  A. 

whole  length  from  the  vertebral  column,  but  ventral  of  the  diges- 
tive tract  (as  the  mediastinum,  omentum  minus,  and  suspensory 
ligament  of  the  liver  of  human  anatomy)  only  reaches  as  far  back 
as  the  liver,  so  that  right  and  lef  b  coeloms  unite  behind.  Some 
other  organs  are  also  suspended  in  the  body  cavity  by  membranes: 
the  testes  by  the  mesorchium,  the  ovary  by  the  mesovarium. 

The  body  cavity  is  frequently  called  the  pleuroperitoneal  cavity, 
since  in  mammals  it  is  divided  by  a  partition,  the  diaphragm,  into 
an  anterior  or  pleural  and  a  posterior  or  peritoneal  (abdominal) 
cavity.  The  lining  membranes  of  these  cavities  are  called  pleura 
and  peritoneum  respectively.  The  pericardial  cavity  is  also  a  de- 
rivative of  the  coelom,  and  the  lining,  the  pericardium,  but  a  part 
of  the  pleuroperitoneal  membrane.  Hence  it  is  that  in  many 
fishes  (sharks,  sturgeon)  a  communication  persists  between  the 
pericardial  ancl  the  other  coelom.  In  most  fishes  and  in  many  rep- 
tiles there  is  a  direct  connexion  of  the  coelom  with  the  exterior  by 
one  or  two  pori  abdominales,  beside  or  behind  the  anus. 

The  alimentary  tract  possesses  the  greatest  systematic  interest 
of  the  vegetative  organs,  for  it  not  only  is  concerned  with  diges- 
tion, but  furnishes,  as  in  all  chordates,  the  respiratory  organs  (gills 
and  lungs)  as  well,  these  arising  in  the  non-chordates  from  the 
ectoderm.  It  begins  with  the  anterior  ventral  mouth  and  ends 
ventrally  with  the  anus,  some  distance  in  front  of  the  tip  of  the 
tail;  it  is  almost  wholly  entodermal  in  origin,  there  being  but 
slight  ectodermal  portions  at  either  end. 

The  first  division  is  spacious  and  consists  of  the  ectodermal 
mouth  cavity  and  the  entodermal  pharynx,  two  spaces  which,  in 
most  vertebrates,  are  not  sharply  marked  off,  but  in  alligators  and 
mammals  are  separated  by  the  soft  palate.  Then  begins  the 
narrow  oesophagus,  widening  behind  to  the  stomach.  From  the 
hinder  or  pyloric  end  of  the  stomach  begins  the  small  intestine, 
which  enlarges  into  the  large  intestine,  separated  from  the  small 
intestine  in  the  higher  vertebrates  by  a  valve  and  one  or  two  caeca. 
The  terminal  portion  in  most  vertebrates  is  called  the  cloaca  be- 
cause it  receives  the  urogenital  ducts.  The  liver  is  the  only  gland 
constantly  present;  it  is  a  large  compact  brown  organ,  generally 
provided  with  a  gall  bladder.  Usually  a  smaller  gland,  the 
pancreas,  occurs.  The  ducts  of  the  liver  (bile  duct,  ductus 
choledochus)  and  pancreas  empty  into  the  small  intestine  near  the 
pylorus.  The  mouth  cavity  may  have  salivary  glands  connected 
with  it,  while  the  rectal  region  occasionally  has  blind  sacs  and 
glands. 


IV.    VERTEBRATA.  547 

A  striking  vertebrate  characteristic  occurs  in  the  dentition. 
In  the  cyclostomes  there  are  horny  teeth — strongly  cornified  epi- 
thelial products  seated  on  connective-tissue  papilla?;  in  the  higher 
groups  occur  true  teeth  of  dentine  and  enamel,  enclosing  a  richly 
vascular  pulp.  They  occur  in  places  where  the  underlying  skele- 
ton affords  them  a  firm  support,  especially  on  the  upper  or  lower 
jaws,  but  they  may  occur  on  other  bones  of  the  mouth  and 
pharyngeal  cavities  (roof  of  the  mouth,  gill  arches).  They  have 
apparently  arisen  from  a  diffuse  dentition,  recalling  the  scales  of 
the  skin,  sinco  many  elasmobranchs  possess,  besides  the  ordinary 
teeth,  rudimentary  teeth  in  mouth  and  pharynx.  Where  teeth  are 
lacking  (birds,  turtles,  baleen  whales)  they  have  been  lost. 

The  respiratory  organs  arise  from  the  pharynx.  In  the  fishes 
and  some  Amphibia  its  walls,  right  and  left,  are  perforated  by 
gill  clefts,  each  of  which  lies  between  two  successive  visceral  arches- 
(fig.  570).  These  are  canals  which  open  internally  into  the^ 
pharynx,  while  the  outer  gill  openings  are  on  the  outer  surface. 
The  anterior  and  posterior  walls  of  the  clefts  bear  delicate  vascular 
folds  of  mucous  membrane,  the  gill  filaments.  These  are  the  in- 
ternal gills,  in  contrast  to  the  external  gills  of  Amphibian  larvae,, 
which  are  dendritic  external  ectodermal  growths  arising  above  and! 
between  the  gill  slits  (figs.  4,  5).  It  is  important  for  the  phylogeny- 
of  the  vertebrates  to  note  that  reptiles,  birds,  and  mammals,  which 
never  breathe  by  gills,  have  gill  clefts  outlined  and  later  lost  with 
the  exception  of  the  Eustachian  cleft. 

Two  problematical  organs,  the  thymus  and  the  lateral  lobes  of  the  thy- 
roid gland,  develop  from  the  epithelium  of  the  gill  clefts.  The  middle- 
unpaired  part  of  the  thyroid  has  been  regarded  as  a  modification  of  the- 
endostyle  of  the  Tunicata  (p.  506).  The  thyroid,  which  produces  iodine- 
compounds,  is  doubtless  very  important ;  disease  or  extirpation  of  itr 
causes  serious  nervous  disturbances. 

The  lungs  also  arise  from  the  pharynx  as  two  sacs  (one  oc- 
casionally remaining  rudimentary),  which  grow  downwards  and 
backwards.  They  retain  their  opening  into  it  either  directly  or 
by  means  of  a  trachea  or  windpipe,  which  just  before  its  entrance^ 
into  the  lungs  usually  divides  into  two  bronchi  (figs.  579,  620). 
At  the  opening  into  the  pharynx  (glottis)  the  supporting  cartilages- 
(remnants  of  the  visceral  skeleton,  p.  524)  are  strong  and  form 
the  larynx,  which  in  mammals  may  be  closed  from  the  pharynx 
by  a  valve,  the  epiglottis.  The  lungs  and  trachea  have  their 
counterparts  in  the  fishes  in  the  swim  bladder,  a  hydrostatic 
apparatus,  and  its  duct. 


548 


CHORD  AT  A. 


The  swim  bladder  of  fishes  and  the  lungs  of  most  amphibia  are  smooth- 
walled  sacs,  but  in  some  have  greater  respiratory  surface  since  folds  ex- 
tend into  the  central  space.  This  peripheral  folding  increases  in  the  rep- 
tiles at  the  expense  of  the  central  chamber,  this  in  some  being  completely 
divided  by  the  partitions,  which  extend  inwards  from  the  walls  to  the 
bronchus.  In  the  mammals  a  central  chamber  is  lacking;  the  bronchi 
extend  into  the  lungs,  brandling  again  and  again  to  the  fine  bronchioles 
which  give  off  alveolar  ducts  lined  with  minute  air  cells  or  alveoli. 

The  circulatory  apparatus  is  easily  derived  from  that  of  annelids, 
and,  like  it,  is  completely  closed.  In  the  annelids  (p.  307,  figs. 

272,  275,  276)  above  and  below  the 
digestive  tract  is  a  longitudinal  blood- 
vessel, these  being  connected  in  each 
somite  by  loops  which  pass  around 
the  intestine.  The  vertebrate  scheme 
varies  in  the  development  of  a  heart 
in  the  ventral  trunk  (the  dorsal  of 
the  annelid).  In  the  lower  verte- 
brates, the  fishes  (figs.  65,  597),  the 
heart  lies  close  behind  the  gills  and 
sends  to  them  the  blood  which  it 
receives  from  the  body.  Hence,  like 
the  whole  ventral  trunk,  it  carries 

FIG.  579.-Lungs  of  man,  ventral  view.  VellOUS    bl°°d-       SinCe    the    anterior 

!»ta!&8^A£fi:%  lo°Ps> the  £m  arteries>  Pass 

viding  below  into  the  two  bronchi;  fV,p     mlla     fhp     dorsal     trnnlr 
Z,  position  of  diaphragm;  I,*,  3,  Sa,  tn<3     £111S>    l  UnK> 

globes  of  right  and  left  lungs.  collects  from  these,  must  contain 
oxygenated  blood,  which  is  sent  by  the  carotids  to  the  head,  and 
by  the  dorsal  aorta  and  the  vascular  loops  to  the  body.  It  thus 
becomes  venous  and  flows  back  into  the  ventral  trunk. 

This  scheme  of  circulation  in  fishes  needs  further  description. 
The  heart,  a  strong  muscular  organ  enclosed  in  a  pericardium,  con- 
sists of  two  parts,  auricle  and  ventricle,  separated  by  valves.  The 
trunk  (ventral  aorta)  arising  from  the  auricle  is  arterial  and  cor- 
responds to  the  ascending  aorta  and  pulmonary  artery  of  man. 
The  arterial  arches  of  the  gill  region  which  arise  from  it  pass  di- 
rectly into  the  dorsal  vessel  only  in  young  fishes  (fig.  597);  later 
they  furnish  the  branchial  circulation  of  gill  arteries,  gill  capillaries, 
and  gill  veins  (fig.  65).  The  dorsal  trunk  is  the  dorsal  aorta 
(aorta  descendens) ;  the  ventral  trunk,  which  only  occurs  in  the 
embryo,  is  the  subintestinal  vein,  from  which  the  portal  vein  arises. 
To  this  are  added  a  system  of  paired  veins,  consisting  of  Cuvierian 


IV.    VERTEBRATA. 


549 


ducts  and  jugular  and  cardinal  veins,  the  latter  with  growth  en- 
croaching more  and  more  into  the  territory  of  the  subintestinal 
vein. 

The  circulation  of  the  fish  type  undergoes  a  great  modification 
with  the  loss  of  gills  and  the  appearance  of  pulmonary  respiration. 
Gills  and  gill  capillaries  disappear,  and  the  branchial  circulation  is 
reduced  to  arterial  arches  leading  direct  from  the  ventral  to  the 
dorsal  aorta.  The  swim  bladder  received  its  blood  from  the  body 
(systemic)  circulation,  but  with  the  functioning  of  the  lungs  pul- 
monary arteries  and  veins  come  into  existence,  while  the  arterial 
arches  in  part  disappear,  in  part  are  divided  between  the  pulmonary 
I  II  111  IV 


FIG.  580. — Diagram  of  modification  of  arterial  arches  in  various  vertebrate  classes.  White, 
vessels  which  degenerate;  cross-lined,  vessels  containing  arterial  blood;  black,  vessels 
containing  venous  blood.  /,  Dipnoi;  //,  Urodeles  with  pulmonary  respiration;  HI, 
Reptiles:  IV,  Birds  (in  mammals  the  left  instead  of  the  right  aortic  arch  persists),  ao1, 
venous  aorta  of  reptiles;  ao2,  arterial  aorta;  ast,  arterial  trunk;  a,  b,  arches  which 
usually  disappear:  ad,  dorsal  aorta:  d.B.  ductus  Botalli;  fc,  gill  capillaries;  pu,  pul- 
monary artery;  1-k,  persistent  arterial  arches. 

and  systemic  circulations  (fig.  580).  Of  the  six  arches  which 
usually  appear  in  the  embryo,  the  first  and  second,  and  the  fifth 
in  animals  with  lungs,  usually  disappear.  The  last  arch  (4),  which 
even  in  the  Dipnoi  supplies  the  swim  bladder,  becomes  a  pulmonary 
artery,  the  other  arches  (1  and  2)  furnish  the  systemic  portions, 
the  dorsal  aorta  (2)  and  the  carotids  supplying  the  head  (1). 
Since  special  pulmonary  veins,  distinct  from  the  systemic  circula- 
tion, carry  the  blood  from  the  lungs  to  the  heart,  the  heart  be- 
comes divided  by  a  septum  which  separates  it  into  right  and  left 
halves.  The  right  half  retains  the  venous  character  of  the  fish 
heart;  since  the  right  auricle  receives  the  systemic  veins,  the  right 
ventricle  gives  off  the  pulmonary  artery.  The  left  half  is  purely 
arterial,  receiving  arterial  blood  by  the  left  auricle  from  the  lungs 
and  sending  it  out  through  the  aorta  ascendens  to  the  body.  A 
complete  separation  of  pulmonary  and  systemic  circulation,  and  a 
corresponding  division  of  the  heart,  occurs  only  in  birds  and  mam- 


550  CUORDATA. 

-mals.  Keptiles  and  amphibia  show  how  the  modification  has  been 
accomplished.  In  these  the  separation  begins  in  the  venous  sys- 
tem and  extends  to  the  auricle,  in  the  reptiles  the  septum  arises  in 
the  ventricle.  In  the  arterial  system  remnants  may  persist,  such 
as  a  connexion  (ductus  Botalli)  of  the  pulmonalis  with  the  aorta 
(//,  d.B),  or  an  aortic  arch  may  arise  with  the  pulmonalis  from 
the  right  side  of  the  heart  (III,  ao). 

Besides  blood-vessels,  lymph  vessels  occur  in  the  vertebrates  as  com- 
plements of  the  venous  system.  The  fluids  which  collect  in  the  spaces  of 
the  connective  tissue  are  taken  by  them  and  carried  into  the  large  venous 
trunks.  Usually  the  action  of  the  heart  and  the  movements  of  the  body 
are  sufficient  to  cause  the  flow  of  this  lymph,  but  special  lymph  hearts 
may  occur.  The  lymph  vessels  distributed  to  the  digestive  tract  play  an 
important  role,  since  they  serve  in  the  resorbtion  of  digested  food.  They 
are  called  chyle  ducts  because  their  contents,  the  chyle,  rendered  white 
by  oil  globules  at  the  time  of  digestion,  distinguishes  them  from  other 
lymphatics.  The  most  important  features  of  lymph  and  blood  have 
already  been  noticed  (p.  88).  In  special  places  small  bodies,  the  lymph 
glands,  are  inserted  in  the  course  of  the  lymph  vessels,  in  which  lymph 
•corpuscles  arise.  Among  these  from  its  structure  is  to  be  enumerated  the 
spleen,  colored  bright  red  by  its  rich  blood  supply. 

The  sexual  and  excretory  organs  are  so  closely  associated  that 
they  are  generally  united  as  the  urogenital  system.  The  sexual 
products  are  formed  in  the  embryo  from  a  special  region  of  the 
peritoneal  epithelium  on  either  side  of  the  vertebral  column. 
These  primordial  cells  early  leave  their  primitive  position,  and  sink 
into  the  underlying  connective  tissue  (fig.  33),  forming  in  the 
male  glandular  tubes,  in  the  female  cords  which  break  up  into 
numbers  of  round  follicles,  each  containing  a  single  larger  cell,  the 
ovum.  In  the  male  the  gonads  thus  formed  are  compact  and  fre- 
•quentlv  oval,  the  testes;  in  the  female  they  are  looser  and  follic- 
ular  ovaries. 

The  deposition  of  the  sexual  cells  occurs  in  many  fishes  by  way 
of  the  body  cavity  and  the  abdominal  pores,  and  in  this  case  a  part 
of  the  ccelom  may  be  cut  off  as  a  special  vas  deferens  or  oviduct. 
In  most  vertebrates  the  ducts  are  formed  from  a  part  of  the 
nephridial  system.  Embryology  shows  that  there  are  three  kinds 
of  nephridia  in  vertebrates:  (1)  the  pronephros,  or  head  kidney; 
(2)  mesonephros,  or  Wolffian  body;  (3)  metanephros,  or  kidney 
proper,  with  the  corresponding  pronephric,  mesonephric  (Wolf- 
fian),  and  metanephric  (uretei)  ducts.  The  first  two  of  these 
ducts  are  genetically  connected,  since  the  development  of  the 
elasmobranchs  shows  that  the  pronephric  duct,  by  splitting,  gives 


IV.    VERTEBRATA. 


551 


rise  to  two  canals,  the  Wolffian  (mesonephric),  and  the  Miillerian 
ducts,  the  latter  retaining  its  relation  to  the  pronephros. 

The  pronephros  is  usually  functional  only  in  embryonic  life 
and  then  only  in  early  stages,  possibly  in  some  cases  not  at  all. 
Its  relations  to  the  other  parts  are  yet  in  question.  In  most 


m/(0d) 


FIG.  581.— Scheme  of  urodele  urogenital  system  based  on  Triton.  (From  Wieders- 
heim,  after  Spengel.)  A.,  male;  B,  female,  a,  excretory  ducts;  gn,  sexual  part 
of  mesonephros;  Ho,  testis;  Zo,  Leydig's  duct  (ureter i;  wgr,  Miillerian  duct 
(oviduct) ;  mg\  its  vestigial  end  in  male  ;  JV,  functional  part  of  mesonephros ; 
Or,  ovary;  Ot,  ostium  tubse;  Ve,  vasa  efferentia ;  *,  collecting  duct  of  vasa  effer- 
entia  (rudimentary  in  B). 

teleosts  the  mesonephros  is  equally  developed  in  nearly  the  whole 
length  of  the  body  cavity,  but  in  the  Amphibia  (fig.  581)  and 
many  elasmobranchs  its  anterior  part  is  smaller  than  the  rest,  a 
condition  which  has  its  explanation  in  its  relations  to  the  sexual 
apparatus. 


552  CHORDATA. 

In  the  males  (excepting  many  fishes)  the  testes  become  con- 
nected with  the  anterior  end  of  the  Wolffian  body  (fig.  581,  A), 
so  that  the  urinary  tubules  of  the  latter  come  to  be  seminal  ducts, 
while  the  hinder  portion  remains  excretory,  this  condition  being 
permanent  in  the  Amphibia.  In  the  amniotes  the  anterior  meso- 
nephros  retains  its  connexion  with  the  testes,  forming  the  vasa 
efferentia,  while  the  Wolffian  duct  forms  the  vas  deferens,  a  por- 
tion of  it  greatly  coiled  being  the  epididymis.  The  remainder 
of  the  Wolffian  body  degenerates,  a  portion  only  persisting  as  the 
paradidymis. 

In  the  females  (fig.  581,  B)  the  mesonephros  is  smaller  in  front, 
as  in  the  males,  but  the  connexion  of  this  with  the  ovary  does  not 
exist,  so  here  the  Wolffian  duct  is  solely  excretory,  and  not,  as  in 
the  males,  excretory  and  seminal  duct.  In  the  female  amniotes 
the  Wolffian  body  almost  entirely  disappears,  for  in  both  sexes  of 
the  reptiles,  birds,  and  mammals  the  metanephros  or  kidney  proper 
is  a  new  formation,  growing  forwards  from  the  posterior  end  of 
the  Wolffian  duct.  In  the  females  of  elasmobranchs,  Amphibia, 
and  Amniotes  the  Miillerian  duct  serves  as  an  oviduct,  its  anterior 
end  opening  by  the  ostium  tubae  into  the  abdominal  cavity  and 
receiving  the  eggs  as  they  escape  from  the  ovary.  In  the  male  the 
Miillerian  duct  disappears  early. 

The  union  of  sexual  and  excretory  organs  to  a  urogenital  system  arises 
from  the  same  relations  as  in  the  annelids ;  both  organs  arise  from  the 
coelomic  epithelium  and  have  temporary  or  permanent  connexion  with  the 
body  cavity.  This  has  already  been  described  for  the  gonads.  The 
urinary  tubules  of  both  pro-  and  mesonephros  are  derivatives  of  the  coelomic 
epithelium  and  possess  an  arrangement  recalling  that  of  the  annelids  in 
a  striking  manner.  As  is  shown  (fig.  70)  in  the  scheme  of  the  embryo 
selachian,  the  nephridial  system  consists  of  numerous  canals,  segmeu tally 
arranged,  connected  by  funnels  (nephrostomes)  with  the  body  cavity; 
and  differs  from  the  segmental  organs  of  the  annelids  in  that  they  do  not 
open  singly  to  the  exterior,  but  by  a  common  duct.  They  also  differ  in 
their  further  development  by  increasing  greatly  in  number  and  forming 
a  compact  organ,  and,  finally,  by  the  formation  in  a  certain  part  of  a 
network  of  blood-vess'els,  the  glomerulus,  which  pushes  into  the  lumen  of 
the  tube. 

The  ducts  of  the  urogenital  system  open  behind  the  anus  in 
most  fishes  on  a  urogenital  papilla;  in  the  elasmobranchs,  amphib- 
ians, birds,  and  most  reptiles  dorsally  into  the  hinder  part  of 
the  digestive  tract,  which  thus  becomes  a  cloaca.  In  turtles  and 
mammals  the  urogenital  canal  opens  into  the  urinary  bladder,  a 
ventral  diverticulum  of  the  rectum  which  first  appears  in  the 


IV.    VEHTEBRATA.  553 

Amphibia.  Urinary  and  sexual  ducts  then  either  open  into  the 
urogenital  sinus,  the  lowest  part  of  the  bladder  leading  to  the 
cloaca  (turtles,  inonotremes),  or  this  part  receives  only  the  geni- 
tal ducts,  while  the  ureters  enter  the  base  of  the  bladder.  The 
urogenital  sinus  remains  in  connexion  with  the  cloaca  in  the 
turtles  and  monotremes;  in  the  other  mammals  a  cloaca  occurs 
only  in  embryonic  life.  Later,  by  formation  of  the  perineum,  the 
cloaca  is  divided  into  a  hinder  digestive  and  an  anterior  urogenital 
canal.  Step  by  step  the  stages  may  be  followed  from  urogenital 
ducts  opening  behind  to  those  opening  in  front  of  the  anus. 

Asexual  and  parthenogenetic  reproduction  are  unknown  in  the 
vertebrates.  The  impregnation  of  the  eggs  in  the  lower  groups 
is  usually  external  and  occurs  during  oviposition;  in  the  higher 
internal  copulation  is  effected  by  opposition  of  the  genital  ori- 
fices or  by  the  development  of  an  intromittent  organ,  the  penis. 
The  fertilized  egg  can  undergo  a  part  or  the  whole  of  its  devel- 
opment in  specialized  parts  of  the  oviduct  (uterus).  Accordingly 
viviparous  and  oviparous  forms  are  distinguished,  and  between 
these  extremes  those  that  are  ovo viviparous  (cf.  p.  161).  Most 
elasmobranchs  are  viviparous,  but  many  are  oviparous.  In  the 
teleosts  oviparous  forms  predominate,  but  there  are  viviparous 
exceptions.  So,  too,  among  the  reptiles  and  Amphibia  there  are 
some  viviparous  species  among  the  egg-laying  majority.  The 
birds  and  mammals  are  most  constant,  the  first  being  exclusively 
ovoviviparous,  while  all  the  mammals  bring  forth  living  young 
with  the  exception  of  the  ovoviviparous  monotremes. 

Three  embryonal  appendages  may  occur  in  the  development, 
the  yolk  sac,  the  amnion,  and  the  allantois.  The  yolk  sac  is  small 
in  those  vertebrates  which  have  some  yolk,  but  not  enough  to 
cause  meroblastic  segmentation  (Amphibia),  yet  it  is  everywhere 
present  and  is  best  developed  in  those  groups  (fishes,  fig.  582, 
reptiles  and  birds)  with  discoidal  segmentation,  and  is  the  result 
of  the  accumulation  of  food  material  in  the  digestive  tract,  which 
forces  out  its  ventral  wall  like  a  hernia.  Its  presence  in  the  mam- 
mals, which  have  small  eggs  lacking  in  yolk,  is  an  indication  that 
these  have  descended  from  large-yolked  forms,  such  as  the  mono- 
tremes yet  are.  The  embryo  either  lies  directly  on  the  yolk  or  is 
connected  with  it  by  a  yolk  stalk. 

While  the  yolk  sac  is  widely  distributed,  the  amnion  and  allan- 
tois are  restricted  to  reptiles,  birds,  and  mammals,  which  are  con- 
sequently spoken  of  as  Amniota  or  Allantoidea,  in  contrast  to  the 
fishes  and  Amphibia,  which  are  frequently  called  Anamnia  or  Anal- 


554 


CHORD  AT  A. 


lantoidea,  from  the  absence  of  these  structures.  The  amnion  is  a 
sac  which  envelops  the  whole  embryo  and  is  connected  with  the 
rest  only  at  the  umbilicus,  that  is,  the  point  where  the  yolk  sac 
projects  from  the  ventral  wall.  In  this  sac  is  an  albuminous 


FIG.  582.  Fm.  583. 

FIG.  582.— Shark  embryo.    (From  Boas.)    y,  part  of  yolk  sac  ;  y,  external  gills  in  front 

of  pectoral  fins. 
FIG.  583.— Embryonic  envelopes  of  a  mammal.    (Diagram  after  Kolliker.)    aft,  amni- 

otic  cavity ;  al,  allantois ;  aw,  amnion  ;  da,  yolk  stalk  ;  ds,  yolk  sac ;  e,  embryo  : 

hh,  ventral  wall  of  embryo;  r,  extra-embryonic  coelom;  sft,  serosa  ;  sz,  serosal 

villi. 

amniotic  fluid.  The  amnion  is  genetically  a  part  of  the  ventral 
surface;  it  develops  ventrally  as  folds — lateral,  anterior,  and  pos- 
terior— which  grow  up  over  the  back  on  all  sides  and  unite  above 
the  embryo. 

The  allantois  is  an  enlargement  of  the  urinary  bladder.  This 
grows  out  from  the  body  cavity  at  the  umbilicus  and  extends  be- 
tween yolk  sac  and  amnion  and  then  grows  in  all  directions  until 
its  folds  meet  above  the  back.  The  part  of  the  allantois  which  re- 
ceives the  urine  may  be  enlarged  or  not.  The  rest  of  the  out- 
growth consists  of  blood-vessels  and  connective  tissue.  The  blood- 
vessels are  the  most  important,  for  the  allantois  forms  the  respira- 
tory apparatus  of  the  embryo,  and  in  the  mammals  it  develops  the 
placenta,  by  which  nourishment  as  well  is  conveyed  to  the  young. 
Yolk  sac,  amnion,  and  allantois  are  enveloped  in  a  common  coat, 
the  serosa. 

Aristotle  and  his  followers  recognized  four  divisions  of  vertebrates,  and 
these  were  retained  by  Linne"  and  Cuvier  under  the  names  Pisces,  Reptilia 
or  Amphibia,  Aves,  and  Mammalia.  Blainville  (1818)  divided  the  second 
of  these  into  two  classes,  retaining  the  name  Reptilia  for  the  one,  Amphibia 
for  the  other.  Milne  Edwards  showed  that  this  division  corresponded 


IV.    VERTEBRATA:   CTCLOSTOMATA.  555 

with  one  between  the  higher  and  lower  groups,  the  amniote  and  the  anam- 
niote  divisions.  Later  Haeckel  divided  the  fishes,  separating  the  Cyclo- 
stomes  from  the  others  as  a  distinct  class,  while  Huxley  pointed  out  the 
close  resemblances  between  the  reptiles  and  birds,  grouping  them  as 
Sauropsida.  Another  division  of  convenience  but  not  of  much  systematic 
importance  contrasts  the  fishes  with  all  other  forms,  the  Tetrapoda,  so 
called  from  the  possession  of  legs  rather  than  fins. 

SERIES  I.  ICHTHYOPSIDA  (ANAMXIA,  ANALLANTOIDA). 

Vertebrates  respiring  for  a  time  or  throughout  life  by  means 
of  gills  ;  neither  amnion  nor  allantois  present  in  the  embryo. 

Class  I.  Cyclostomata  (Marsipobranchii,  Agnatha). 

The  class  of  Cyclostomes  contains  but  few  species,  among 
which  the  lamprey  eels  and  the  slime  or  hag  fishes  are  best  known. 
In  shape  they  are  eel-like.  They  are  distinctly  vertebrate  in  the 
possession  of  large  liver  and  nephridia;  of  a  muscular  heart  with 
auricle  and  ventricle,  lying  in  a  pericardium;  olfactory  lobes, 
epiphysis  and  hypophysis,  and  the  higher  sense  organs.  In  the 
brain,  cerebrum  and  cerebellum  are  not  so  prominent  as  are  the 
optic  lobes  and  medulla.  The  inner  ear  is  not  divided  into  utric- 
ulus  and  sacculus,  and  it  has  but  one  or  two  semicircular  canals, 
but  always  two  ampullae.  The  skin  (fig.  26)  consists  of  derma 
and  a  stratified  epidermis. 

The  cyclostomes  are  distinguished  from  the  true  fishes  by  the 
lack  of  a  vertebral  column.  The  axial  skeleton  of  the  trunk  consists 
either  of  the  notochord  alone  or  of  it  and  small  neural  arches.  A 
cranium  and  a  basket-like  gill  skeleton  are  present,  but  so  different 
are  these  from  those  of  other  vertebrates  that  homologies  are  dif- 
ficult. The  absence  of  paired  fins  is  important.  Since  the  median 
fins  are  supported  by  horny  threads  alone,  the  cartilaginous  appen- 
dicular  skeleton — alone  of  importance — is  entirely  wanting.  Then 
the  skin  lacks  scales,  and  the  mouth,  true  dentine  teeth,  for  the 
pointed  brown  teeth  arranged  in  circles  in  the  mouth  of  the  lam- 
prey (fig.  584),  and  the  fewer  teeth  of  the  myxinoids,  are  purely 
epidermal  products  and  cannot  be  compared  with  the  teeth  of 
other  vertebrates.  Other  important  differences  have  given  rise  to 
names  applied  to  the  group. 

The  name  Cyclostomata  refers  to  the  circular  mouth,  an  ex- 
ternal feature,  which,  however,  rests  on  the  important  fact  that 
the  jaws  are  absent  or  extremely  rudimentary,  and  do  not  close  on 
each  other  as  do  the  jaws  of  other  vertebrates.  This  cyclostome 
•condition  is  of  value  to  the  animals,  as  it  aids  them  in  sucking  on 


556 


CHORD  AT  A. 


to  other  animals.  At  the  base  of  the  dome-like  mouth  cavity  is 
the  so-called  tongue,  which  is  the  sucking  apparatus,  since  it  can  be 
drawn  backwards  like  a  piston  (fig.  584). 

The  name  Marsipobranchs  has  been  given  on  account  of  the 
form  of  the  gills,  which  are  usually  six  or  seven  in  number,  but  in 
Bdellostoma  may  be  twelve  or  fourteen  on  either  side.  Each  gill 
cleft  consists  of  three  parts,  the  gill  sac  (marsupium),  which  alone 
contains  gills,  and  the  afferent  and  efferent  ducts  (fig.  585).  These 
canals  arise  separately,  and  may  continue  so  (Bdellostoma),  but  in 
Petromyzon  the  afferent  ducts  unite  to  a  single  tube  which  opens 
ventrally  in  the  pharynx.  In  Myxine  (fig.  585)  the  conditions  are 
reversed,  the  efferent  canals  uniting  to  empty  through  a  single 
external  opening. 

A  third  name,  Monorhina,  has  been  given,  since  these  forms, 
in  contrast  to  all  other  vertebrates,  have  an  unpaired  olfactory 
organ.  The  single  nostril,  lying  in  the  mid  line  of  the  head,. 


FIG.  584.  FIG.  585. 

FIG.  584. — Mouth  of  Petromyzon  marinus  with  horny  teeth  and  tongue.    (From  Gegenbaur.)- 

FIG.  585.— Gill  apparatus  of  Myxine  glutinosa.    (After  J.  Muller.)    a,  atrium;  «6,  gill  artery 

and  gill  arch :  6r,  gill  sac  (the  lines  show  the  gills) ;  for',  efferent  canal;  c,  cesophageo- 

cutaneus   duct;  rf,  skin  turned  away;  t,  afferent  gill  canal;  o,  oesophagus;  s,  mouth 

of  atrium ;  v,  ventricle  of  heart. 

opens  into  a  nasal  sac,  from  the  bottom  of  which  a  canal  descends 
towards  the  roof  of  the  mouth,  ending  blindly  in  Petromyzontes 
(Hyperoartia),  or  penetrating  it  in  the  Myzontes  (Hyperotretia), 
so  that  an  inner  nasal  opening  (choana)  occurs.  A  paired  olfac- 
tory nerve  supplies  the  organ. 

Sub  Class  I.  Myzontes  (Hyperotretia). 

Semiparasitic  cyclostomes  with  cirri  around  the  mouth,  very  primitive 
nephridia,  right  and  left  rows  of  slime  sacs,  eyes  rudimentary  (lens,  sclera, 


IV.    VERTEBRATA:  PISCES.  557 

and  choroid  lacking).  From  the  large  amount  of  mucus  they  are  known 
as  slime  eels.  They  bore  into  fishes  and  eat  the  flesh.  Myxine*  on  the 
east  coast,  Bdellostoma  *  (Polistotrenid)  on  the  west. 

Sub  Class  II.  Petromyzontes  (Hyperoartia). 

Several  American  species  of  lampreys,  all  belonging  to  Petromyzon* 
(with  sub  genera),  have  well-developed  dorsal  fins,  and  seven  branchial 
openings.  They  occur  in  salt  and  fresh  water,  some  marine  species 


FIG.  586.— Petromyzon  marinus,*  sea  lamprey.    (After  Goode.) 

ascending  streams  to  lay  their  eggs.  The  young  pass  through  a  larval 
(Ammoccetes)  stage  with  rudimentary  eyes  and  slit-like  mouth.  Many  of 
the  species  live  on  the  mucus  and  blood  which  they  rasp  from  fishes. 

Here  may  be  mentioned  a  group  of  fossils,  the  OSTRACODERMI,  of 
uncertain  position.  They  have  fish-like  bodies,  but  no  skeleton  or  jaws  are 
known.  They  flourished  in  paleozoic  seas.  Pteraspis,  Cephalaspis, 
Ptericlitliys. 

Class  II.  Pisces  (Fishes). 

The  term  fish  is  used  in  a  wider  and  a  narrower  sense.  In  the 
first  it  includes  any  aquatic  vertebrate  swimming  by  means  of  fins 
and  breathing  by  gills;  in  the  more  strict  sense,  as  used  here,  it 
means  aquatic  branchiate  forms  with  vertebral  column,  cranium, 
and  well-developed  visceral  skeleton;  with  paired  as  well  as 
unpaired  fins,  these  supported  by  a  cartilaginous  or  bony  skeleton 
in  addition  to  horny  rays;  with  double  nasal  pits;  with  a  skin  and 
oral  mucous  membrane  which  can  produce  ossifications,  the  scales 
and  teeth.  The  cyclostomes  are  thus  excluded.  The  fishes  are 
the  best  adapted  of  all  vertebrates  for  an  aquatic  life,  and  their 
whole  organization  must  therefore  be  considered  from  this  stand- 
point. 

The  epidermis  consists  of  numerous  layers  of  protoplasmic  cells 
with  an  extremely  thin  external  cuticle.  Cornifications  of  this 
epidermis  are  lacking  under  ordinary  conditions,  with  the  excep- 
tion-of  a  thin  portion  of  the  external  subcuticular  layer.  At  the 
time  of  sexual  maturity  cornifications  increase  greatly  in  most 
Cyprinoids  and  many  Salmonids,  producing  hard  bodies  in  the  skin, 


558 


CHORD  AT  A. 


the  l  pearl  organs/  Enormous  numbers  of  large  slime  cells  give 
the  fishes  their  well-known  slippery  skins.  Since  the  epidermis 
contributes  nothing  to  the  firmness  of  the  body  walls,  all  protective 
structures  arise  from  the  derma,  which  is  composed  of  many  layers 
of  dense  connective  tissue  and  furnishes  the  characteristic  dermal 
skeleton,  the  scales.  These  lie  at  the  boundary  of  epidermis  and 
derma,  commonly  imbedded  in  pockets  of  the  latter,  and  are,  on 
account  of  their  different  structure,  of  systematic  value,  although 
the  classification  based  entirely  upon  them  is  no  longer  retained. 
The  placoid  scales  (fig.  554,  587,  4)  nave  already  been  men- 
tioned, because  they  form  the 
starting  point  for  dermal  ossifica- 
tions and  teeth  (p.  515).  They 
are  rhombic  bony  plates,  usually 
close  together  like  a  mosaic,  but 
not  overlapping.  In  the  centre 
of  each  is  a  spine,  directed  back- 
wards, in  which  is  a  pulp  cavity, 
while  the  tip  of  the  spine  is  cov- 
ered with  a  cap  of  hard  substance, 
variously  called  enamel  or  vitro- 
dentine. 

The  ganoid  scales  (fig.  587, 
3)  are  usually  rhomboid  and 
arranged  like  parquetry.  In  the 
early  stages  they  may  bear  teeth, 
The  outer  surface  is  always  covered 
with  a  thick  layer  of  'ganoin/  which  gives,  even  in  fossils,  an 
iridescent  effect,  a  most  characteristic  feature.  The  ganoin  is  no 
longer  regarded  as  enamel,  but  the  most  superficial  layer  of  dentine 
(vitrodentine). 

Cycloid  and  ctenoid  scales  are  closely  related.  They  are  always 
more  loosely  placed  in  the  pockets,  from  which  they  are  easily  with- 
drawn as  in  '  scaling '  a  fish.  They  are  arranged  in  oblique,  trans- 
verse, and  longitudinal  rows,  and  overlap  like  shingles,  one  scale 
covering  the  parts  of  two  scales  behind.  The  cycloid  scales  (fig. 
587,  1)  are  approximately  circular  with  a  middle  point,  surrounded 
by  concentric  lines,  from  which  go  radiating  lines.  The  ctenoid 
scale  (2}  has  the  radial  and  concentric  lines  of  the  cycloid,  but  has 
the  hinder  edge  truncate  and  the  free  portion  bearing  small  spines 
or  teeth,  processes  of  the  concentric  ridges. 

Besides  these   types  of  scales  many  fishes  bear    considerable 


FIG.  587.— Scales  of  fishes.     1,  cycloid; 
ctenoid;  3,  ganoid;  A,  placoid. 

but  these  are  lost  in  the  adult. 


IV.    VERTEBRATA:   PISCES.  559 

spines  (strongly  developed  single  scales)  and  larger  bony  plates, 
these  last  usually  resulting  from  the  fusion  of  numerous  scales. 

The  coloration  of  fishes  is  threefold  in  origin.  The  silvery  lustre  is 
due  to  crystals  of  guanin  which  occur  not  only  in  the  skin  but  in  the  peri- 
toneum and  pericardial  walls.  In  some  fishes  from  their  iridescence 
(Alburnus  lucidus)  these  crystals  become  of  commercial  value.  They  are 
freed  from  the  skin  by  boiling  with  ammonia  and,  suspended  in  the  fluid, 
form  the  important  part  of  essence  of  pearl  (essence  d'orient)  which  is 
used  in  making  artificial  pearls,  being  either  applied  to  the  outside  of  ala- 
baster balls  (Roman  pearls)  or  as  a  coating  to  the  inside  of  glass  beads 
(Paris  pearls).  The  other  colors  of  fishes  are  due  in  part  to  the  numerous 
strongly  pigmented  fat  cells,  in  part  to  *  chromatophores '  in  the  derma, 
which,  under  control  of  the  nervous  system,  can  alter  their  form  and 
extent  and  thus  produce  color  changes  in  the  fish.  It  is  by  means  of 
these  chromatophores  that  fishes  adapt  themselves  to  their  surroundings. 
It  is  of  interest  to  note  that  destruction  of  the  eyes  results  in  loss  of  power 
to  change  color. 

The  axial  skeleton  shows  many  conditions  which  are  unknown 
outside  the  class,  and  varies  in  character  from  group  to  group,  the 
most  important  differences  consisting  in  its  cartilaginous  or  bony 
character.  The  vertebrae  are  nearly  always  amphiccelous,  the 
notochord  persisting  in  the  cavities  between  the  successive  centra 
(fig.  557).  Neural  and  haemal  arches  occur,  these  having  as  key- 
stones the  unpaired  spinous  processes.  The  neural  arches  extend 
throughout  the  columns;  the  haemal  are  complete  only  in  the  tail; 
in  the  trunk  the  haemal  spines  are  absent  and  the  haemal  processes, 
divided  into  basal  processes  and  ribs,  surround  the  viscera.  A 
sternum  is  everywhere  lacking.  When  ossification  is  lacking  or 
is  incomplete,  two  pairs  of  arches  may  occu»r  in  each  segment,  the 
anterior  being  the  stronger  and  alone  persisting  in  fishes  with  ossi- 
fied vertebrae ;  the  second  is  much  smaller,  so  that  its  elements  are 
not  called  arches,  but  intercalaria  (figs.  556,  588). 

The  great  number  of  visceral  arches,  and  their  independence 
from  the  cranium,  are  characteristic  of  fishes.  After  removal  of 
these  the  cranium  in  all  cartilaginous  fishes  is  very  simple  (fig. 
588),  but  in  the  teleosts,  with  the  appearance  of  ossification,  be- 
comes very  complicated,  since  the  bones  are  very  numerous  and 
are  not,  as  in  mammals,  in  part  fused  to  larger  bones.  There  are 
also  great  differences  between  the  different  families  of  fishes,  some 
having  bones  which  are  lacking  in  others  (figs.  560,  589).  The 
large  membrane  bones  of  the  cranial  roof  (parietals,  p,  frontals,  /, 
and  nasals,  no)  and  the  large  ventral  parasphenoid  (ps)  are 
especially  constant.  The  vomer  in  front  of  the  parasphenoid  is 


560 


CHORD  ATA. 


unpaired,  while  in  all  other  vertebrates  it  is  paired.  Most  con- 
stant of  the  cartilage  bones  are  the  ethmoids  (the  paired  ecteth- 
moids,  ee,  and  the  sometimes  paired  mesethmoid),  and  the  four 
occipitals.  On  the  other  hand  the  otic  and  optic  regions  vary 
considerably;  the  otic  region,  from  its  great  size,  has  several  bones, 
usually  (fig.  589)  five  in  number:  pterotic,  pto,  often  called 


ol     ic.     ns. 

<        \        . 


v  g/i.Jf.  jio 


CO. 


S.  7. 


FIG.  588.— Cranium,  visceral  arches,  and  part  of  vertebral  column  of  Mustelus  vulgaris.  ao» 
antqrbital  process;  co,  copula;  gp,  foramen  for  glossopbaryngeal;  H,  otic  capsule  and 
hyoid;  Hm,  hyomandibular;  ic,  intercalare;  Md,  mandible  (Meckel's  cartilage); 
JV,  nasal  capsule;  o,  optic  foramen;  06,  neural  arcb  ;  po,  postorbital  process;  Pq, 
ptery goquadrate ;  ps,  spinous  process ;  J?,  rostrum  ;  r,  ribs ;  tr,  trigeminus  foramen  ; 
v,  vagus  foramen;  1-8,  visceral  arches:  1,  labial;  2,  mandibular;  3,  byoid;  U-8,  gill 
arches. 

squamosal;  sphenotic,  spho,  frequently  called  postfrontal;  epiotic, 
epo;  prootic,  pro;  and  opisthotic,  00,  the  last  sometimes  lacking. 
In  the  region  of  the  eye  the  cartilaginous  sphenoids  are  rarely 
well  developed,  the  large  parasphenoid  taking  their  place.  The 
same  is  true  of  the  ali-  and  orbitosphenoids,  tliese  sometimes  form- 
ing an  interorbital  septum  (fig.  560)  or  a  more  or  less  wide  in- 
terorbital  fenestra  (fig.  589). 

The  character  of  the  visceral  skeleton  is  related  to  the  aquatic 
life.  All  fishes  have  numerous  gill  arches  (five  to  seven,  mostly 
five),  which,  since  their  function — gill  supporting — is  similar,  are 
similar  in  structure.  So  far  as  they  are  not  degenerate  they  con- 
sist each  of  four  parts  and  are  connected  by  unpaired  copulas,  these 
often  being  fused.  The  upper  ends  are  frequently  toothed  and, 
in  chewing,  are  opposed  by  the  rudimentary  last  arch,  on  which 
account  these  are  spoken  of  as  the  superior  and  inferior  pharyngeal 
bones.  The  anterior  visceral  arches  are  greatly  different  in  car- 
tilaginous and  bony  fishes.  In  the  former  (fig.  588)  the  pterygo- 
quadrate  (pq)  and  the  Meckelian  cartilage  bear  teeth  and  oppose 
each  other  in  biting.  In  the  bony  fishes  (fig.  589)  the  teeth  of 


IV.    VERTEBRATA:  PISCES. 


561 


the  lower  jaw  oppose  the  tooth-bearing  elements,  premaxillary 
and  maxillary,  of  the  maxillary  series,  while  the  pterygoquadrate 
elements  —  the  palatine  and  the  series  of  pterygoids  —  are  the  an- 
tagonists of  the  hyoid. 

A  second  characteristic  of  the  bony  fishes  is  already  outlined  in 
the  cartilaginous  fishes  :  the  modification  of  the  hyomandibular  to 


qio    jiLo 


we. 


FIG.  589.— Skull  of  haddock.  Infraorbital  ring  and  operculum.  outlined  in  red.  n, 
angulare;  ar,  articulare  ;  as,  alisphenoid;  de,  dentary;  ee,  ectethmoid;  ekt, 
ectopterygoid;  eng,  os  entoglossum ;  ent}  entopterygoid;  epo,  epiotic;  /r,  frontal; 
h'-ft3,  hyoid  elements:  Jim,  hyomandibular;  ih,  interhyal;  ma,  maxilla;  me, 
mesethmoid;  mt,  metapterygoid;  na,  nasal;  neb,  ncl,  ocs,  basi-,  ex-,  and  supra- 
occipital  ;  oo,  opisthptic  ;  p,  parietal ;  pa,  palatine  ;  y>rm,  premaxillary  ;  pro,  pro- 
otic  ;  ps,  parasphenoid  ;  pto,  pterotic ;  qit,  quadrate;  rbr,  branchiostegals ;  spfto, 
sphenotic;  xy,  symplectic ;  w,  vomer;  w,  vertebra.  Bones  outlined  in  red:  in/, 
inf raorbital ;  to,  interoperculum  ;  o,  operculum  ;  pro,  preoperculum ;  .so,  suboper- 
culum  ;  1,  2,  3,  axes  of  ..abial,  mandibular,  and  hyoid  arches. 

a  suspensor  of  the  jaws.  In  the  elasmobranchs  (especially  the 
skates)  the  parallelism  of  hyoid  and  mandibular  arches  is  lost,  the 
hyomandibular  separating  from  the  hyoid  and  attaching  itself  to 
the  hinge  of  the  jaws.  In  the  teleosts  the  hyomandibular  is  thus 
brought  in  connexion  with  the  quadrate,  and  lies  between  it  and 
the  cranium,  the  joint  being  thus  indirectly  supported  from  the 
cranium,  a  bone,  the  symplectic  (known  only  in  fishes)  helping 
out  the  suspensor,  while  another  bone,  the  interhyal,  connects  this 
with  the  hyoid,  which  itself  divides  into  two,  so  that  the  hyoid 
arch,  like  a  gill  arch,  consists  of  four  elements. 


562  CHORDATA. 

The  opercular  apparatus  does  not  occur  in  all  fishes.  It  is  a 
number  of  bony  plates  and  processes  which  arise  from  the  hyoid 
arch  and  extend  backwards  over  the  gills,  protecting  them.  It 
arises  in  part  (opercular  bones — (9,  Pro,  So,  lo,  fig.  589)  from 
the  hyomandibular,  in  part  (branchostegal  rays)  from  the  hyoid 
bone.  The  significance  of  this  apparatus  will  be  spoken  of  in  con- 
nexion with  the  gills;  it  gives  the  fish  head  a  definite  character, 
but  covers  its  structure,  on  which  account  it,  like  the  infraorbital 
ring,  is  shown  in  red  in  the  figure  589. 

The  appendages  are  also  influenced  by  the  aquatic  life.  In 
contrast  to  the  cyclostomes,  there  are  two  pairs  of  paired  fins,  the 
thoracic  or  pectoral,  and  the  pelvic,  ventral,  or  abdominal  fins;  in 
contrast  with  Amphibia,  reptiles,  and  mammals,  which  occasionally 
have  fin-like  structures,  the  fishes  have  three  unpaired  fins,  dorsal, 
caudal,  and  anal  fins.  Only  rarely,  as  in  the  eels,  the  ventral  fins 
are  lacking;  more  rarely  (Maraenidae)  the  pectorals  are  lost.  The 
function  of  the  fins  in  swimming  and  in  balancing  makes  it  neces- 
sary that  they  be  broad  and  well-supported  plates.  Hence  it  is 
that  numerous  skeletal  parts  are  present;  besides  those  preformed 
in  cartilage,  numerous  horny  or  bony  rays;  further,  that  all  parts 
should  be  similar  and  closely,  even  if  flexibly,  bound  to  each  other. 
Joints  are  unnecessary  except  at  the  base  where  the  fins  join  the 
supports  and  move  upon  the  body.  The  supports  of  the  paired 
fins  are  the  girdles,  arched  skeletal  parts,  which  in  the  sharks  are 
held  only  by  muscles,  a  statement  which  is  true  for  the  pelvic 
girdle  of  all  fishes.  This  is  why  the  ventral  fins  so  readily  change 
their  place.  Their  primitive  position  is  at  the  hinder  end  of  the 
body  cavity  (Pisces  abdominales,  figs.  598,  601).  From  this  point 
they  can  move  forward  to  beneath  the  pectorals  (Pisces  thoracici, 
fig.  602),  or  may  even  come  to  lie  in  front  of  them  (Pisces  jugu- 
lares)  in  the  throat  region  (fig.  606).  The  pectoral  arch  is  united 
to  the  vertebral  column  in  the  skates;  to  the  skull  by  a  series  of 
bones  in  the  teleosts. 

The  dorsal  and  anal  fins  are  supported  by  elements  preformed 
in  cartilages  which  rest  upon  the  neural  or  haemal  spines  and  in 
turn  support  the  fin  rays.  In  the  caudal  fin  the  rays  rest  directly 
upon  the  spinous  processes.  Three  types  of  caudal  fin  are  rec- 
ognized— diphycercal,  heterocercal,  and  homocercal  (fig.  10), 
distinctions  of  great  importance.  The  primitive  type  is  the  diphy- 
cercal, in  which  the  vertebral  column  extends  directly  into  the 
middle  of  the  fin,  dividing  it  into  symmetrical  halves.  In  the 
heterocercal  type  the  vertebral  axis  binds  slightly  upwards  at  the 


IV.    VERTEBRATA:   PISCES. 


563 


base  of  the  fin,  so  that  the  dorsal  part  is  reduced,  the  ventral  greatly 
enlarged,  the  result  being  extremely  asymmetrical,  as  seen  from 
the  exterior.  The  homocercal  fin  is  symmetrical  externally,  but 
in  reality  is  extremely  asymmetrical.  The  end  of  the  vertebral 
column,  the  unossified  notochord,  is  bent  abruptly  upwards,  and 
hence  the  fin  is  almost  entirely  formed  of  the  ventral  portion, 
which  is  usually  divided  by  a  terminal  notch  into  upper  and  lower 
halves.  The  homocercal  fin  begins  with  a  diphycercal  and  passes 
through  a  heterocercal  stage  in  development. 

In  correspondence  with  the  simple  motions  the  musculature  is  simple 
and  consists  largely  of  longitudinal  muscles  divided  into  myotomes,  which 
are  conical  with  the  apex  in  front,  and  are 
so  inserted  in  each  other  that  a  cross-sec- 
tion gives  concentric  circles.  In  a  section 
there  are  at  least  two  such  systems,  the 
muscles  being  divided  by  a  lateral  in- 
cision into  dorsal  and  ventral  halves. 
There  are  also  smaller  groups  of  muscles 
related  to  fins,  gill  arches,  jaws,  etc.,  but 
of  much  smaller  size,  derivatives  from  the 
larger  mass.  Electric  and  pseudelectric 
organs,  which  occur  in  different  fishes, 
sometimes  in  the  trunk,  at  others  in  the 
tail,  are  formed  by  the  modification  of  mus- 
cles. Each  organ  consists  of  numerous 
closely  packed  vertical  or  horizontal  col- 
umns, each  column,  like  a  Voltaic  pile, 
consisting  of  layers  of  gelatinous  plates 
(equivalents  of  muscle  bundles)  in  which 
the  nerves,  with  special  end  plates,  termi- 
nate. The  discharge  is  electronegative. 

The  brain  shows  the  low  position  of  the  class  in  the  slight  development 
of  the  cerebrum  (fig.  591).  This  is  especially  true  of  the  teleosts,  in  which, 
in  place  of  a  cortex,  there  is  only  a  thin  epithelial  layer  (Pall),  what  was 
formerly  called  cerebrum  being  only  the  corpora  striata.  The  independent 
olfactory  lobes  lie  either  close  to  the  cerebrum  (most  teleosts,  Lol)  or  are 
separated  from  it  by  an  olfactory  tract  (fig.  592,  h).  The  optic  thalami 
are  small  (d),  but  below  them  are  enlargements  characteristic  of  fishes,  the 
lobi  inferiores,  and  between  them  the  sacculus  vasculosus.  Both  optic 
lobes  and  cerebellum  are  greatly  developed. 

The  nose  consists  of  two  preoral  pits,  the  opening  being  divided  by 
a  bridge  of  skin  into  anterior  and  posterior  nostrils.  In  many  selachians 
the  nostrils  are  connected  with  the  mouth  by  a  groove  covered  by  a  fold  of 
skin,  and  in  the  Dipnoi  there  is  a  choana.  The  eye  has  several  peculiari- 
ties. The  lens  is  very  convex,  almost  conical,  due  to  the  slight  refraction 
caused  by  the  passage  of  light  from  the  water  into  the  cornea.  Further, 


EP 


FIG.  590.— Driagrammatic  section 
of  electrical  apparatus.  (From 
Wiedersheim.)  The  arrow  points 
dorsally  or  anteriorly.  BG,  con- 
nective-tissue framework;  EP^ 
electrical  plates;  G,  gelatin- 
ous tissue;  N,  nerves  entering 
through  the  septa;  NN,  nerve 
terminations. 


564  CHORD  ATA. 

the  eye  is  very  short-sighted  because  light  is  so  absorbed  by  water  that 
objects  forty  feet  away  are  invisible.  With  this  is  connected  the  cam- 
panula Halleri.  The  processus  falciformis,  a  sickle-shaped  outgrowth  of 
the  choroid,  extends  from  the  entrance  of  the  optic  nerve  into  the  vitreous 
body  as  far  as  the  lens,  swelling  out  into  the  campanula  ;  this  contains  a 
muscle  which  draws  back  the  lens  and  so  is  an  apparatus  of  accommoda- 
tion. Near  the  entrance  of  the  optic  nerve  is  a  problematic  organ,  the 


L.Oi: 


a 


FIG.  591.  FIG.  592. 


FIG.  591.—  Brain  of  trout.  (After  Wiedersheim.)  BG,  corpus  striatum  ;  GP,  pine- 
alis  ;  HH,  cerebellum  ;  Lol,  olfactory  lobes  ;  MH,  optic  lobes  ;  NH,  Medulla 
oblongata  ;  PalL  pallium,  in  part  cut  away  :  VH,  cerebrum  ;  I-XII^  nerves. 
(See  p.  536.) 

FIG.  592.  —  Brain  and  nasal  capsules  of  Scyllium  catulus.  (From  Gegenbaur.)  a,  me- 
dulla ;  6,  cerebellum  ;  c,  optic  lobes  ;  d,  'twixt  brain  ;  0,  cerebrum  ;  /i,  bulbus  and 
tractus  olf  actorius  ;  o,  nasal  capsules. 

choroid  gland,  consisting  largely  of  blood-vessels  (rete  mirabile).  Chon- 
drifications  and  ossifications  of  the  sclera  are  common.  Lids  are  weakly 
developed  or  absent,  and  only  some  elasmobranchs  have  a  nictitating  mem- 
brane. 

The  ear  has  a  relative  size  found  in  no  other  vertebrates,  the  labyrinth 
corresponding  well  with  fig.  575.  The  labyrinth  contains  in  many  teleosts 
two  otoliths,  the  asteriscus  and  sagitta,  the  first  being  especially  large. 
Experiments  show  that  the  ears  are  primarily  for  balance,  and  hearing  is 
doubtful.  Strychninized  fish  do  not  respond  to  sound,  if  in  its  production 
mechanical  vibrations  are  avoided. 

Of  all  sense  organs  the  most  noticeable  are  those  of  the  skin,  especially 
those  of  the  lateral  line,  which  are  nowhere  else  so  well  developed  and 
which  occur  elsewhere  only  in  cyclostomes  and  aquatic  amphibia.  In 
fishes  a  line  on  either  side  usually  begins  at  the  tail  and  extends  to  the 
head,  where  it  divides  into  several  curved  lines  (fig.  602,  81).  Its  position 
is  marked  by  a  groove  or  a  canal  in  the  scales  which  opens  to  the  exterior 
by  numerous  canals  through  the  scales.  Branches  of  trigeminus,  facialis. 
glossopharyngeus,  and  especially  the  lateral  branch  of  the  vagus  (fig.  570)  go 
to  these  organs,  the  latter  extending  back  to  the  tail.  These  supply  special 


IV.    VERTEBRATA:  PISCES.  565 

sense  organs,  which  may  be  grouped  in  several  lines  or  occur  in  pits  (am- 
pullae) in  the  skin  in  other  places.  Their  function  is  obscure,  since  noth- 
ing of  the  sort  occurs  in  man  or  mammals.  They  are  specific  organs  of 
aquatic  vertebrates  and  possibly  have  to  do  with  the  perception  of  water 
pressure. 

The  alimentary  tract  is  spacious  only  in  the  oropharyngeal 
region.  Then  it  narrows  to  a  tube  in  which  the  various  regions 
are  not  sharply  marked  off  from  each  other.  Mouth  and  pharynx 
frequently  bear  teeth.  In  the  teleosts  the  bones  of  the  floor  of 
the  cranium  and  those  of  the  visceral  arches  may  be  covered  with 
coalesced  heckel-like  teeth.  In  the  elasmobranchs  the  teeth  are 
mostly  confined  to  the  lower  jaw  and  the  pterygoquadrate,  but  are 
in  rows,  the  anterior  row  alone  being  functional;  but  as  these  are 
loosely  held  they  are  easily  torn  out,  when  they  are  replaced  by 
the  row  behind.  Liver  and  spleen  are  always  present;  pancreas 
and  gall  bladder  usually  occur.  In  many  fishes  blind  sacs,  the 
pyloric  caeca,  occur  at  the  junction  of  stomach  and  intestine  (fig. 
593,  B)',  others  have  a  spiral  valve  (A),  a  fold  of  mucous  mem- 
brane, which  extends  like  a  spiral  stairway  into  the  lumen  of  the 
intestine,  increasing  the  digestive  surface.  Caeca  and  spiral  valve 
rarely  occur  in  the  same  fish. 


B 


FIG.  593.— Digestive  tracts  of  (A)  Squatina  vulgaris  (partly  opened)  and  (B)  Tra- 
chinus  radiatiis.  (From  Gegenbaur.)  ap,  pyloric  caeca ;  c,  rectum  ;  d,  bile  duct ; 
dp,  duct  of  air  bladder;  i,  intestine  ;  oe,  oesophagus  ;  p,  pylorus ;  i\  stomach  ;  vs, 
spiral  gland  ;  #,  rectal  gland. 

Gills  of  two  types  occur  (fig.  594,  A  and  B).  In  both  the  gill 
clefts,  which  lie  between  successive  branchial  arches,  begin  by 
openings  in  the  pharynx,  but  differ  in  their  external  openings. 
In  the  elasmobranch  type  (A)  the  external  openings  are  a  series  of 
slits  separated  by  broad  dermal  bridges  which  cover  the  gills  and 
gill  clefts  (fig.  598).  The  gills  are  vascular  folds  of  mucous  mem- 


566 


CHORD  AT  A. 


brane  with  secondary  folds  which  extend  on  anterior  and  posterior 
sides  of  the  cleft.  Each  arch  except  the  last,  as  the  sections  (fig. 
594,  A,  and  595)  show,  bears  two  rows  of  gill  folds  (demi- 


FIG.  594.— Pharynges  of  (.4)  Elasmobranch  (Zygoma)  and  CB)  Teleost  (Gadus\  the 
skull  removed  and  on  the  left  the  gill  slits  cut  across,  a,  attachment  of  upper 
jaw  to  cranium;  as,  outer  gill  slit;  Jb,  gill  arch;  fol1,  foZ2,  anterior  and  posterior 
gills  (demibranchs) ;  ft,  dermal  projection ;  tow,  hyomandibular  ;  is,  inner  gill 
cleft ;  m,  mouth  ;  ma,  maxillare  ;  o,  oesophagus ;  op,  operculum  ;  ops,  opercular 
opening;  pa,  palatine;  phi,  inferior  pharyngeal  bones;  pq,  pterygoquadrate ; 
pnn ,  premaxilla;  s,  shoulder  girdle  ;  uk,  lower  jaw;  z,  tongue. 

branchs)  which  belong  to  different  clefts  and  are  separated  from 
each  other  by  tissue  containing  the  cartilaginous  gill  rays. 

In  the  second  type  (B),  which  occurs  in  all  other  fishes,  the 
dermal  bridges  are  lacking,  and  the  tissue  between  the  demi- 
branchs has  more  or  less  completely  disappeared,  so  that  the 
demibranchs  of  one  arch  become  connected^  their  free  ends  pro- 
jecting into  the  water  like  the  teeth  of  a  double  comb.  Here, 
on  account  of  their  very  delicate  structure,  they  would  be  ex- 
posed to  serious  injury  were  they  not  protected  by  the  operculum 
or  gill  cover.  This  is  a  fold  of  skin  arising  from  the  hyoid  arch 
and  extending  back  over  the  gill  region.  It  is  supported  by  two 
groups  of  bones,  the  opercular  bones  proper  (fig.  589,  0,  /Sfc,  lo, 
Pro),  attached  to  the  hyomandibular,  and  the  branchiostegals 


IV.    VERTEBRATA:  PISCES. 


567 


Fio.  595.— Sections  of  gill 
arches  of  Gadm  (left) 
and  Zygcena  (right), 
slightly  enlarged,  a, 
artery  ;  ft,  gill  arch  ; 
W*f  bi2,  demibranchs ; 
h,  dermal  projection ; 
r,  cartilage  ray;  V, 
vein  ;  z,  tooth. 


(r&r)  from  the  hyoid,  these  latter  supporting  the  branchiostegal 
membrane.  Between  the  free  edge  of  the 
operculum  and  the  branchiostegal  mem- 
brane and  the  skin  of  the  body  behind  is 
the  opercular  cleft  (fig.  594,  ops),  which  is 
obviously  not  identical  with  a  gill  cleft,  but 
leads  into  an  atrium  into  which  the  gill 
clefts  empty.  In  many  elasmobranchs  and 
ganoids  there  is  a  rudimentary  cleft,  the 
spiracle,  between  the  pterygoquadrate  and 
hyomandibular,  in  which  a  rudimentary  gill, 
or  pseudobranch,  may  occur,  this  often  per- 
sisting when  the  spiracle  is  closed. 

Besides  gills,  fishes,  with  .the  exception 
of  elasmobranchs  and  some  teleosts,  have  a 
swim  bladder  which  is  usually  regarded  as 
the  homologue  of  the  lungs.  It  is  often 
shaped  like  an  hour  glass,  filled  with  air,  and 
may  open  into  the  oesophagus  by  a  pneumatic 
duct  (Physbstomi),  or  this,  appearing  in  development,  may  be  lost 
in  the  adult  (Physoclisti).  The  air  bladder  serves  for  respiration 
in  the  Dipnoi  and  possibly  in  some  ganoids  (Lepidosteus  and 
Amia),  but  is  usually  a  hydrostatic  apparatus,  its  enlargement  or 
compression  altering  the  specific  gravity  of  the  fish.  In  fishes 
brought  up  from  great  depths  the  expansion  of  air  in  the  swim 
bladder  frequently  forces  the  viscera  out  through  the  mouth. 

The  heart,  enclosed  in  the  pericardium,  lies  immediately 
behind,  the  gill  region,  and  is  protected  by  the  shoulder  girdle. 
It  always  consists  of  auricle  and  ventricle  (fig.  596),  separated  by 
a  pair  of  valves  to  prevent  back-flow  of  the  blood;  it  sends  the 
blood  to  the  gills  by  the  arterial  trunk  (ventral  aorta),  and  receives 
it  from  the  body  through  a  thin-walled  sac,  the  venous  sinus,  in 
which  the  hepatic  veins  and  the  Cuvierian  ducts  (formed  by  union 
of  jugular  and  cardinal  veins)  empty  (figs.  65,  597). 

The  most  important  differences  lie  in  the  development  of  conus 
and  bulbus  arteriosus.  These  are  muscular  accessory  organs,  the 
first  arising  from  the  heart,  the  other  from  the  arterial  trunk;  and 
correspondingly  the  conus  has  striped,  the  bulbus  smooth  muscle 
fibres.  The  anterior  end  of  the  heart  contains  <  semilunar '  valves, 
which,  like  the  auriculo-ventricular  vjilves,  prevent  the  back-flow 
of  the  blood.  When,  by  increase  in  the  number  of  valves,  this  part 
becomes  elongate,  a  conus  arteriosus  (fig.  596,  A)  is  formed.  The 


568 


CHORDATA. 


bulbus  ( (?)  is  a  muscular  swelling  in  front  of  the  conus,  in  the 
arterial  trunk. 

The  connexion  of  ventral  and  dorsal  aortas  is  effected  in  young 
fishes  (fig.  597)  by  the  gill  arteries  directly;  later  by  means  of  the 
complicated  loops  of  the  gill  circulation.  When  these  are  de- 


FIG.  596,— Forms  of  hearts  of  fishes  in  schematic  long  section.    (After  Boas.)    A, 

chian  and  most  ganoids;  B,  Amia;  C,  Teleost.    a,  auricle;  6,  bulbus  arteriosus;  c,  conus 
arteriosus;  fc,  valves;  s,  sinus  venosus;  t,  truncus  aortse;  v,  ventricle. 


FIG.  597.— Head  of  embryo  teleost.  (Diagram  from  Gegenbaur.)  a,  auricle  ;  abr,  ventral 
aorta  with  arterial  arches;  ad,  dorsal  aorta;  c,  carotid  ;  dc,  Cuvierian  duct,  formed  by 
union  of  jugular  and  posterior  cardinal  veins  ;  n,  nostril ;  *,  gill  clefts  ;  sv,  sinus  veno- 
sus; v,  ventricle. 

veloped,  afferent  branchial  arteries,  gill  capillaries,  and  efferent 
arteries  can  be  recognized,  the  latter  uniting  to  form  the  dorsal 
aorta  and  also  giving  off  the  arteries  (carotids),  which  go  to  the 
head. 

The  nephridia  are  a  pair  of  large  reddish-brown  organs  lying 
outside  the  body  cavity  to  the  right  and  left  of  the  vertebral 
column,  usually  extending  from  heart  to  anus.  Their  ducts  empty 
behind  the  anus  or  in  the  dorsal  wall  of  the  intestine  and  are  often 
provided  with  enlargements  called,  from  their  functions,  urinary 
bladders,  although  totally  different  morphologically  from  the 
urinary  bladder  of  the  higher  vertebrates.  The  gonads,  suspended 


IV.    VERTEBRATA  :   PISCES.  569 

by  mesorchia  or  mesovaria,  are  large  and  project  into  the  body 
cavity.  They  are  rarely  unpaired.  In  the  elasmobranchs  and 
most  ganoids  their  products  pass  out  by  the  urogenital  system  (p. 
552),  in  other  forms  by  the  pori  abdominales  or  by  special  ducts. 

Cuvier  divided  the  fishes  into  cartilaginous  and  bony  groups,  an  im- 
portant step  so  far  as  the  extremes  (elasmobranchs  and  teleosts)  were 
concerned.  Agassiz  recognized  a  middle  group  which  he  named  Ganoidei, 
from  the  character  of  the  scales,  but  his  account  was  modified  and  made 
more  accurate  by  Johannes  Miiller,  who  also  included  the  Dipnoi  among 
the  fishes.  At  present  the  group  of  ganoids  is  retained  largely  as  a  matter 
of  convenience.  Its  members  are  more  closely  related  with  the  teleosts 
than  with  the  elasmobranchs,  and  in  America  Ganoids  and  Teleosts  are 
united  under  the  head  Teleostomi,  the  name  alluding  to  the  presence  of 
a  true  upper  jaw  comparable  to  that  found  in  higher  vertebrates. 

Sub  Class  I.  Elasmobranchii  (Plagiostomi,  Chondropterygii). 

The  elasmobranchs,  the  shark-like  fishes,  are  almost  exclu- 
sively marine,  varying  in  length  from  a  foot  and  a  half  to  sixty 
feet,  living  almost  exclusively  on  other  vertebrates,  and  noted  for 
their  voracity.  Sometimes  slender  and  cylindrical,  as  in  the  sharks 
(fig.  598),  sometimes  flattened  dorsoventrally,  as  in  the  skates  (fig. 


Spl 


FIG.  598.— Acanthias  vulgaris*  dogfish.    (From  Claus.)*"/?,  ventral  fin;  Br,  pectoral  fin; 
A'.s,  gill  clefts  ;  n,  nostril  ;  R',  R*,  dorsal  fins  ;  6',  heterocercal  caudal  fin  ;  Spl,  spiracle. 

599),  they  agree  in  form  in  that  the  head  is  prolonged  into  a  snout, 
which  is  usually  supported  by  a  cartilaginous  prolongation  of  the 
cranium,  the  rostrum  (fig.  588,  R).  The  mouth  lies  ventrally,  at 
more  or  less  distance  from  the  anterior  end,  and  is  transverse, 
whence  the  name  Plagiostomi — transverse  mouth.  This  position 
makes  it  necessary  that  a  shark  approaching  its  prey  from  below 
must  turn  on  its  back  before  biting.  The  tail  is  heterocercal  or  is 
drawn  out  in  a  long  filament.  The  skin  is  covered  with  placoid 
scales,  usually  close  together,  these  being  so  small  in  some  cases 
that  the  skin — shagreen — is  used  instead  of  sandpaper  for  polish- 
ing. More  rarely  the  scales  are  larger,  and  the  spines,  which 
project  from  the  skin,  justify  in  size  and  form  the  term  dermal 
teeth.  Such  strong  spines  occur  especially  at  the  front  of  the 


570  CHORD  ATA. 

dorsal  fins  (ichthyodolurites  of  paleontologists).  The  skeleton  is 
cartilaginous,  frequently  calcified  on  the  outside.  The  calcifica- 
tion can  also  extend  into  the  vertebrae,  producing  star-like  figures. 
Since  bone  is  lacking,  the  sharks  have  no  upper  jaws,  but  bite  with 
the  pterygoquadrate.  The  amphicoelous  vertebras  (lacking  in  the 
Holocephali  and  the  extinct  Cladoselachii,  Ichthyotomi,  and 
Acanthodidae),  have  neural  arches,  small  ribs,  and  intercalaria. 
The  number  of  gill  arches  and  clefts  varies  between  five  and  seven, 
the  first  cleft  lying  between  the  hyoid  and  the  first  branchial  arch. 
Besides,  most  elasmobranchs  have  a  spiracle  and  pseudobranch 
(fig.  598,  Spl}.  Except  in  the  Holocephali  the  gill  clefts  open  sep- 
arately, the  hyoid  arch  being  without  an  operculum. 

In  the  visceral  anatomy  these  points  are  of  importance  as  dis- 
tinguishing elasmobranchs  from  Teleostomes.  (1)  The  heart  has  a 
large  conus,  with  several  rows  of  valves  (fig.  596,  A),  but  lacks  a 
bulbus.  (2)  The  alimentary  tract  (fig.  593,  A)  has  a  spiral  valve, 
but  lacks  swim  bladder  and  pyloric  caeca.  (3)  The  sexual  products 
are  carried  to  the  exterior  by  the  urogenital  ducts.  The  eggs 
escape  from  the  follicles  of  the  ovary  (occasionally  unpaired)  by 
dehiscence  into  the  body  cavity,  and  from  thence  by  the  unpaired 
ostium  tubae  and  the  paired  Miillerian  ducts  to  the  exterior.  The 
spermatozoa  traverse  the  anterior  part  of  the  Wolffian  body  ('  kid- 
ney'). Sexual  and  reproductive  ducts  open  dorsally  into  the 
cloaca. 

Male  elasmobranchs  are  distinguished  by  the  presence  of  a  copu- 
latory  structure  (mixipterygium)  developed  by  enlargement  of  some  radii 
of  the  ventral  fin  (fig.  599,  c).  The  large  eggs,  rich  in  yolk,  are  fertilized 
in  the  oviducts  and  usually  develop  in  uterine  enlargements  of  the  ducts. 
The  embryos  (fig.  582),  with  long  gill  filaments  protruding  from  the  gill 
slits,  are  nourished  by  the  yolk  in  the  yolk  sac.  In  Mustelus  and  Car- 
charias,  as  Aristotle  knew,  there  is  the  formation  of  a  placenta,  which 
differs  from  that  of  the  mammals  in  that  the  embryonic  blood  supply 
arises  from  the  blood-vessels  of  the  yolk  sac  and  are  not  allantoic.  There 
are  oviparous  elasmobranchs,  and  in  these  the  egg  is  surrounded  by 
albumen  and  a  shell,  but  these  eggs  differ  from  those  of  birds  in  that  the 
skull  is  horny  and  is  usually  drawn  out  at  the  four  corners,  sometimes 
with  threads  for  attaching  the  egg  to  plants,  etc. 

Order  I.  Sjelachii. 

With  the  notochord  more  or  less  completely  replaced  by  verte- 
bral centra;  no  dermal  bones. 

Sub  Order  I.  DIPLOSPONDYLI.  Gill  slits  lateral,  six  or  seven  in 
number,  a  single  dorsal  fin.  Chlamydoselaclms  with  terminal  mouth. 
Hexanchus,*  mouth  normal,  six  gill  slits  ;  Heptanchus,  seven  gill  slits. 


IV.    VERTEBRATA:  PISCES,  SELACHIL  571 

Sub  Order  II.  SQUALI  (Euselachii).  Normal  sharks,  with  cylindrical 
bodies,  free  thoracic  fins,  heterocercal  tail,  lateral  gill  slits.  Most  of  them 
are  fast  swimmers  and  are  rapacious,  the  teeth  being  usually  pointed,  with 
sharp  or  toothed  edges,  but  in  some  the  teeth  are  pavement-like  and  are 
used  for  crushing  shell  fish.  The  numerous  families  are  distinguished  by 
vertebral  characters,  number  of  dorsal  fins,  presence  of  nictitating  mem- 
brane, etc.  In  the  GALEID.E,  in  which  the  nictitating  membrane  is 
present,  belong,  besides  the  dog-sharks  (Mustelus  *  and  Galeus*),  the  largest 
of  all  sharks,  Carcharinus*  some  of  which  have  man-eating  reputations. 
The  hammer  heads  (Zygcena  *)  are  closely  allied.  The  mackerel  sharks 
Lamna*)  and  the  great  white  *  man-eater,'  Carcharodon,*  lack  nictitating 
membranes.  All  of  the  foregoing  have  star-shaped  figures  in  the  verte- 
brae (p.  570).  In  the  dog-fishes,  represented  by  Acanthias  vulgaris*  (or 
Squalus  acanthias,  fig.  598),  there  is  a  spine  in  front  of  each  dorsal  fin. 

Sub  Order  III.  RAI^E.    In  the  skates  the  body  is  flattened  horizontally 
fig.  599),  and  the  pectoral  fins,  also  flattened,  are  united  to  the  sides  of 


FIG.  599.— Bawi  batw,  male,  ventral  view.  (After  Mo'bius  and  Heincke )  B,  ventral, 
Br,  pectoral  fin;  B,  rostrum;  a,  anus;  c  copulatory  part  of  ventral:  to  sill  clefts; 
wi,  mouth;  /i,  nostril;  between  them  the  oronasal  groove. 

the  body,  the  union  usually  extending  clear  to  the  tip  of  the  snout,  and 
frequently  back  to  the  pelvis,  giving  the  body  a  rhombic  appearance  from 
above.  The  animals  swim  by  undulating  motions  of  these  fins.  They 
mostly  lie  quiet  on  the  bottom,  and  hence  the  lower  surface  is  white, 
the  upper  colored.  The  union  of  the  fins  to  the  side  has  resulted  in  trans- 


572  CHORD  ATA. 

fer  of  the  gill  slits  to  the  lower  surface,  the  spiracles  to  the  upper.  The 
teeth  are  usually  pavement-like.  The  PRISTINE,  or  sawfishes,  are  the  most 
shark-like,  but  are  readily  recognized  as  belonging  here  by  the  position  cf 
the  gill  slits.  The  common  name  is  due  to  the  fact  that  the  snout  is  pro- 
longed into  a  paddle-shaped  blade,  the  edges  armed  with  teeth.  Pristis* 
RAIID^E;  the  typical  members  of  the  group  ;  Raia.*  Closely  allied  are  the 
TRYGONID^:,  or  sting  rays,  with  whip-like  tail  with  one  or  two  spines,  the 
' stings,'  at  the  base ;  Vasyatis*  The  torpedos  (TORPEDINID^E)  have 
smooth  skins,  and  have  electrical  organs,  kidney-shaped  bodies,  on  either 
side  between  gill  arches  and  pectoral  skeleton.  Torpedo* 

Order  II.  Holocephali. 

These  forms,  which  have  no  common  English  names,  differ  from 
the  selachii  in  having  the  pterygoquadrate  arch,  which  bears  a  few 
large  chisel  teeth,  fused  with  the  cranium  without  a  suspensor;  in 


FIG.  600.— ChimoRra  monstrosa.    (From  Kingsley.) 

having  a  dermal  fold  constituting  an  operculum,  which  covers  the 
gill  slits;  and  corresponding  with  this,  the  gills  more  on  the  teleost 
type  (p.  566).  Lastly,  the  vertebral  centra  are  not  developed. 
Chimcera.*  Fossils  appear  in  the  Devonian. 

The  CLADOSELACHII  (Cladoselache),  ICHTHYOTOMI  (Pleur  acanthus),  and 
ACANTHODID^E  are  paleozoic  forms  in  which  vertebral  centra  were  lacking. 
In  Cladoselache  the  skeleton  of  the  paired  fin  consisted  of  numerous  simi- 
lar radii  and  was  more  primitive  than  the  archipterygium;  Pleuracanthus 
was  diphycercal,  and  the  head,  as  in  Acanthodes,  bore  dermal  bones. 

Sub  Class  //,  Ganoidei. 

The  ganoids  form  a  transition  group  in  which  elasmobranch 
and  teleost  characters  are  mingled  in  a  notable  manner.  They 
have  the  spiral  valve  of  the  sharks,  the  swim  bladder  of  the  telosts; 
the  heart  with  the  conus  is  selachian,  the  respiratory  structures — 
the  comb-like  gills  and  the  operculum — are  as  distinctly  teleostean. 
The  hyoid  arch,  with  the  development  of  the  operculum,  has  not 
entirely  lost  its  respiratory  function,  since  in  garpike  and  sturgeon 
it  bears  an  opercular  gill,  and  often  there  is  a  pseudobranch  in 
the  spiracle.  The  skeleton  is  always  ossified  in  certain  parts;  large 


IV.    VERTEBRATA:  PISCES,  GAN01DEI.  573 

membrane  bones  lie  on  the  shoulder  girdle,  on  the  roof  and  floor 
of  the  skull  (parasphenoid) ;  the  horny  threads  of  the  fins  are  bony 
rays.  In  general  the  skeleton  ranges  between  two  extremes — an 
extremely  primitive  cartilaginous  condition  with  persistent  noto- 
chord,  and  one  with  a  more  than  ordinary  degree  of  ossification. 
It  is  important  for  the  systematist  to  find  characters  in  all  ganoids 
which  occur  only  in  the  group.  The  ganoid  scales,  used  by  Agassiz, 
are  not  sufficient,  since  the  sturgeon  has  bony  plates  free  from 
ganoin,  while  the  paddle  bill  (Polyodon*)  has  almost  no  dermal 
skeleton,  and  Amia  has  cycloid  scales.  Most  recent  and  fossil 
forms  possess  fulcra,  bony  plates  with  forked  ends  lying  shingle- 
like  in  front  of  the  fins  (fig.  10,  B),  but  these  are  not  universal, 
and  are  absent,  e.g.,  in  Amia  and  Polypterus  (fig.  10,  A  and  C). 
The  group  is  largely  American.  The  few  recent  ganoids  fall  into 
three  distinct  groups. 

Order  I.  Crossopterygii. 

These  are  largely  extinct,  but  two  genera  persisting  to-day.  The  tails 
are  diphycercal  or  heterocercal;  the  pectoral  fins  have  the  basal  portion 
scaled ;  broad  gular  plates  beneath  the  jaws  in  place  of  branchiostegals;  the 
skeleton  well  ossified.  Polypterus  and  Calamoichthys  from  Africa.  The 
order  was  probably  ancestral  to  the  Amphibia. 

Order  II.  Chondrostei. 

These  forms  resemble  the  sharks  externally  in  the  heterocercal  tail, 
spiracle,  ventral  position  of  the  mouth;  internally  in  the  cartilaginous 
skull  and  (except  Polyodori)  in  the  pterygoquadrate  serving  as  upper  jaw. 
In  the  vertebral  column  they  are  more  primitive  than  most  selachians, 
since  centra  are  lacking,  the  neural  and  haemal  arches  and  the  intercalaria 
resting  direct  on  the  notochordal  sheath  (Sg.  556).  ACIPENSERIIME,  with 


FIG.  601.— Acipenser  sturio*  common  sturgeon.    (After  Goode.) 
large   bony   dermal   plates.      Acipenser,*   sturgeon.     The  swim   bladder 
furnishes    isinglass,    the   ovaries    make    caviare.      POLYODONTIDJB,    with 
naked   skin   and    long    paddle-like    snout,    toothed   maxillaries  present. 
Polyodon*  paddle  fish. 

Order  III.  Holostei. 

In  these  the  skull  is  ossified  as  in  teleosts;  maxillary  and  premaxillary 
bones  are  present,  the  pterygoquadrates  reduced  and  not  meeting  in  front, 
and  the  mouth  terminal.  The  body  may  be  covered  either  with  ganoid  or 


574 


CHORD  AT  A. 


•cycloid  scales.    The  living  forms  (the  group  appears  in  the  trias)  have  ossi- 
fied opisthocoelous  vertebrae  and  diphy-  or  homocercal  tails. 

LEPIDOSTEULE.  Scales  rhomboid,  branchiostegal  rays  present,  a  pseudo- 
branch,  but  no  spiracle.  Lepidosteus*  garpike.  AMIID.E,  distinctly  teleos- 
tean  in  appearance  with  cycloid  scales,  amphicoalous  vertebras,  and  heart 
with  reduced  conus  (fig.  596,  B).  Amia*  bow  fin. 

Sub  Class  III.   Teleostei. 

The  teleosts  owe  their  name  to  the  extensive  ossification  of 
the  skeleton,  which  consists,  in  the  trunk,  of  amphicoelous  vertebrae, 
and  in  front  a  skull  with  numerous  primary  and  secondary  bones, 
already  enumerated  (p.  560,  fig.  589).  Maxillaries  and  premaxil- 
laries  are  present,  but  these  are  frequently  without  teeth,  since 
other  bones  of  the  mouth  (vomers,  palatines,  liyoid,  gill  arches, 
superior  pharyngeals — the  latter  alone  in  Cyprinoids)  may  bear 
teeth.  Frequently  there  are  present  small  bones,  usually  forked, 
lying  in  the  intermuscular  septa  above  the  ribs,  which  are  not  pre- 
formed in  cartilage.  These  are  the  epipleurals,  and  are  distinct 
from  the  ribs.  In  the  fins  both  cartilage  and  dermal  rays  are  ossi- 
fied, the  former  remaining  small,  the  rays  forming  most  of  the 
support.  These  rays  may  either  be  soft  and  flexible  (Malacopteri) 
or  hard  and  spine-like  (Acanthopteri),  a  matter  of  classificatory 
value.  In  the  first  case  they  consist  of  numerous  small  threads 


FIG.  602.— Perca  fluviatilis.  (From  Ludwig-Leunis.)  A,  anal  fin  ;  B,  ventral  fin;  Br, 
pectoral  fin ,  K,  operculum ;  JV,  nostrils ;  R\,  R^,  spinous  and  soft  dorsal  fins  ;  tf, 
caudal  fin  ;  67,  lateral  line. 

(fig.  602,  Br,  A,  B,  Rz),  in  the  other  the  parts  of  a  ray  are  fused 
to  a  spine  which,  sometimes  provided  with  poison  glands  (Scorpcena, 
Amphacantlie,  etc.),  become  good  defensive  weapons.  The  tail  is 
usually  homocercal;  the  diphycercy  of  eels  and  other  fishes  is  sec- 
ondary. The  dermal  skeleton  consists  of  ctenoid  or  cycloid  scales, 
sometimes  of  spines  or  body  plates.  In  rare  instances  the  skin  is 
naked. 


IV.     VERTEBRATA:  PISCES,  TELEOSTEI.  575 

The  hyoid  arch  always  bears  an  operculum  and  branchiostegal 
membrane,  but  there  is  no  opercular  gill.  The  gills  of  the 
comb-like  type,  are  confined  to  the  four  anterior  gill  arches,  but 
they  may  be  reduced  to  even  two  and  one-half  pairs  of  demi- 
branchs.  Instead  of  a  conus  (present  in  Butrinus),  the  bulbus 
arteriosus  is  well  developed;  a  spiral  valve  is  lacking,  but  pyloric 
appendages  are  common.  A  swim  bladder  is  usually  present,  but 
its  duct  is  frequently  closed. 

The  teleosts  are  distinguished  from  all  vertebrates  except  the  cyclo- 
stomes  and  perhaps  some  ganoids  in  that  the  nephridial  system  does  not 
form  part  of  the  sexual  ducts.  The  eggs  and  milt  are  deposited  through 
the  abdominal  pores  or  by  special  canals  developed  from  the  body  cavity. 
Copulation  occurs  in  only  a  few  viviparous  forms  (Embiotoeidae,  Garribu- 
sia,  etc.).  The  rule  is  that  males  and  females  deposit  their  reproductive 
products  in  the  water  at  the  same  time,  and  this  leads  to  the  enormous 
schools  of  herring  and  other  fishes  which  occur  yearly  at  certain  times. 
This  also  explains  the  ease  with  which  artificial  impregnation  in  fish 
culture  is  performed. 

In  rare  instances  the  males  care  for  the  young,  as  in  the  case  of  the 
sticklebacks ;  more  noticeable  are  the  conditions  in  the  lophobranchs  (sea 
horses  and  pipe  fish),  where  the  males  receive  the  eggs  in  a  brood  pouch  on 
the  ventral  surface.  A  metamorphosis  is  known  only  in  the  eel-like  fishes, 
the  larvae  of  which — originally  described  as  distinct  under  the  name  Lepto- 
cephalus — are  flat,  transparent  forms  with  colorless  blood,  enormous  tails, 
and  extremely  small  trunk.  These  larvae  normally  occur  in  the  sea  at  the 
depth  of  some  hundred  fathoms.  The  fresh-water  eels  go  to  the  ocean 
for  propagation.  On  the  other  hand  many  salt-water  fish  go  to  fresh 
water  for  reproduction. 

The  classification  of  the  fishes  is  yet  in  an  unsettled  state,  partly  owing 
to  the  large  number  of  forms,  partly  to  the  fact  that  the  groups  intergrade. 
Most  European  writers  recognize  six  divisions,  Physostomi,  Anacanthini, 
Pharyngognathi,  Acanthopteri,  Chaetognathi,  and  Lophobranchii.  Our 
authorities  separate  the  Ostariophysi  from  the  Physostomi,  the  Pediculati 
and  Hemibranchii  from  the  Acanthopteri,  and  unite  the  Anacanthini  and 
some  of  the  Pharyngognathi  with  the  Acanthopteri  and  make  a  distinct 
group,  Synentognathi,  of  the  others.  The  characters  on  which  these  divi- 
sions are  based  are  less  convenient  for  the  tyro  than  those  adopted  here. 

Order  I.  Physostomi. 

The  character  to  which  this  name  refers  is  not  readily  seen 
without  dissection,  the  persistence  of  the  duct  of  the  swim  bladder. 
This  is,  however,  correlated  with  the  soft  character  of  the  fin 
rays  (few  exceptions)  and  the  abdominal  position  of  the  ventral 
fins.  The  Ostariophysi  are  remarkable  in  having  a  chain  of  bones 
connecting  the  swim  bladder  with  the  ear.  More  than  a  third  of 
the  food  fishes  and  nearly  all  of  the  fresh-water  fishes  belong  here. 


576 


CHORD  ATA. 


The  Ostariophysial  families  are  the  SILURID^E  (1000  species), 
or  cat-fish,  with  barbies  about  the  mouth,  of  which  Malapterurus, 


FIG.  603.— Salmo  solar,*  Atlantic  salmon.    (After  Goode.) 

the  electric  cat  of  Africa,  is  most  noteworthy.  The 
or  carp  (1000  species),  and  the  suckers,  CATOSTOMID^E,  have  little 
food  Value.  The  electric  eel  of  South  America  belongs  to  the 
GYMNONOTI.  The  other  families  are  true  Physostomes.  The  SAL- 
MONICA  are  easily  recognized  by  the  'adipose  dorsal/ a  fin  formed 
of  a  fold  of  skin  without  fin  rays.  The  trout  and  salmon  (Salmo  *) 
belong  here  and  are  among  the  most  important  food  fishes. 
Osmerusi*  smelt;  Coregonus,*  white  fish;  CLUPEID^:,  herring, 
shad;  ANGUILLID^;,  eels,  the  breeding  habits  referred  to  above. 
ESOCID^;,  pike  and  pickerel.  AMBLYOPSID^E,  blind  fish  of  Mam- 
moth Cave. 

Order  II.  Paryngognathi. 

In  many  fishes  the  inferior  pharyngeal  bones  (i.e.,  the  last 
rudimentary  gill  arch)  fuse  to  form  a  single  bone,  and  these  forms 
are  called  Pharyngognathi.  Some  have  spiny  fins,  among  the 
;,  including  Ctenolabrus,*  the  cunners,  and  Tautoga,*  the 


FIG.  604.— Ctenolahrus  cceruleus,*  cunner.    (After  Goode.) 

tautog.  These  are  placed  among  the  Acanthopteri  by  American 
authors.  Others  have  only  soft  fin  rays.  These  are  the  Synento- 
gnathi  and  include  the  EXOCCETID^:,  or  some  of  the  flying  fishes, 
in  which  the  pectoral  fins  are  very  large,  acting  as  parachutes 
when  the  fish  leap  from  the  water.  Exocc&tus.* 


IV. 


VERTEBRATA:  PISCES,  TELEOSTEL 


57T 


Order  III.  Acanthopteri  (Acanthopterygii). 

This  is  the  largest  group  of  fishes,  its  members  usually  having 
the  ventral  fins  thoracic  in  position  and  more  than  three  rays  spiny 
in  dorsal,  anal,  and  ventral  fins.  The  sticklebacks  (GASTERO- 
STEID^E)  and  some  other  forms  have  the  pharyngeal  bones  reduced, 
the  ventral  fins  farther  back,  and  form  the  group  Hemibranchii. 
Gasterosteus.*  The  perch  of  fresh  water  (PERCID^E),  Perca* 
and  Micropterus*  (black  bass),  and  the  marine  SERRANID.E,  some 
of  which  are  hermaphroditic,  have  ctenoid  scales.  The  SCOMBRID^E, 
with  Scomber*  the  mackerel,  and  TJiynnus*  the  horse  mackerel,  and 


Fio.  605. — Scomber  scomLrus,  mackerel. 

the  XIPHIID^E,  or  sword  fishes,  in  which  the  snout  is  prolonged  into 
a  long  sword,  are  the  most  important  edible  fishes  of  the  group.  The 
LORICATI,  including  the  sculpins  (Coitus,*  Hemitripterus,*)trQ- 
quently  have  the  body  armored  with  bony  plates.  The  EMBIOTOCID^E, 
or  surf  perches  of  the  Pacific,  are  viviparous.  The  suck  fishes, 
Remora,*  Echeneis,*  have  the  first  dorsal  modified  into  a  sucker  on 
the  top  of  the  head. 

Order  IV.  Anacanthini. 
These  are  soft-finned  fishes  in  which  the  ventral  fins  lie  in 


FIG.  606.— Gadus  morrhua*  cod.    (After  Storer.) 

front  of  the  pectorals.     Structure  goes  to  show  that  these  have 
descended    from    Acanthopteran    forms.     With   few   exceptions 


578 


CHORD  AT  A. 


(Lota,*  burbot),  all  are  marine.  The  GADID^E,  with  Gadus,*  in- 
cluding the  cod  and  haddock,  and  the 
PLEURONECTID^E,  with  Hippoglossus,* 
the  halibut  and  other  genera,  the  floun- 
ders,, turbot,  and  sole,  make  this  the 
most  important  group  of  marine  fishes. 
The  Pleuronectidae,  from  their  asym- 
metry, need  a  word.  The  young  are 
perfectly  symmetrical,  but  the  animals 
turn  on  one  side,  the  lower  becoming 
white.  The  eye  of  this  side  gradually 
works  over  to  the  upper  side,  twisting 
the  bones  of  the  skull  in  its  progress. 

Order  V.  Lophobranchii. 
A  small  group  of  marine  species, 
having  in  common  gills  composed  of 
small  rounded  tufts,  the  body  covered 
with  a  segmented  armor  of  bony  plates 
and  peculiar  breeding  habits,  the  male 
carrying  the  eggs  and  young  in  a  brood 
pouch.  The  sea  horses,  Hippocampus,* 
with  their  horse-like  heads,  and  the 

FIG.  GOT. -Hippocampus  hepta-  slender  pipe  fishes,  Syngnathus,*  belong 


gon 
Goc 


us,*     sea     horse.       (After 
code.) 


here. 


Order  VI.  Plectognathi. 

A  small  group  of  peculiar  compact  fishes,  in  which  the  bones 
in  each  jaw  are  coossified,  the  ventral  fins  reduced  or  absent.  In 
the  trunk  fishes,  Ostracodermi,  the  body  is  enclosed  in  a  firm  angu- 


FIG.  608.— Chilomycterus  geometricus,*  swell  fish.    (After  Goode.) 

lar  box  of  bony  plates.  The  G-ymnodonta,  or  swell  fishes  (fig.  608), 
have  the  power  of  inflating  the  body  to  spherical  sacs.  The  flesh 
is  poisonous. 


IV.    VERTEBRATA:  DIPNOI.  579 

Sul  Class  IV.  Dipnoi  (Dipneusti). 

The  lung  fishes  have  the  form  of  true  fishes,  with  scales  and 
paired  fins,  supported  by  a  single  or  a  doubly  pinnate  archiptery- 
gium.  The  median  fin  is  not  separated  into  dorsals,  caudal  and 
ventral,  and  the  caudal  part  is  diphycercal.  The  skeleton  is  very 
primitive,  consisting  largely  of  cartilage,  the  notochord  being  re- 
tained to  a  great  extent.  The  animals  live  in  fresh  water  and, 
under  ordinary  conditions,  breathe  by  gills  which  are  covered  by 
an  operculum.  In  the  gills  there  are  some  peculiarities  recalling 
amphibian  structures,  Protopterus,  and  the  young  of  Lepidosiren 
having  external  as  well  as  internal  gills.  The  resemblances  are 
strengthened  by  the  periodic  appearance  of  pulmonary  respira- 
tion. The  lung  fishes  live  in  the  tropics  in  pools  and  swamps 
which,  during  the  hot  season,  may  be  more  or  less  completely  dried 
up.  When  the  water  becomes  too  foul  for  branchial  respiration, 
the  swim  bladder  is  used.  This  is  a  paired  or  unpaired  sac  with 
a  duct  leading  to  the  oesophagus,  and  the  interior  has  its  respira- 
tory surface  increased  by  the  development  of  air  cells.  Protopterus 
indeed  can  live  out  of  water;  it  burrows  in  the  mud  at  the  dry 
season,  and  builds  a  cocoon  lined  with  mucus  in  which  it  remains 


rifi.  oiU. — Protojjterus  annectens,  lung  fish.    (From  Boas.; 

quiescent  until  the  wet  season.  The  nose  is  respiratory,  with  a 
choana  opening  into  the  mouth  cavity.  The  last  gill  vessels  give 
off  pulmonary  arteries,  and  there  are  veins  carrying  the  blood  back 
to  the  heart.  The  heart  itself  shows  the  beginning  of  division 
into  arterial  and  venous  halves,  especially  in  the  regions  of  the 
conus  and  auricle. 

The  few  species  now  living  have  a  wide  and  discontinuous  distribution, 
and  are  the  remnants  of  a  much  richer  group  which  appeared  in  the 
paleozoic.  MONOPNEUMONIA,  with  one  swim  bladder :  Ceratodus  of  Aus^ 
tralia.  D'IPNEUMONIA,  with  two  bladders :  Protopterus,  Africa ;  Lepido- 
siren, South  America.  Possibly  the  paleozoic  ARTHRODIRA,  some  of 
gigantic  size  (Dinichthys),  belong  here. 


580 


CHORD  ATA. 


Fe 


Class  III.  Amphibia. 

There  are  two  views  as  to  the  origin  of  the  Amphibia.  Accord- 
ing to  the  one  they  have  descended  from 
Crossopterygian  ganoids  (and  this  seems  the 
better  supported);  the  other  is  that  they 
have  come  from  the  Dipnoi.  The  group  is 
distinguished  at  once  from  the  fishes  by  the 
absence  of  fins.  There  is,  it  is  true,  a  median 
fin  in  larval  life,  and  this  may  persist  (Peren- 
nibranchs,  Triton],  but  it  is  never  divided 
into  dorsal,  caudal,  and  anal,  and  it  lacks 
any  skeletal  support  (figs.  4,  5).  The  paired 
fins  are  replaced  by  pentadactyle  feet  (p.  529). 
These  are  often  webbed  and  are  used  for 
swimming;  they  are  also  used  for  creeping 
and  leaping,  and  are  consequently  jointed 
between  the  separate  skeletal  elements  (fig. 
610).  Besides  the  shoulder  and  hip  joints, 
which  alone  occur  in  fishes,  there  occur  also 
elbow  (knee),  wrist  (ankle),  and  finger  joints. 
The  number  of  digits  is  not  always  five,  for 
a  reduction  to  four,  three,  or  eve  a  two 
occurs. 

The  connexion  of  the  girdles  with  parts 
of  the  axial  skeleton  (lacking  in  most  fishes) 
is  of  importance.     The  pelvic  girdle  is  con- 
c,  oentrale;  F,   nected  with  the  vertebral  column  by  means 
;  i  intermedium;  f~  of  the  ilium,  which  articulates  either  directly 


metacarpals  and  digits. 


or  by  a  sacral  rib  with  the  single  sacral  ver- 

tebpa>       yentrall  y  the  t  WQ  halveg  of  the  gj  rd](> 

fuse,  and  usually  the  limits  of  ischium  and  pubis  cannot  be  traced. 

The  attachment  of  the  pectoral  girdle  is  less  firm  (fig.  564,  A}. 
The  dorsal  portion,  the  scapula,  ends  free  in  the  muscles;  the 
ventral,  differentiated  into  coracoid  and  clavicle,  is  often  connected 
with  the  sternum,  but  this  is  'not  connected  with  the  vertebral 
column,  since  the  ribs  are  too  short  to  reach  it.  The  sternum  is 
frequently  connected  with  an  episternum. 

The  vertebral  column  often  (Perennibranchs,  Derotremes, 
Caecilians,  and  many  Stegocephali)  resembles  that  of  fishes  in 
amphicoelous  centra  and  persistence  of  notochord.  The  notochord 
may  disappear,  there  then  occurring  opisthocoelous  (Salamandrina) 


IV.    VERTEBEATA:  AMPHIBIA. 


581 


or  procoelous  centra  (most  Annra).  There  is  also  an  articulation 
of  sknll  with  vertebral  column,  rare  in  fishes  but  characteristic  of 
land  animals,  by  which  the  first  vertebra  (atlas)  becomes  distinct 
from  the  rest. 

The  skull  is  remarkable  for  the  extent  to  which  the  chondrocra- 
nium  is  retained  and  the  consequent  small  number  of  primary 
bones  (figs.  611,  612).  The  bones  of  the  orbital  region  are  repre- 


PP 


TIG.  611.— Frog  skull  from  below.    (From  Wiedersheim.)    For  letters  see  fig.  612. 

sented  by  a  pair  each  of  ali-  and  orbitosphenoids  in  the  urodeles,  by 
a  ring  of  bone,  the  sphenethmoid  (os  en  ceinture),  in  the  anura. 
The  auditory  region  usually  contains  only  prootics,  the  occipital 
only  exoccipitals.  The  absence  of  other  occipitals  is  often  of  value 
in  distinguishing  between  amphibian  and  reptilian  skulls,  since  in 
the  former  the  articulation  with  the  atlas  is  consequently  by  double 
occipital  condvles.  Of  secondary  cranial  bones  are  to  be  men- 
tioned the  nasals,  frontals  (in  many  pref rentals  also),  and  parietals, 
the  latter  two  fused  in  anura  to  f rontoparietals ;  ventrally  the  large 
parasphenoids. 

The  cranium  is  increased  by  the  addition  of  the  large  quadrate 
cartilage,  which  becomes  applied  to  the  otic  capsule  and  (Anura) 
fuses  with  it,  while  the  rest  of  its  arch  (pterygoquadrate)  extends 
forward  in  a  more  or  less  complete  condition,  reaching  the  nasal 
capsule  in  the  Anura.  The  quadrate  cartilage  is  covered  externally 
by  the  squamosal  (paraquadrate),  and  supports  the  lower  jaw,  com- 
posed of  Meckel's  cartilage  surrounded  by  membrane  bones 


582 


CHORD  AT  A. 


(dentary,  splenial,  angulare,  etc.);  its  articular  portion,  like  the 
quadrate,  being  rarely  incompletely  ossified.  Vomers,  palatines, 
and  pterygoids  appear  in  the  base  of  the  skull,  all  three  forming  a 
continuous  arch  in  the  Anura;  in  front  of  them  lie  the  premaxil- 


fo.  os. 


FIG.  612.— Lateral  and  hinder  views  of  frog  skull.  (After  Parker.)  Letters  for  this 
and  611 :  an,  angulare ;  As,  alisphenoid  cartilage;  co  (Cocc),  occipital  condyles; 
col,  columella;  d,  dentary;  E  (e),  sphenethmoid;  fo,  foramen  magnum;  FP, 
frontoparietal;  Gk,  otic  capsule:  h',h",  hyoid  and  copula;  jg,  jugal;  M  (m), 
maxillary  (in  lower  jaw  mento-Meckelian) ;  ink,  Meckel's  cartilage;  N,  Nl, 
nasal  capsule;  na,  nasal;  ofo,  os,  cartilages  from  which  basi-  and  supraoccipitals 
arise  elsewhere  ;  ol  (Olcrt),  exoccipital ;  p/,  frontoparietal ;  Pal,  palatine  ;  p  (PP), 
palatine  arch;  Pmx,  premaxillary ;  Pro,  prootic;  Ps,  parasphenoid:  Pt,  pterygoid; 
Qw,  quadrate;  Qjg,  jugal;  s</,  squamosal;  Fo,  vomer.  Cartilages  dotted. 

laries,  and  in  most  cases  maxillaries.  Between  the  hinder  end  of 
the  maxillaries  and  the  quadrate  there  may  be  a  gap  or  it  may  be 
bridged  by  a  jugal.  By  the  modification  of  the  quadrate  into  a 

suspensor  the  hyomandibular  loses 
its  function,  and  if  represented  at 
all,  it  is  as  part  of  the  columella. 
The  character  of  the  remaining  vis- 
ceral skeleton  depends  upon  the 
respiration  (fig.  613).  Where  gills 
occur,  the  copula  and  hyoids — repre- 
FTG.  613.— Hinder  visceral  skeleton  of  sentingr  body  and  cornua — as  well  as 

(A)  larva  of  a  salamander  ;  (B)  of  °  • 

toad.   (From  Gegenbaur.)  a,  body  lour  gill  arches  are  present,  but  with 

of  hyoid;  fo,  anterior  horn  (hyoid);         _  . 

c,  rest  of  branchial  skeleton.  pulmonary  respiration  the  hyoid  ap- 

paratus is  reduced  to  a  hyoid  with  anterior  and  posterior  cornua, 
the  gill  arches  being  contained  in  the  posterior  horns. 

With  the  assumption  of  a  terrestrial  life  changes  occur  in  the 
sense  organs.  The  organs  of  the  lateral  line,  which  occur  in  all 
larvae  and  are  persistent  in  the  aquatic  perennibranchs,  and  the 
nerves  which  supply  them,  disappear;  the  eyes  in  the  Salamandrina 
have  upper  and  lower  lids;  in  the  frogs  an  under  lid  (really  nicti- 
tating membrane).  The  nose  becomes  respiratory  and  is  provided 
with  choanse  opening  into  the  mouth.  Especially  noteworthy  is 
the  auditory  apparatus.  This,  in  the  urodeles  and  caecilians,  is 


IV.    VERTEBRATA:  AMPHIBIA. 


583 


rzr 


very  primitive,  even  the  tympanum  being  absent,  but  in  the  Anura 
a  sound -conducting  apparatus  appears. 
The  spiracular  cleft  persists  as  a  canal, 
opening  into  the  pharynx  by  the  Eu- 
stachian  tube,  its  outer  end  expanded  into 
the  tympanic  cavity  and  closed  externally 
by  the  tympanic  membrane,  supported  by 
the  cartilaginous  tympanic  annulus  (dot- 
ted circle  in  fig.  612,  B).  The  connexion 
of  the  labyrinth  with  the  tympanum  is 
by  an  opening  in  the  otic  capsule,  the 
fenestra  ovalis,  in  which  is  the  stapes 
(?  part  of  capsule),  the  columella  extend- 
ing from  this  to  the  tympanic  membrane 
and  carrying  the  sound  waves  across  to 
the  inner  ear.  The  brain  (fig.  614)  has 
advanced  above  that  of  the  fishes  in  the 
stronger  development  of  the  cerebrum, 
but  remains  behind  in  the  small  size  of  the 
cerebellum,  which  is  but  a  thin  lamella. 

The  respiratory  organs  afford  impor-  FIG.  6U.-Brain  of  frog.  /,  line 

i  •  ••     ,  -i       .-11  j  -•  between  olfactory  lobes  and 

tant  characters,  Since  both  gills  and  lungs      cerebrum;  Frh,  fossa  rhom- 

occur.     Of  gills  there  are  two  kinds,  inter-    £™ifStory™ve-! 


ZH'  'twixt  brain- 


nal  (found  only  in  Anura),  of  entodermal 
origin,  and  ectodermal  gills,  external  in 
position  (figs.  4,  5),  occurring  in  all.  These  ectodermal  gills,  three 
in  number,  are  richly  vascular  and  arise  from  the  skin  at  the 
upper  part  of  the  gill  clefts.  The  paired  lungs  open  into  the 
hinder  part  of  the  pharynx,  either  directly  through  the  glottis  or 
more  rarely  by  a  short  trachea.  Cartilages,  the  remnants  of  gill 
arches,  may  support  trachea  and  glottis,  and  on  the  latter  support 
vocal  cords  (larynx).  Breathing  is  accomplished  by  a  kind  of 
swallowing,  the  air  being  forced  into  the  lungs  by  the  muscles  of 
the  floor  of  the  mouth  and  the  pharynx.  Persistent  gills  and 
lungs  are  found  only  in  the  Perennibranchs.  Usually  the  young 
breathe  by  gills,  the  adults  by  lungs,  the  origin  of  the  metamor- 
phosis to  be  described  below. 

Besides  gills  and  lungs  the  skin  is  an  important  respiratory 
organ,  as  are  pharynx  and  mouth  cavity,  in  which  the  air  must  re- 
main for  some  time  on  account  of  the  respiratory  mechanism.  This 
renders  intelligible  the  fact  that  many  Salamandrina  (Spelerpes, 
DesmognatJius,  Pletlwdon,  Gyrinophilus,  etc.)  have  neither  gills 


584: 


CHORD  ATA. 


nor  lungs,  but  have  only  pharyngeal  and  cutaneous  respiration. 
The  capillary  network  in  these  parts  is  greatly  developed  and  may 
extend  into  the  epithelium.  Thus,  also,  it  happens  that  in  the 
Anura  the  skin  receives  as  large  an  artery  as  the  lungs  (fig.  616,  cii). 

The  skin  is  thin  and  slimy  from  the  numerous  mucous  glands, 
which  not  infrequently  produce  a  poisonous  secretion  (so  called 
parotid  gland  in  the  ear  region).  The  epithelium  bears  a  thin 
horny  layer  which  at  intervals  is  molted  as  a  continuous  sheet. 
The  derma  in  the  Anura  is  undermined  by  large  lymph  spaces, 
the  presence  of  which  makes  the  skinning  of  a  frog  such  an  easy 
matter.  Ossifications  in  the  skin — enormously  developed  in  the 
fossil  Stegocephali  —  occur  but  rarely  (Gymnophiona)  in  recent 
Amphibia.  On  the  other  hand  the  abundance  of  chromatophores  is 
noticeable,  these,  under  the  influence  of  the  nerves,  changing  their 
shape  and  thus  producing  color  changes  in  many  Amphibia. 

The  heart  (figs.  615,  616)  has  two  auricles,  distinctly  sep- 
arated in  Anura,  a  right  with  venous  blood,  a  left  which  with 


FIG.  615. 


FIG.  616. 


FIG.  615.— Heart  and  arterial  arches  of  salamander  larva.  (After  Boas.)  a1  a2,  right 
and  left  auricles;  an,  arterial  trunk;  ad,  dorsal  aorta;  as,  left  aortic  arch;  />, 
direct  connexion  between  afferent  and  efferent  arteries;  c,  carotid;  I,  afferent 
artery;  p,  pulmonary  artery;  v,  ventricle;  l~U,  afferent  arteries;  l'-3\  gills. 

FIG.  616. — Heart  and  arches  of  frog  (diagram),  a/  a,,,  right  and  left  auricles ;  aa, 
ventral  aorta;  ad,  as,  right  and  left  aortic  arches  (radices  aortse);  c,  carotid;  cu, 
cutaneus;  ?,  lingualis;  p,  pulmonary  artery;  ss,  subclavian;  v,  ventricle;  ve, 
vertebralis;  1,  2,  4,  persisting  arches. 

pulmonary  respiration  receives  arterial  blood.  There  is,  however, 
but  a  single  ventricle,  and  the  arterial  trunk  is,  at  least  externally, 
single.  The  arterial  arches  show  different  relations  and  have 
different  fates.  With  branchial  respiration  the  first  three  afferent 
and  efferent  arteries  are  connected  in  two  ways,  the  one  by  the 


IV.    VERTEBRATA:  AMPHIBIA.  585 

capillaries  of  the  gills,  the  other  direct  (fig.  615,  b).  In  the  fourth 
arch  there  is  no  gill  system,  but  on  the  other  hand  this  arch  gives 
off  the  pulmonary  arteries  (p)  to  the  lungs. 

With  the  loss  of  gills  (fig.  616)  the  third  arch  frequently  dis- 
appears entirely  (Anura),  as  well  as  the  gill  circulation  of  the 
others,  while  the  direct  circulation  persists,  at  least  in  part.  The 
first  arch  gives  rise  to  the  carotids,  supplying  the  head  (c) ;  the 
second  unites  with  its  fellow  of  the  opposite  side  to  form  the  dor- 
sal aorta;  the  fourth  forms  the  pulmonary  artery  and,  in  the 
Anura,  gives  off  a  cutaneus  artery  (ou)  to  the  skin.  A  longitu- 
dinal fold  inside  the  arterial  trunk  is  so  arranged  that  the  venous 
blood  from  the  body  coming  to  the  heart  through  the  right  auricle 
is  mostly  sent  out  through  the  fourth  arch  to  the  lungs  and  the 
skin,  while  the  blood  returned  from  the  lungs  by  the  pulmonary 
vein  passes  through  the  left  auricle  and  then  through  the  first 
and  second  arches  (carotid  and  aortic  arches).  So  there  is  here  a 
separation  of  pulmonary  and  systemic  circulations,  although  the 
blood  all  passes  through  a  common  ventricle. 

The  sexual  organs  (fig.  581)  are  similar  to  those  of  selachians. 
The  eggs  pass  from  the  ovary  to  the  oviducts  (Miiller's  duct),  and 
in  this  are  enveloped  with  a  gelatinous  layer.  The  spermatozoa, 
on  the  other  hand,  pass  through  the  anterior  part  of  the  Wolffian 
body  (' kidney')  and  thence  out  through  the  ureter.  The  distinc- 
tion from  the  selachians  lies  in  the  fact  that  a  urinary  bladder, 
lying  ventrally  to  the  rectum,  is  present,  at  some  distance  from  the 
urinary  ducts,  which  open  dorsally  into  the  cloaca.  Besides  sexual 
organs  fat  bodies  frequently  occur,  lobed  and  often  brightly  col- 
ored structures,  best  developed  between  the  reproductive  seasons. 

A  sort  of  copulation  occurs,  and  internal  impregnation  is  effected  in 
many  urodeles  and  in  the  Gymnophiona,  but  not  in  the  Anura.  The  Anura 
and  most  other  forms  are  oviparous,  but  occasionally,  as  Salamandra 
maculosa  and  S.  atra  of  Europe,  viviparous  species  occur.  Many  inter- 
esting brooding  habits  are  known.  The  male  of  Alytes  obstetricans  wraps 
the  cords  of  eggs  about  his  legs  and  crawls  into  a  hole  until  the  young  are 
hatched,  while  the  females  of  Amphiuma  and  Ichthyophis  watch  over  the 
eggs.  The  male  of  Rhinoderma  darwinii  has  a  large  sac  arising  from  the 
pharynx  in  which  the  eggs  and  young  are  cared  for  until  the  completion 
of  the  metamorphosis.  In  Pipa  americana  the  male  places  the  eggs  on  its 
back,  the  skin  thickening  around  them  so  that  each  lies  in  a  separate 
pocket,  from  which  the  young  escape  at  length  in  nearly  the  adult  form. 
In  Nototrema  and  Notodelphys  there  are  dermal  sacs  upon  the  back  for  the 
reception  of  the  eggs. 


586  CHORDATA. 

The  development  of  the  Amphibia  possesses  special  interest, 
since  it  affords  the  only  easily  observable  instances  of  a  metamor- 
phosis among  the  vertebrates.  This  metamorphosis  is  the  more 
marked  the  wider  the  adults  are  from  the  fishes.  In  the  Anura  a 
larva,  the  tadpole  (fig.  4)  escapes  from  the  egg.  It  lacks  lungs, 
but  has  three  pairs  of  external  gills,  no  legs,  but  a  swimming  tail 
with  a  fin-like  fold.  In  the  metamorphosis  the  gills  and  tail — larval 
organs — are  lost,  while  lungs  and  legs  are  formed.  A  complica- 
tion is  introduced  into  the  metamorphosis  in  that,  for  a  time  after 
the  loss  of  the  external  gills,  internal  branchiae,  lying  in  gill  slits, 
occur.  These,  however,  are  not  visible  from  the  exterior,  since  a 
fold  of  skin  grows  back  over  them,  thus  forming  a  cavity,  the 
atrium,  into  which  the  gill  slits  open,  and  which  in  turn  opens  to 
the  exterior  by  an  opening  (rarely  paired),  usually  on  the  left  side 
(fig.  617).  In  the  tailed  forms  the  metamorphosis  is  simplified, 


Fia.  617.— Side  view  of  tadpole,   e,  eye;  g,  opening  of  atrium;  I,  hind  leg;  w,  mouth; 

u,  vent. 

usually  consisting  in  the  loss  of  the  external  gills  and  sometimes 
in  the  change  of  form  of  the  tail,  which  may  lose  its  fin  fold  and 
become  cylindrical.  The  last  traces  of  a  metamorphosis  disappear 
in  the  perennibranchs,  where  lungs  occur  and  the  gills  persist 
(Siren  is  said  to  lose  the  external  gills  and  then  re-form  them).  In 
the  Anura  the  metamorphosis  is  lost  when,  as  in  Hylodes  mar- 
tinicensis,  the  whole  development  occurs  in  the  egg,  the  young 
hatching  in  the  adult  form. 

Order  I.  Stegocephali. 

Extinct  forms  with  well-developed  tail,  numerous  membrane 
bones  in  the  skull,  and  frequently  a  bony  armor,  at  least  on  the 
ventral  surface.  Some  were  of  gigantic  size,  and  some  from  the 
folded  condition  of  the  enamel  of  the  teeth  are  known  as  Laby- 
rinthodonta.  The  group  appears  in  the  carboniferous  (footprints 
in  the  Devonian),  and  died  out  in  the  trias. 


IV.    VERTEBRATA:   AMPHIBIA. 


587 


Order  II.  Gymnophiona  (Caecilise,  Apoda). 

These  are  the  nearest  of  living  forms  to  the  Stegocephali, 
but  fossils  are  entirely  unknown.  The 
group  is  exclusively  tropical,  occurring 
in  Ceylon,  African  islands,  and  America, 
a  discontinuous  distribution  indicative 
of  great  age.  They  are  burrowing  ani- 
mals and  feed  on  small  invertebrates. 
As  a  result  of  this  subterranean  life  the 
eyes  are  small  and  concealed  under  the 
skin,  the  legs  are  entirely  lost,  so  that 
the  animals  are  snake-like  in  appearance. 
In  the  skin  there  are  usually  small  bony 
scales;  the  drum  of  the  ear  is  lacking; 
the  vertebrae  are  amphicrelous.  Inside 
the  egg  many  species  have  three  pairs 
of  beautifully  feathered  gills  (fig.  618),  a 

•nrrknf    nf     fVimr     r»^H-  inpnoA     fn     fVm     Am 

tneir   pertmenc<     to  tne  Am- 
phibia.     Later,   for  a  time,  there  is  an 
external  gill  opening  which  finally  closes. 
ffypoffeophis,  Seychelles;  C'acilia,  America. 


FIG.  618.—  Larva   of  Ichthyophis 

uiutinfafu*.  (From  Boas,  after 
Sarasins.) 

Ichihyophis,  Ceylon; 


Order  III.  Urodela  (Gradientia). 

Of  recent  forms  of  Amphibia  the  urodeles  are  the  most  fish- 
like.  The  vertebral  column  consists  of  numerous  vertebrae,  and 
of  these  a  large  part  are  behind  the  sacrum  and  consequently 
belong  to  the  tail.  Ribs  are  present,  but  so  short  that  they  do  not 
reach  the  sternum,  which  is  weakly  developed  or  is  entirely  absent. 
Tympanum,  and  Eustachian  tube  are  entirely  lacking,  as  are  the 
vocal  chords  and  the  production  of  sound. 

Sub  Order  I.  PERENNIBRANCHIATA.  Two  or  three  gill  slits,  three 
bushy  gills,  and  a  swimming  tail  persist  throughout  life.  Necturus,*  mud 
puppy,  with  legs  and  two  gill  slits.  Siren,*  three  gill  slits,  hind  legs  lack- 
ing. Proteus,  of  Austrian  caves,  much  like  Necturus,  but  nearly  blind. 

Sub  Order  II.  DEROTREMA.  External  gills  lost,  but  an  opening  in 
the  neck  leading  to  the  gill  slits.  Meiwpoma*  (Cryptobranchus),  hell- 
bender, legs  strong  ;  Amphiuma*  legs  rudimentary. 

Sub  Order  III.  SALAMANDRINA  (Hyetodera).  After  the  loss  of  gills 
the  gill  slits  close.  Amblystoma,*  remarkable  for  the  length  of  time  the 
larvae  retain  their  gills,  A.  tigrinum  (fig.  5)  and  the  Mexican  axolotl  even 
breeding  in  the  larval  stage.  The  adult  of  the  true  axolotl  is  unknown. 
Pletliodon*  Spelerpes.*  The  European  Salamandra  atra  and  8.  macu- 


588  CHORD  AT  A. 

lata  are  viviparous,  the  former  undergoing  its  metamorphosis  inside  the 
mother. 

Order  IV.  Anura. 

The  anura  have  the  compact  bodies  familiar  in  frogs  and  toads, 
with  a  small  number  (7-9)  of  trunk  vertebrae  and  complete  absence 
of  tail ;  the  caudal  vertebras  being  represented  by  a  long  bone,  the 
urostyle.  Ribs  are  sometimes  distinct,  sometimes  fused  to  the 
transverse  processes;  the  limbs  are  larger  than  in  other  Amphibia, 
and  are  frequently  used  for  leaping  and  climbing.  Ear  drum  and 
tympanic  membrane  are  lacking  only  in  the  Pelobatidae;  their 
presence  is  correlated  with  the  existence  of  vocal  cords  and  the  pro- 
duction of  sound.  The  metamorphosis  includes  a  tadpole  stage. 

Sub  Order  I.  AGLOSSA.  Toad-like  anura  with  degenerate  tongue  and 
unpaired  opening  of  the  Eustachian  tube.  Pipa  (p.  585),  South  America  ; 
Dactylethra,  Africa. 

Sub  Order  II.  ARCIFERA.  Tongue  present,  Eustachian  tubes  widely 
separate,  coracoids  of  the  two  sides  overlapping.  BUFONID^E,  toads,  tooth- 
less ;  Bufo*  the  dermal  glands  poisonous.  PELOBATIDAE,  with  teeth, 
usually  no  tympanum.  Scaphiopus,*  burrowing  toad,  with  tympanum. 
HYLID^E,  tree  toads,  toothed  ;  tips  of  toes  with  sucking  discs  ;  Hyla,*Acris* 

Sub  Order  III.  FIRMISTERNIA.  Tongue  present,  Eustachian  tubes 
distinct,  coracoids  firmly  united  in  the  middle  line.  RANMLE,  frogs. 
Rana  catesbiana,*  bull  frog,  the  largest  frog  known  ;  numerous  other 
American  species. 

SERIES  II.    AHNIOTA. 

Vertebrates  with  amnion  and  allantois  (p.  554)  in  embryonic 
life;  with  the  pro-  and  mesonephros  functional  only  in  the  em- 
bryos, and  replaced  in  the  later  stages  by  the  true  kidney  (meta- 
nephros) ;  ducts  of  the  embryonic  excretory  system  retained  only 
so  far  as  they  have  genital  functions;  gill  slits  appearing  as  trans- 
itory structures,  but  without  gills  .and  never  functional.  There 
:are  two  great  divisions  of  the  Amniotes,  the  Sauropsida  and  the 
Mammalia.  The  Sauropsida  include  the  Eeptilia  and  the  Aves, 
which  agree  with  each  other  and  differ  from  the  mammals  in 
having  a  single  occipital  condyle,  the  quadrate  acting  as  suspensor 
of  the  jaws  ;  ankle  joint  between  the  first  and  second  rows  of 
tarsals;  the  presence  of  epidermal  scales,  nucleated  red  blood 
corpuscles,  and  a  cloaca. 

Class  I.  Reptilia, 

On  account  of  similarity  of  form,  the  reptiles  and  Amphibia 
were  long  united.  They  form  parallel  groups:  urodeles  and  liz- 
ards, frogs  and  turtles,  csecilians  and  snakes.  Hence  the  points 


IV.    VERTEBRATA:  REPTILIA. 


589 


of  distinction  must  be  emphasized.  The  most  important  are  two : 
the  reptiles  belong  to  the  Amniota  and,  as  such,  have  the  em- 
bryonal features  of  the  group;  second,  although  often  aquatic, 
they  are,  in  the  entire  absence  of  branchial  respiration,  in  character 
of  skin  and  skeleton,  in  their  entire  structure,  like  the  true  land 
animals. 

The  skin,  the  better  to  withstand  desiccation  by  the  air,  is 
strongly  cornified,  so  that  in  the  epidermis  a  many-layered  stratum 
corneum  and  a  many-layered  stratum  Malpighii  can  be  distin- 
guished. At  the  tips  of  the  toes  the  stratum  corneum  develops 
strong  claws.  Further  protection  is  afforded  by  the  thick  derma, 
often  capable  of  being  tanned  into  leather,  in  which  not  infre- 
quently bony  plates  occur.  Dermal  glands  are  very  rare,  the 
femoral  pores  of  the  lizards  (fig.  625,  &),  which  appear  like  the 
ducts  of  glands,  being  produced  by  the  ends  of  cornified  epithelial 
cones.  The  axial  skeleton,  both  skull 
and  vertebral  column,  is  nearly  always 
ossified;  only  exceptionally  (Splieno- 
don  and  the  amphiccele  Ascalabotae) 
are  considerable  parts  of  the  noto- 
chord  retained.  The  vertebrae  are 
usually  proccelous. 

In  the  skull  of  reptiles  (as  in  the 
allied  birds)  are  many  characters 
which  they  share  with  Amphibia  and 
which  distinguish  them  from  mam- 
mals. This  is  especially  the  case 
with  the  visceral  skeleton.  As  in- 
the  Amphibia,  the  hinder  end  of  the 
pterygoquadrate  is  attached  to  the 
otic  capsule;  the  quadrate  is  ossified 
and  affords  the  articulation  for  the 
lower  jaw,  which  is  composed  of  many 
bones.  The  squamosal  lies  at  the  base 
of  the  quadrate  and,  in  the  Squamata, 
is  intercalated  between  it  and  the 
cranium.  Behind  it  is  the  columella, 
its  inner  end  inserted  in  the  fenestra 
ovalis.  From  the  quadrate  the  palatine 
series  of  bones — pterygoid,  palatine, 
vomer — extends  forward,  these  being 
frequently  toothed;  and  in  front  of  and  parallel  to  it  the  pre- 


FIG.  619.— Ventral  view  of  skull  of 
Tropidonotus.  (From  Wieders- 
heim.)  Bp.basioccipital;  B*,  basi- 
sphenoid  (in  front  also  parasphe- 
noid);  C7i,  choana;  Cocc,  occipital 
condyles;  Eth, ethmoid  cartilage; 
F,  frontal;  Fo,  fenestra  ovalis; 
M,  maxillary ;  of,  exoccipital;  7, 
parietal;  P/,  pref rental;  P7,  pal- 
atine ;  Pmx,  premaxillary  ;  Pf, 
pterygoid  r  Qu,  quadrate;  Squ, 
squamosal;  Ts,  transversum ;  Vb, 
vomer  ;  II,  optic  foramen. 


590  CHORDATA. 

maxillaries  and  maxillaries.  Extremely  characteristic  of  the 
reptiles,  the  turtles  excepted,  is  an  os  transversum,  which  appears 
in  no  other  vertebrates.  It  extends  from  the  hinder  end  of  the 
maxillary  to  the  pterygoid  (figs.  619,  626,  627,  630,  Ts,  tr).  A 
jugal  is  also  frequently  present.  Of  the  other  visceral  arches, 
since  gills  are  lacking,  only  the  hyoid  bone  and  laryngeal  carti- 
lages persist. 

In  the  cranium  the  complete  ossification  of  the  occipital  region 
is  noticeable,  the  four  occipital  bones  being  present.  The  basi- 
occipital  forms  the  larger  part  of  the  single  occipital  condyle,  in 
which  parts  of  the  exoccipitals  participate,  the  single  condyle 
being  the  sharpest  distinction  between  the  reptilian  and  amphibian 
skull.  The  basisphenoid,  which  lies  in  front  of  the  basioccipital, 
has  an  anterior  process  or  rostrum,  representing  the  rudimentary 
parasphenoid  (possibly  presphenoid).  Above,  the  skull  is  roofed 
in  with  membrane  bones :  parietals  (frequently  fused  and  perforated 
by  the  parietal  foramen  for  the  pineal  eye),  frontals,  nasals,  as 
well  as  pre-  and  postfrontals  and  postorbitals,  and  usually  lachry- 
mals as  well. 

The  ethmoidal  region  is  largely  cartilaginous  ;  ali-  and  orbitosphenoids 
are  small  and  variable.  Only  the  prootic  is  constant  in  the  otic  region ; 
epiotic  and  opisthotic  usually  fusing  with  the  occipitals,  the  opisthotic 
being  large  and  distinct  only  in  the  turtles.  The  zygomatic  arch  (lost  in 
snakes)  is  formed  of  jugal  and  quad  rat  ojugal,  while  above  it  may  be  a 
second  arch  formed  of  postorbital  and  squamosal. 

The  convex  occipital  condyle  forms,  with  the  concave  surface 
of  the  first  vertebra  (atlas),  an  articulation  for  motion  in  the  ver- 
tical plane  and  lateral  motions,  while  a  twisting  around  the  long 
axis  of  the  body  is  permitted  by  the  joint  between  the  atlas  and 
the  second  vertebra,  the  axis  or  epistropheus.  The  atlas  is  a  bony 
ring,  its  centrum  having  separated  and  united  with  the  body  of 
the  axis,  forming  a  pivot  around  which  the  atlas  turns.  There  are 
two  sacral  vertebrae,  and  the  vertebras  of  the  trunk  are  divided  into 
thoracic  and  lumbar,  the  former  bearing  long  ribs  which  reach  to 
the  sternum,  while  the  shorter  ribs  of  the  neck  end  freely. 

Limbs  are  lacking  in  snakes  and  some  lizards.  When  present 
the  number  of  digits  varies  between  three  and  five  (usually  four  or 
five).  In  the  pelvis  ischium  and  pubis  are  separated  by  an  obturator 
foramen  and  are  united  with  the  corresponding  bones  of  the  oppo- 
site side  by  a  double  symphysis.  In  the  shoulder  girdle  scapula 
and  coracoid  alone  are  constant,  a  clavicle  occurring  in  turtles  and 
lizards,  in  the  latter  an  episternum  (fig.  564)  as  well.  Of  con- 
siderable systematic  importance  is  the  position  of  the  ankle  joint. 


IV.    VERTEBRATA:  REPT1LIA. 


591 


This  is  intertarsal  in  character,  in  that  it  occurs  between  the  first 
and  second  rows  of  tarsal  bones  (fig.  636,  C). 


MD 


FIG.  620.— Viscera  of  Alligator.  (From  Wiedersheim.)  ED,  rectum;  If,  heart;  L, 
liver;  Lg,  lung ;  M,  stomach;  MD,  intestine;  Oe,  oesophagus;  P,  pylorus;  Tr, 
trachea  ;  ZB,  body  of  hyoid;  ZH,  its  cornua  ;  *,  perforations  of  hyoid. 

Since  reptiles  lack  even  transitory  gills,  the  gill  slits  are  com- 
pletely degenerate  before  the  young  escapes  from  the  egg.     Dermal 


592 


CHORD  ATA. 


respiration  is  far  less  important  than  with  the  Amphibia,  lungs, 
as  in  birds  and  mammals,  being  the  respiratory  organs,  and  in 
these  a  progressive  development  may  be  followed.  The  larynx  is 
followed  by  a  trachea  with  cartilage  supports  in  its  wall,  and  this 
either  opens  directly  into  the  two  lungs  or  divides  into  two  bronchi, 
which,  in  Varanus,  may  divide  again  inside  the  lungs.  The  lungs 
in  the  more  primitive  forms  are  subdivided  only  peripherally,  but 
in  the  higher  groups  the  whole  is  chambered,  partitions  extending 
inwards  to  the  intrapulmonary  bronchus. 

Since  the  respiration  is  entirely  pulmonary,  the  heart  is  divided 

into  a  left  arterial  and  a  right 
venous  half,  and  a  corresponding 
separation  of  systemic  and  pulmonary 
blood-vessels  occurs  (fig.  621).  The 
two  auricles  are  completely  separated, 
while  a  septum  extends  into  the  ven- 
tricle, complete  in  the  crocodiles, 
but  not  in  turtles,  lizards,  and  snakes. 
Yet  even  in  the  crocodiles  a  mixing 
of  arterial  and  venous  blood  occurs 
since  in  the  large  aortic  trunks  which 
arise  from  both  ventricles  a  commu- 
nication, the  foramen  Panizzae,  per- 
sists. The  arterial  trunk  is  divided 
by  internal  partitions  into  three  ves- 
sels, which  are  but  rarely  visible  from 
the  exterior.  One  of  these  arises 
from  the  right  ventricle,  carries 
venous  blood,  and  takes  over  the 
fourth  arterial  arch,  which  gives  off 

FIG.  621.—  Heart  of  crocodile  with  ar-  the   pulmonary   arteries  (4,   p).     A 

second  vessel  arises  from  the  right 
ventricle,  is  purely  arterial  and  con- 
nects  with  most  of  the  remaining 

arterial  arches'  the  first>  which 


vessels    from    the    heart,  .and  the   (aortic      arch,     ad)      of  the      SCCOnd 
connexion  (foramen   Panizzae)  be-   V                               '  .         ' 

tween  the   arterial  trunk  and  the  arch.       The  third  Vessel  Connects  Oil 
left  aortic  arch,  just  in   front  of                                              .  .     . 

the  heart.  the  one   hand   with   the   remaining 

(left,  second)  arch  and  on  the  other  with  the  right  or  venous  half 
of  the  heart.  The  foramen  Panizzse  occurs  between  this  and  the 
right  aortic  arch. 


IV.    VERTEBRATA:   REPTILIA.  593 

The  venous  character  of  the  left  aortic  arch  and  the  incomplete 
ventricular  septum  (or  presence  of  foramen  Panizzae)  prevent  a 
complete  separation  of  systemic  and  pulmonary  circulations.  In 
the  turtles  a  third  element  enters,  the  persistence  of  a  ductus 
Botalli  (as  in  Urodeles,  fig.  580,  II,  dB). 

To  the  foregoing  adaptations  to  a  terrestrial  life  may  be  added 
indications  of  higher  development.  The  brain  shows  two  advances. 
The  cerebellum,  especially  in  turtles  and  alligators,  has  be- 
come large,  and  the  cerebrum  grows  dorsally  and  backwards  over 
the  'twixt  brain  and  forms  the  temporal  lobes  of  the  hemispheres. 
The  parietal  organ  is  developed  as  nowhere  else.  In  many  lizards 
it  forms  an  unpaired  dorsal  eye  lying  beneath  the  skin  in  the 
parietal  foramen.  The  paired  eyes  possess  lids  (usually  upper  and 
lower  as  well  as  a  nictitating  membrane),  and  frequently  (turtles, 
lizards,  and  many  fossils)  a  ring  of  bony  plates  (sclerotic  bones) 
in  the  sclera.  A  new  opening  in  the  petrosal,  the  f  enestra  rotunda, 
places  the  tympanic  cavity  and  the  labyrinth  in  close  relations. 

In  the  excretory  system  amniote  characters  prevail.  The 
Wolffian  body  with  its  duct  is  functional  in  the  embryo.  Later 
there  arises  behind  it  the  permanent  kidney  (metanephros)  with 
the  ureter,  while  the  embryonic  structures  disappear  with  the  ex- 
ception of  those  retained  as  accessory  to  the  genital  apparatus. 

Thus  in  the  male  the  vas  deferens  and  epididymis  are  formed 
from  the  Wolffian  duct;  in  the  female  the  Mullerian  duct  (early 
lost  in  the  male)  becomes  the  oviduct.  Usually  the  urogenital 
canals  open  dorsally  in  the  cloaca,  rarely  in  an  elongation  of  the 
urinary  bladder  (Chelonia).  This  latter  is  lacking  in  snakes  and 
crocodiles. 

Almost  all  reptiles  lay  eggs;  only  in  the  Squamata  (some  snakes 
and  lizards)  are  viviparous  or  ovoviviparous  forms  present.  The 
eggs  much  resemble  those  of  birds,  in  that  the  large  yolk  is  sur- 
rounded with  a  layer  of  albumen  and  enclosed  in  a  fibrous,  often 
calcified  shell.  To  open  the  egg  the  embryo  has  an  egg  tooth  on 
the  tip  of  the  snout ;  this  consists  of  dentine  in  the  Squamata,  but 
elsewhere,  as  in  birds,  is  horny.  From  these  relations  it  follows 
that  internal  impregnation  must  occur;  the  eggs  undergo  a  discoidal 
(meroblastic)  segmentation.  Copulatory  organs  to  accomplish 
this  internal  fertilization  occur,  and  these  are  of  classificatory  im- 
portance, since  they  differ  in  character  in  the  Squamata  on  the  one- 
hand,  the  turtles  and  crocodiles  on  the  other.  These  differences 
are  correlated  with  differences  in  the  form  of  cloacal  opening  and 
in  structure  of  skull  and  skin,  so  that  all  living  species  may  be- 


594:  CHORDATA. 

divided  into  two  groups,  the  Lepidosauria,  containing  the  lizards, 
snakes  and  Sphenodon,  and  the  Hydrosauria  with  turtles  and  croc- 
odiles. This,  however,  ignores  the  fossil  forms.  When  these  are 
taken  into  consideration  another  grouping  must  be  adopted. 

Order  I.  Theromorpha. 

Extinct  reptiles  from  the  Permian  and  triassic  which  are  closely  re- 
lated to  the  stegocephalous  amphibia;  with  amphiccelous  vertebrae,  im- 
movable quadrate,  and  from  two  to  six  sacral  vertebras.  The  ANOMODON- 
TLA,  with  partial  or  complete  loss  of  teeth,  stand  near  the  turtles,  while 
the  THERIODONTA,  in  which  a  heterodont  dentition  is  developed,  resemble 
in  this  and  some  other  respects  the  mammals,  which,  by  many,  are  sup- 
posed to  have  descended  from  them. 

Order  II.  Plesiosauria. 

Extinct  aquatic  forms  from  the  triassic  to  the  cretaceous,  some  forty 
feet  in  length.  They  had  long  necks,  and  the  limbs  were  modified  into 
.swimming  paddles  recalling  the  flippers  of  the  whales.  The  quadrate  was 
immovable,  and  the  jaws,  with  numerous  teeth  in  sockets,  were  long. 

Order  III.  Ichthyosauria. 

These  forms  resembled  the  Plesiosaurs  in  skin,  swimming  feet,  elongate 
jaws,  and  quadrate,  but  had  the  teeth  (sometimes  absent)  in  grooves  rather 


' 


FIG.  622.— Restoration  of  Plexiosaur.    (After  Dames.) 

than  in  sockets,  and  short  necks.    Some  species  at  least  were  viviparous. 
Their  range  in  time  was  like  that  of  the  preceding  order. 

Order  IV.  Chelonia  (Testudinata). 

The  turtles  form  in  external  appearance  a  sharply  circumscribed 
group,  with  the  short  and  compact  body  enclosed  in  a  bony  case, 
from  which  only  head,  tail,  and  legs  protrude  (fig.  623).  The 
case  consists  of  a  convex  dorsal  portion,  the  carapace  and  a  flat- 
tened ventral  plastron,  the  two  being  united  in  most  forms  at  the 
margins.  Each  consists  of  bony  plates,  the  positions  and  names 
of  which  may  be  learned  from  the  adjacent  cut.  It  only  needs 
mention  that  the  neural  plates  are  united  with  the  spinous  pro- 
cesses, the  costals  with  the  ribs,  and  that  the  entoplastron  is  re- 


IV.    VERTEBRATA:  REPTILIA,   CHBLONIA. 


595 


garded  as  an  episternum.  It  is  not  connected  with  the  internal 
skeleton,  since  the  sternum  is  lacking.  The  pelvis  is  only  rarely 
fused  with  the  plastron.  This  bony  case  is  usually  covered  with 
horny  shields,  their  number  and  arrangement  usually  agreeing 
with  the  plates  of  the  case,  although  without  their  contours  exactly 
coinciding. 

More  important  are  the  great  firmness  of  the  skull  and  the 
immovable  condition  of  the  quadrate,  the  lack  of  an  os  transver- 
sum  and  of  any  but  basisphenoid  of  the  sphenoidal  bones,  and  by 

A 


FIG.  623.— Carapace  (A)  and  Plastron  (B)  of  Testudo  grceca.  (From  Wiedersheim.)  C, 
costal  plates;  E,  entoplastron  ;  Kp.  epiplastron:  H,  posterior:  Hp,  hypoplastron; 
Hy,  hyoplastron  ;  M,  marginal  plates  ;  J\T,  neural  plates  ;  JVp,  nuchal  plate  ;  Py, 
pygal  plate  ;  .R,  ribs  ;  V,  anterior  ;  Xi,  xiphisternum. 

growth  forward,  and  backwards  by  which  the  girdles  are  brought 
inside  the  ribs.  The  teeth  are  entirely  lost,  and,  as  in  birds,  the 
jaws  are  enclosed  in  sharp  horny  beaks,  in  many  cases  efficient 
weapons  against  larger  vertebrates.  The  cloacal  opening  is  oval, 
its  major  axis  corresponding  to  that  of  the  body,  and  in  its  anterior 
end  is  an  unpaired  erectile  penis  used  in  copulation.  Turtles 
appeared  in  the  Permian,  and  the  group  has  persisted  until  now. 

Characters  of  armor  and  legs  serve  to  contrast  sharply  the  land  and 
sea  turtles;  the  first  with  well-developed  legs,  five-toed  in  front,  four- 
toed  behind,  the  toes  with  claws;  the  carapace  arched,  into  which  legs, 
head,  and  tail  may  be  retracted.  In  the  sea  turtles  the  feet  are  flipper- 
like  (fig.  624),  claws  mostly  absent,  and  the  carapace  weakly  united  to  or 
free  from  the  plastron,  flat  and  incapable  of  covering  head  or  appendages. 
The  fresh-water  species  are  intermediate  in  position. 

Sub  Order  I.  ATHECA.  Carapace  of  numerous  mosaic  scales  and  not 
connected  with  ribs  and  vertebrae;  skin  leathery.  Dermochelys  (Sphargis) 
coriacea,*  the  leather-back  tortoise  of  warmer  seas,  reaches  a  weight  of 
1500  pounds. 


596  CHORD  AT  A. 

Sub  Order  II.  TRIONYCHIA.  Fresh-water  forms  with  poorly  ossified 
carapace,  but  ribs  and  vertebrae  connected  with  it.  Our  leather  turtles. 
(Amyda*)  and  soft- shelled  turtles  (Aspidonectes*)ot  savage  habits  belong 
here. 

Sub  Order  III.  CRYPTODIRA.  Carapace  well  developed  and  united 
with  ribs  and  vertebrae,  but  the  pelvic  arch  free.  The  species  are  numer- 
ous, including  terrestrial,  fresh-water,  and  marine  forms.  CHELYDRID^, 
fresh  water,  tail  long.  Chelydra  serpentina,  *  snapping  turtle ;  Machrochelys 


FIG.  &24.—  Eretmochelys  imbricata,  tortoise-shell  turtle.      (From  Hajek.) 

lacerti?ia,*  alligator  turtle.  CHELONID^E,  marine,  paddle-like  feet.  Tha- 
lassoclielys  caretta,*  loggerhead;  Chelone  my  das*  green  turtle,  the  favorite 
of  epicures;  Eretmoclielys  imbricata,  whose  horny  shields  furnish  tortoise 
shell.  TESTUDINID.E,  terrestrial,  including  Xerobates*  the  '  gopher  turtle  * 
of  the  South,  the  giant  Testudoot  the  Galapagos  Islands,  and  the  enormous 
fossil  Colossochelys  atlas  of  India,  18-20  feet  long,  8  feet  high.  Other 
families  contain  our  mud  turtles  (Kinosternon  *),  box  turtles  (Cistudo*), 
and  terrapins  (Malaclemmys*). 

Sub  Order  IV.  PLEURODIRA.  Pelvis  united  to  carapace  and  plastron. 
All  belong  to  the  southern  hemisphere. 

Order  V.  Rhynchocephalia. 

These  resemble  the  lizards  not  only  in  body  form  (four  five- 
toed  feet)  and  in  scaly  skin,  but  in  certain  anatomical  matters  as 
well:  lack  of  hard  palate,  presence  of  epipterygoid,  transverse 
cloacal  opening,  and  heart,  lungs,  and  brain.  On  the  other  hand 
they  recall  the  crocodiles  in  having  two  postorbital  arches  and 
immovable  quadrate.  The  large  abdominal  sternum  and  abdominal 
ribs  are  noticeable  as  well  as  the  uncinate  processes  of  the  true 
ribs.  The  notochord  is  but  incompletely  replaced.  The  group 
appears  in  the  Permian  and  is  thus  one  of  the  oldest  of  reptilian 
types,  and  is  usually  regarded  as  ancestral  to  all  the  orders  yet  to 
be  mentioned.  The  only  living  species,  Sphenodon  (Hatteria) 
punctata,  belongs  to  the  New  Zealand  region. 

Order  VI.  Dinosauria. 

This  order  included  some  of  the  largest  land  animals  which  have  ever 
existed.  Some  of  them  were  from  forty  to  one  hundred  feet  long  and 
twelve  to  twenty  feet  high  (Amphiccelias,  Camarasaurus).  In  some  there 


IV.    VERTEBRATA:   REPTILIA,  S  QUA  MAT  A. 


597 


•was  an  exoskeleton,  some  of  the  plates  of  which  in  the  stegosaurs  measured 
a  yard  across.  Among  the  characters  of  the  group  are  the  fixed  quadrate, 
jugal  and  postorbital  arches,  three  to  ten  sacral  vertebra,  and  ilium 
•elongate  in  front  of  and  behind  the  acetabulum.  Some  of  these  forms 
(Orthopoda)  in  pneumaticity  of  bones,  in  having  the  pubic  bones  directed 
backwards,  and  in  the  formation  of  an  intratarsal  joint,  resembled  the 
birds,  and  have  been  regarded  as  the  ancestors  of  that  group.  The  Dino- 
saurs were  confined  to  mesozoic  time. 

Order  VII.  Squamata  (Lepidosauria,  Plagiotremata). 

One  of  the  characters  which  unite  lizards  and  snakes  and  which 
has  given  the  name  Plagiotremata  is  the  transverse  form  of  the 
cloacal  opening  (fig.  625),  behind  which,  in  the  male,  are  the 

nS'Jr 
-na  '  • 
fir 


ar 


FIG.  625. 


FIG.  626. 


FIG.  625.— Hinder  trunk  and  hind  limbs  of  a  lizard.  (From  Ludwig-Leunis.)  a, 
cloacal  slit ;  b,  femoral  pores  ;  sea,  anal  shield. 

FIG.  626.— Skull  of  Ameiva  vulgaris.  an,  angulare  ;  ar,  articulare  ;  co,  epipterygoid  ; 
cr,  coronoid  ;  d,  dentary  ;  /r,  frontal ;  j,  jugal  ;  la,  lachrymal ;  m,  maxillary  ;  na, 
nasal ;  p,  postorbital,  above  and  behind  it  the  parietal ;  p/,  pref rontal ;  pr,  pre- 
maxilla ;  pt,  pterygoid  ;  g,  quadrate  ;  oj,  quadratojugal ;  sq,  squamosal ;  tr,  trans- 
versum. 

paired  copulatory  organs,  each  lying  in  a  sac  from  which  they  can 
be  everted  like  the  finger  of  a  glove.  The  names  Squamata  and 
Lepidosauria  refer  to  the  scaly  condition  of  the  skin.  These 
scales  are  horny  structures  and  somewhat  distinct  from  the  bony 
scales  of  fishes.  The  derma  forms  flattened  papilla  which  resemble 
the  scales  of  fishes  in  that  in  many  species  they  contain  bony 
plates.  These  papillae  determine  the  character  of  the  epidermis. 
Since  the  stratum  corneum  is  especially  thick  on  the  top  of  the 
papillae  and  thinner  between  them,  rhomboid  and  oval  plates  occur, 
which  either  lie  flush  with  each  other  (shields)  or  overlap  like 
shingles  (scales).  The  rule  is  that  the  head  is  covered  with  regu- 
larly arranged  shields,  each  with  its  name,  the  trunk  with  scales 
in  longitudinal,  transverse,  and  oblique  lines.  Outside  these  is  a 
layer  of  cornified  cells,  the  pseudocuticula,  and  outside  of  all  an 
inconspicuous  true  cuticle.  Since  all  cornified  cells  are  dead  and 


598  CHORDATA. 

require  periodic  removal,  the  horny  layers  are  cast  yearly  and  re- 
placed by  new.  During  this  periodic  molting,  which  recalls  that 
of  arthropods,  the  animals  are  sickly  and  apt  to  die  in  captivity. 

All  Squamata  are  characterized  by  the  slenderness  of  the 
cranial  bones  (fig.  619,  626,  627),  which,  especially  in  the  Lacertilia, 
incompletely  close  in  the  cranium.  The  quadrate  is  movable, 
and  the  squamosal  is  intercalated  between  it  and  the  cranium.  A 
hard  palate  is  lacking,  and  the  choanse,  as  in  the  amphibia,  lie  far 
forward  (fig.  619,  Ch).  There  is  a  wide  gap  in  the  partition 
between  the  two  ventricles  of  the  heart. 

Sub  Order  I.  LACERTILIA  (Saurii).  The  lizards  are  usually  distin- 
guished from,  the  snakes  by  the  possession  of  limbs,  but  a  few  forms, 
undoubted  lizards,  like  the  glass  snakes  and  Amphisbsenaa.  lack  limbs. 
These  are  distinguished  by  the  existence  of  the  scapula  and  the  iliac  bone 
united  to  the  vertebra,  and  especially  by  the  presence  of  a  sternum,  which 
never  occurs  in  snakes.  In  the  skull  is  a  peculiar  bone  (lacking  only 
in  Chameleons  and  Amphisbsenae),  found  nowhere  else,  the  epipterygoid 
(fig.  626,  co);  it  reaches  from  the  pterygoid  to  the  parietal,  and  from  its 

fffrffr 


FIG  627.— Skull  of  rattlesnake.  (From  Boas.)  F>;  frontal;  ft,  hypmandibular  (09111- 
mella);  MX,  maxillary:  iV,  nasal:  Os,  supraoccipital;  Fa,  parietal;  Pal,  palatine; 
P/,  postfrontal;  PC/,  pref rontal ;  Pt,  pterygoid  ;  Px,  premaxilla ;  Q,  quadrate;  3g, 
squamosal;  2V,  transversum;  1,  dentary;  3,  articulare. 

slender  shape  is  sometimes  called  columella,  but  is  not  to  be  confounded 
with  the  true  columella  of  the  ear.  The  bones  of  the  jaws  are  firmly  united, 
so  that  the  mouth  has  no  special  capacity  for  opening  widely.  The  jugal- 
quadratojugal  arch  is  present. 

In  external  appearance  the  presence  of  eyelids,  nictitating  membrane, 
tympanic  membrane,  and  Eustachian  tube  are  noticeable,  these  being 
absent  only  in  the  Amphisba3na3.  In  the  Ascalabota?,  as  in  snakes,  the  lids 
grow  together,  forming  a  transparent  covering  over  the  eyes.  Fossil 
lizards  are  rare,  but  the  group  dates  back  to  the  cretaceous. 

Section  I.  ASCALABOT^E  (geckos).  Skeleton  incompletely  ossified,  noto- 
chord  persistent,  amphicoele  vertebrae;  skin  granular  rather  than  scaly, 
usually  adhesive  discs  on  the  toes  by  which  they  climb  vertical  surfaces  or 
can  walk  upon  ceilings.  Two  hundred  species.  Phyllodactylus* 


IV.    VERTEBRATA:    REPTILIA,  SQUAMATA.  599 

Section  II.  CRASSILINGUIA.  Tongue  thick,  fleshy,  not  protrusible  from 
the  mouth,  or  only  slightly  so.  IGUANIOE  ;  American,  often  a  comb  of 
spines  on  the  back,  teeth  pleurodont,  i.e.,  firmly  united  to  the  inner  side 
of  the  jaw.  Three  hundred  species.  Anolis,*  Sceleporus*  Phrynosoma* 
*  horned  toads.'  AGAMID.E;  Old  World,  teeth  acrodont,  i.e.,  seated  on  the 
angle  of  the  jaw  bones.  One  hundred  and  fifty  species.  Chlamydosaurus, 
Draco  volans,  with  ribs  greatly  elongate  and  supporting  a  dermal  fold 
which  acts  as  a  parachute. 

Section  III.  FISSILINGUIA.  Tongue  long  and  thin,  divided  at  the  tip, 
and  capable  of  wide  protrusion  from  the  mouth,  and  in  Varanus  retractile 
into  a  sheath.  TEJID^E  ;  American,  teeth  acrodont ;  Cnemidophortts* 
Tejus.  HELODERMATIDJS,  pleurodont ;  Heloderma,*  the  '  Gila monsters,'  are 
the  only  poisonous  lizards.  LACERTILID^E  (Lacerta)  and  VARANID^E  (Vara- 
nus, the  monitors)  are  Old  World  forms,  Lacerta  vivipara  bringing  forth 
living  young. 

Section  IV.  BREVILINGUIA.  Tongue  short,  slightly  notched  at  the  tip, 
slightly  protrusible.  Four  hundred  species.  SCINCID^,  with  tendency  to 
reduction  of  the  limbs.  Eumeces,*  Oligosoma*  In  Anguis  and  Typhline 
the  legs  are  absent.  ZONURID.E,  with  a  finely  scaled  groove  along  the  side; 
all  Old  World  except  our  Ophisaurus  ventralis,*  the  glass  snake,  a  limb- 
less form  with  brittle  tail. 

Section  V.  ANNULATA.  In  many  respects  snake-like ;  legs  and  epi- 
pterygoid,  tympanum,  and  movable  eyelids  lacking  and  usually  girdles  ; 
tropical  or  subtropical.  In  Chirotes  sternum  and  reduced  fore  legs 
retained.  AmpliisbcBna. 

Section  VI.  VERMILINGUIA  ;  includes  the  Old  World   chameleons  (our 


FlG.628. — Head  of  chameleon  with  tongue  extended. 

'  chameleon '  is  Anolis, — supra}  with  long  fleshy  tongue,  lying  rolled  up  in 
the  mouth,  but  protrusible  and  used  for  catching  insects,  its  end  being 
covered  with  a  sticky  mucus.  Other  characteristics  are  the  ring-like  eye- 
lids functioning  as  an  iris,  the  climbing  feet  in  which  the  toes  are  united 
into  two  opposable  groups;  epipterygoids,  clavicle,  sternum,  and  tympanic 
membrane  lacking.  The  chameleons  are  best  known  from  their  changes 
of  color,  produced  by  rapid  alterations  in  the  size  and  shapes  of  the 
chroinatophores.  Color  changes  occur  in  other  lizards,  but  not  to  such  an 
extent  as  here. 


600 


CHORD  ATA. 


Sub  Order  II.  PYTHONOMORPHA.  Large,  extinct,  extremely  elon- 
gate reptiles  with  four  flipper-like  limbs  and  strong  swimming  tail. 
Flourished  in  the  cretaceous.  Mosasaurus,  CUdastes. 

Sub  Order  III.  OPHIDIA.  The  snakes  are  distinguished  from  most 
lizards  by  the  absence  of  limbs,  and  connected  with  this  the  similar  verte- 
brae in  which  only  trunk  and  caudals  can  be  distinguished.  The  caudals 
lack  ribs,  but  these  are  present  and  long  in  the  trunk  region,  serving  for 
locomotion  and  supporting  the  body  on  their  distal  ends.  .  Since  there  are 
legless  lizards,  it  is  further  necessary  to  say  that  in  the  Ophidia  the  girdles 
and  sternum  are  lost,  only  the  Peropoda  having  remnants  of  the  hinder 
appendages  and  pelvis,  but  these  not  connected  with  the  vertebral  column. 
Further  distinctions  exist  in  sense  organs  and  jaws.  The  columella  is 
indeed  present,  but  tympanum  and  Eustachian  tube  are  lacking.  The  eye- 
lids also  seem  to  be  wanting,  but  examination  shows,  in  front  of  the  cornea 
and  separated  from  it  by  a  lachrymal  sac,  a  transparent  membrane,  com- 
posed of  the  fused  eyelids  (outer  cornea).  The  apparatus  of  the  jaws  (figs. 
619,  627)  is  remarkable  for  its  great  extensibility,  which  enables  snakes  to 
swallow  animals  larger  than  themselves,  after  coiling  around  them  and 
crushing  them.  This  extensibility  is  in  part  due  to  the  fact  that  the  bones 
of  the  lower  jaw  are  bound  together  at  the  symphysis  by  elastic  ligaments, 
in  part  to  the  freedom  of  motion  of  the  bones  of  the  upper  jaw  (excepting 
the  small  premaxillaries)  and  the  palate.  Further,  the  sqtiamosal  (&g), 
quadrate  (Q),  and  transversum  (Tr)  are  elongate  and  slender,  the  quadrate 
being  widely  separated  by  the  squamosal  from  the  skull,  while  the  zygo- 
matic  arch  is  entirely  absent.  The  food  is  forced  down  the  throat  by 
hook-shaped  bones  on  palatines  and  pterygoids.  A  wide  distension  of 
the  stomach  is  rendered  possible  by  the  elasticity  of  its  walls  and  the  great 
mobility  of  the  ribs,  which  are  not  united  ventrally  by  a  sternum. 

In  the  non-poisonous  snakes  the  dentition  is  similar  on  jaws  and 
palate  bones  (fig.  619).  The  vomer  and,  usually,  the  premaxilla  are  tooth- 

less. In  the  poisonous  serpents  poison 
fangs  appear  on  the  maxilla  (fig.  627) 
and  are  distinguished  from  the  other 
teeth  by  their  greater  size  and  connex- 
ion with  a  large  poison  ghuid.  The 
duct  of  the  gland  opens  at  the  base  of 
the  tooth  ;  the  poison  which  is  pressed 
out  by  the  pressure  of  the  jaw  muscles 
is  led  to  the  tip  of  the  tooth  either  by  a 
groove  (proteroglyphic  tooth,  fig.  629,  A) 
or,  when  the  groove  is  closed  to  a  cnnal 
(solenoglyphic  tooth,  B),  through  this 
.  pro-  canal  which  opens  at  base  and  tip  of 

teroglyphic    (grooved)   tooth    of  co-  tl      tnnth 
bra,  and  section  of  same;  #,  #,,  so-  tue  1      tn- 

The    asymmetrical   character  of  the 

lungs  is  interesting.     In  the   Peropoda 
one  lung  (apparently  the  left)  is  much 
smaller  than  the  other  ;  in  the  poison  snakes  and  some  others  it  is  rudi- 


Fm.  629—  Pison   fangs. 


lenoglyphic  tooth  (tubular)  of  rattle- 
snake ;  g,  poison  canal ;  jo,  pulp 
cavity. 


IV.    VERTEBRATA:  REPTILIA,   CROCODILIA. 


601 


mentary  or  even  absent.  In  the  Typhlophidae,  on  the  other  hand,  the  right 
appears  to  be  degenerate.  'The  urinary  bladder  is  always  absent.  The 
excreta,  chiefly  uric  acid,  accumulate  as  a  solid  mass  in  the  cloaca  and 
form  the  chief  part  of  the  excrement ;  the  faeces,  on  account  of  the 
extraordinary  digestive  powers,  being  small  in  amount. 

Section  I.  OPOTERODONTA  (Angiostoma).  Burrowing  blind  tropical 
snakes  with  the  mouth  incapable  of  distension,  the  animals  living  on 
small  insects.  Typhlops. 

Section  II.  PEROPODA.  These  large  snakes  have  paired  lungs  and  rudi- 
ments of  hind  extremities  ;  lack  poison  fangs,  and  kill  their  prey  by  mus- 
cular power.  Python,  Africa ;  Boa  and  Eunectes  (anaconda),  South 
America. 

Section  III.  COLUBRIFORMIA.  Ordinary  snakes  (over  500  species)  with 
numerous  teeth  in  the  upper  jaw,  but  with  appendages  entirely  absent. 
Some  are  poisonous,  some  not,  but  no  structural  lines  can  be  drawn  be- 
tween them.  The  AGLYPHA  have  no  grooved  teeth.  Tropidonotus,*  water 
snakes;  Bascanion,*  black  snakes;  Eutainia,*  garter  snakes.  The  PRO- 
TEROGLYPHA,  with  grooved  teeth,  perma- 
nently erect,  are  poisonous.  Most  are 
brightly  colored.  Elaps,*  the  coral  snake; 
Naja  tripudians,  the  cobra  of  India  ;  N. 
haje,  Cleopatra's  asp.  Here  belong  the 
pelagic  sea  snakes  of  the  Indo-Pacific, 
which  are  viviparous. 

Section  IV.  SOLENOGLYPHA.  With  the 
maxilla  reduced  and  serving  as  a  socket 
for  the  single  large  tubular  tooth  with 
one  or  more  reserve  teeth  (fig.  627). 
VIPERID^,  Old  World,  no  pit  between 
nostril  and  eye.  CROTALID.E,  New  World 
and  Asia,  with  a  pit  between  nose  and 
eye.  Crotalus*  with  the  tail  ending  in 
a  rattle  formed  by  remnants  of  cast  skins, 
is  common  throughout  the  United  States. 
Agliistrodon  contortrix*  copperhead, 
and  A.  piscivorus,  moccasin,  lack  the 
rattle.  Bothrops  lanceolatus  of  the  An- 
tilles, possibly  the  most  poisonous  snake. 


Order  VIII.  Crocodilia  (Loricate). 

The  crocodiles,  alligators,  etc., 
ome  of  the  forms  already 
mentioned  in  the  oval  cloacal  open- 
ing with  single  copulatory  organ, 
immovable  quadrate,  and  the  bony 
plates  in  the  skin.  In  shape  they  are 
lizard-like,  but  in  structure  they  differ  from  all  other  living  reptiles 


Coee 

FIG.  630.— Ventral  surface  of  skull 
of  crocodile.  (From  Wiedersheim.) 
Cocc,  occipital  condyle:  C/i,  cho- 
ana  ;  jg,  jugal ;  M,  maxillary  ;  O/>, 
basioccipital ;  Or/>,  orbit ;  Qi,  quad- 
;  Pi,  pala- 


ratojugal ;  Qtt,  quadrate 
tine  ;  Pmx,  premaxilla; 
goid;  Ts,  transversum. 


ptery- 


602 


CHORD  AT  A. 


and  approach  most  nearly  to  the  Theromorphs.  The  maxillaries, 
palatines,  and  ptery golds  have  united  in  the  living  species  in  the 
middle  line,  forming  a  hard  palate  and  forcing  the  vomers  upwards 
into  the  nasal  region.  This  same  process  has  carried  the  choana 
(fig.  630,  Cli)  to  the  back  of  the  skull.  Some  of  the  ribs  have 
two  heads;  the  ears  and  nostrils  are  provided  with  valves.  A 
sternum  is  present  and,  farther  back,  abdominal  ribs  and  an  ab- 
dominal sternum.  The  jaws  are  extended  into  a  long  snout,  and 
the  teeth,  which  occur  only  on  the  margins,  are  placed  in  sockets 
(alveoli).  The  four-chambered  heart  has  already  been  described 
(p.  592).  The  animals  move  slowly  on  land,  but  in  the  water, 
thanks  to  their  strong,  keeled  tail,  they  are  very  active.  They  have 
a  strong  smell,  owing  to  musk  glands  in  the  cloaca  and  on  the 
under  jaw.  The  group  appeared  in  the  trias,  and  of  the  three  sub 
orders  two,  the  Pseudosuchia  and  Parasuchia,  are  extinct. 

Sub  Order  EUSUCHIA.  External  nostrils  united,  choana  posterior; 
five  toes  in  front,  four  behind.  Gavialis,  India,  snout  long  and  slender. 
Alligator  lucius*  alligator  ;  Crocodilus,*  most  species  Old  World,  one,  C. 
americanus,*  occurring  in  our  southern  waters. 

Order  IX.  Pterodactylia  (Pterosauria). 

Extinct  reptiles  of  the  Jurassic  and  cretaceous,  adapted  for  flight. 
The  bones  were  hollow  and  the  wings  were  broad  membranes,  supported, 
like  those  of  a  bat,  by  the  body  and  the  greatly  elongated  fifth  digit  of  the 


FlG.  631. — Dimorpliodon,  a  pterodactyle.    (After  Woodward.) 

fore  limbs.  Some  were  sparrow-like  in  size  and  some,  Pteranodon,  had  a 
wing  expanse  of  twenty  feet.  Yet  one  of  these  large  forms  from  Kansas 
had  its  pelvic  opening  so  small  that  its  eggs  could  not  have  been  more 
than  half  an  inch  in  diameter. 


IV.    VERTET3RATA:  AVES.  603 


Class  II.  Aves. 

While  structurally  the  birds  stand  very  near  the  reptiles,  yet  by 
the  development  of  wings  and  the  feathering  of  the  body  the  group 
is  one  strictly  circumscribed.  The  skin  is  in  some  places,  as  the 
lower  part  of  the  legs,  covered  with  horny  scales  and  shields,  on 
the  toes  are  claws,  but  as  a  rule  the  fingers  are  feathered.  On 
most  places  the  skin  is  soft  and  thin,  since  the  derma  and  stratum 
corneum  are  poorly  developed.  Periodic  molts  of  the  integument 
do  not  occur,  since  the  horny  layer,  as  in  mammals,  undergoes  a 
constant  renewal.  These  peculiarities  of  the  skin  are  correlated 
with  the  appearance  of  the  protecting  plumage. 

The  feather,  like  the  hair  of  mammals,  is  exclusively  epithelial 
in  character,  but  of  a  much  more  complicated  structure.  The  cor- 
nified  epithelium  forms  a  firm  axis,  the  scape,  from  which,  right 
and  left,  arise  branches,  or  barbs.  The  scape  is  solid  as  far  as  the 
barbs  extend  (rachis,  or  shaft),  while  below  it  is  hollow  (quill,  or 
calamus).  The  quill  is  inserted  deep  in  the  derma,  in  a  follicle, 
and  is  provided  with  muscles  for  its  movement.  Its  hollow  in 
most  fully  developed  feathers  is  empty  save  for  the  '  pith/  a  small 
amount  of  dried  tissue.  In  young  growing  feathers  it  is  occupied 
by  a  richly  vascular  connective  tissue,  the  feather  papilla,  which, 
for  purposes  of  nourishment,  extends  inwards  from  the  derma. 
The  feather  may  therefore  be  regarded  as  a  cornified  outgrowth 
from  the  skin  which  has  arisen  on  a  papilla  of  the  derma,  a  view 
which  corresponds  well  with  its  development  and  shows  its 
homology  with  the  scales.  In  many  birds  (cassowaries)  two  well- 
developed  feathers  arise  from  the  same  follicle — a  fact  which 
explains  the  existence  of  a  rudimentary  feather,  the  hyporachis, 
or  after-shaft,  attached  to  the  scape  below. 

In  contour  feathers  the  barbs  are,  to  a  great  extent,  united  into  a 
vane.  Right  and  left  of  the  shaft  they  lie  close  together  and  parallel, 
each  repeating  in  miniature  the  entire  feather,  the  barb  having  branches 
or  barbules,  which,  overlapping  the  barbules  of  adjacent  barbs,  give  the 
vane  its  close  texture.  The  vane  is  held  together  by  minute  hooks  on  the 
barbules  of  one  barb  interlocking  with  those  of  the  next.  Down  feathers 
(plumes)  differ  from  contour  feathers  in  the  absence  of  hooks  and  the 
loose  arrangement  of  the  barbs.  Since  feathers  consist  of  cornified  epithe- 
lium and  these  cells  are  held  firmly  (only  in  powder  down  is  there  a 
gradual  loss),  they,  like  the  scaly  coat  of  the  snakes  and  lizards,  must  be 
molted  yearly  and  replaced  by  new. 

Young  birds  or  embryos  have  only  down  feathers.  Later  the  contour 
feathers  arise  in  regular  order  in  the  feather  tracts,  or  pterylae,  between 


604: 


CHORD  AT  A. 


which  are  apteria  in  which  no  contour  feathers  appear  (fig.  632).  Since 
the  contour  feathers  overlap  like  shingles,  they  form  a  firm  coat  of 
plumage  beneath  which  the  down  and  semiplumes  form  a  warm  coat. 


FIG.  632.  FIG.  633. 

FIG.  632. — Feather  tracts  and  apteria  of  pigeon,  dorsal  view.  (From  Ludwig-Leunis.) 
FIG.  633.— Regions  and  feathers  of  Falco  lanarius.  (From  Schmarda.)  As,  secondaries  ; 

Ba<  belly  ;   Br,  breast ;  Bz<  rump  ;  D'-D'",  wing  coverts  ;  Di,  gonys  of  bill ;  EF, 

alula  ;  F,  culmen  of  bill ;  H,  occiput ;  HS,  primaries ;  K",  throat  ;  L,  legs  ;  JV,  neck  ; 

Sch,  crown ;  SF,  parapterium ;  St,  forehead,  lower  tail  coverts ;  Sz,  rectrices  ;  W, 

cheek;  WH,  cere  with  nostril;  Zh,  toes. 

Besides  these  covering  feathers  (coverts,  or  tectrices,  fig.  633,  D)  there  are 
the  longer  feathers  of  the  wing,  the  remiges,  and  the  tail  feathers,  or 
rectrices  (8z).  The  larger  remiges  form  the  chief  part  of  the  wing;  they 
spring  from  the  part  of  the  limb  corresponding  to  the  hand  (carpus, 
metacarpus,  phalanges)  and  are  known  as  primaries  (HS),  while  the 
secondaries  (As),  arising  from  the  forearm,  are  shorter.  These  are  over- 
lapped at  the  base  by  the  coverts  (D,  D',  D'1)  and  by  the  parapterium  (SF) 


FIG.  634.— Wing  skeleton  of  stork.  (From  Gegenbaur.)  c,  c',  carpalia  of  first  row; 
ft,  humerus  ;  wi,  fused  metacarpals  and  carpals  of  second  row ;  p-p'\  phalanges  of 
first  three  fingers  ;  r,  radius ;  w,  ulna. 

springing  from  the  shoulder.  A  few  feathers  arising  from  the  first  finger 
remain  distinct  from  the  remiges  and  form  the  alula  (EF).  In  the  water 
birds  especially  the  feathers  are  oiled  by  the  secretion  of  a  pair  of  glarrds 
at  the  base  of  the  tail  above  the  coccyx. 

Since  the  feathers  are  not  only  for  protection,  but  give  to  most 
birds  the  power  ol  prolonged  flight,  they  predicate  a  special  mode 


IV.    VERTEBRATA:   AVES. 


605 


S/- 


of  life,  under  the  influence  of  which  all  of  the  other  organs  exist. 
The  character  of  the  skeleton,  the  respiratory  organs,  and  in  part 
the  sense  organs  and  brain,  are  connected  with  the  powers  of  flight. 

As  the  feathers  of  the  wings,  like  the  fins,  form  what  may 
be  called  a  paddle  working  as  a  whole,  the  skeleton  of  these  limbs  is 
simplified  (fig.  634),  first,  by  the  reduction  of  the  fingers,  of  which 
only  three  with  a  small  number  of  phalanges  persist  (j>,p',p")\ 
second,  by  fusion  of  the  corresponding  metacarpals  (m)  with  each 
other  and  with  the  adjacent  carpal 
bones.  On  the  other  hand,  in  order 
that  there  may  be  the  necessary  en- 
ergy and  the  most  complete  transfer 
of  the  same  to  the  body,  the  con- 
nexion with  the  skeletal  axis  is 
strengthened  by  special  development 
of  the  parts.  In  the  shoulder  girdle 
(fig.  635)  all  three  elements  are  firm, 
a  sword-shaped  scapula  (s),  a  colum- 
nar coracoid  (c),  and  clavicles  which 
are  usually  united  to  a  i  wish-bone/ 
or  furcula  (/).  Clavicles  and  furcula 
are  united  directly  or  by  ligaments 
to  the  broad  sternum,  the  anterior 
face  of  which  is  developed  into  a 
strong  keel,  the  carina,  or  crista 
sterni,  in  order  to  give  the  largest 
surface  for  attachment  of  the  large 
muscles  of  flight.  Usually  the  greater 
the  powers  of  flight  the  more  devel- 
oped the  carina,  yet  in  some  cases 
(albatross)  the  weak  carina  is  com- 
pensated for  by  the  enormous  width 
of  the  sternal  plate.  In  running  birds  (ostriches,  etc.)  the 
carina  is  entirely  gone.  The  thoracic  framework  is  rendered 
more  firm  by  the  development  of  uncinate  processes  from  the  ver- 
tebral parts  of  the  ribs  (u)  which  overlap  the  succeeding  ribs. 

Since  the  fore  limbs  are  no  longer  used  for  walking,  the  sup- 
port of  the  body  depends  upon  the  hinder  extremities,  and  this 
has  brought  about  two  striking  characteristics — the  broad  union  of 
the  pelvis  with  the  vertebral  column,  and  the  simplification  of  the 
leg  skeleton.  In  the  embryo  the  ilium  (fig.  635,  il)  is  connected 
only  with  the  two  sacral  vertebrae  present  in  most  reptiles,  but 


Fm.  635.— Trunk  skeleton  of   stork. 
(From     Gegenbaur.)     as,    sternal 

rrt  of  rib  ;  c,  coracoid  ;  era,  keel ; 
furcula  (fused  clavicles) ;  /p, 
fused  spinous  processes  of  thoracic 
vertebrae;  il,  ilium;  is,  ischium;  oc, 
vertebral  part  of  ribs;  p,  pubis;  8, 
scapula;  sp.  spinous  process;  st,st', 
sternum  and  abdominal  processes; 
?t,  uncinate  processes;  x,  acetabu- 
lum. 


606 


CHOIWA  TA. 


later  it  extends  forward  and  back,  uniting  with  at  least  nine  ver- 
tebrae and  sometimes  with  as  many  as  twenty-three;  while  the  iliac 
bones  of  the  two  sides  meet  dorsal  to  the  vertebral  column.  This 
extensive  union  of  pelvis  and  axial  skeleton  is  understood  when 
we  recall  that  in  walking  or  at  rest  the  vertebral  column  is  not 
vertical  as  in  man,  but  is  inclined.  Ischium  and  pubis  are  peculiar 
in  that  they  extend  backwards,  parallel  to  each  other,  from  the 
acetabulum,  and  that  only  exceptionally  (ostrich)  are  the  bones 
of  the  two  sides  united  by  a  symphysis. 

In  the  hind  limbs  occur  conditions  similar  to  those  which  will 


PIG.  636.—^!,  leg  of  Buteo  vulgarte.  a,  femur;  7>,  tibio-tarsus;  b',  remains  of  fibula;  c, 
tarso-metatarsus ;  c',  same,  front  view;  dl-tia,  toes.  J3,  lower  leg  of  bird  embryo; 
C,  of  lizard.  /,  femur;  t,  tibia;  jj,  fibula  ;  £x,  tarsales  of  first  row  (talus);  ti,  tar- 
sales  of  second  row;  between  these  intertarsal  joint;  I-F,  digits.  (From 
Gegenbaur.) 

be  repeated  in  the  ungulates.  The  weight  of  the  body  makes  it 
necessary  that  the  simplification  found  in  the  wing  should  be  re- 
peated in  the  lower  leg  and  foot,  and  that  the  numerous  bones 
usually  occurring  in  these  regions  be  replaced  by  one  which  shall 
support  the  pressure  (fig.  636).  Therefore  the  fibula,  well  de- 
veloped in  the  embryo  (B),  becomes  reduced  to  an  inconspicuous 
rudiment;  the  metatarsals,  distinct  in  the  embryo  (B),  fuse  to  a 


IV.    VERTEBRATA:   AVES.  607 

single  tarso- metatarsus  (A,  c),  which  has  below  as  many  articular 
surfaces  as  there  are  toes  (since  the  fifth  toe  only  appears  in  the 
embryo,  at  most  four,  in  some  three  or  even  two,  d-d'").  At  the 
same  time  the  tarsals  disappear  by  fusion  with  adjacent  parts. 
Even  in  reptiles  (C)  a  part  of  the  tarsals  unite  with  the  bones  of 
the  shank,  and  the  remainder  with  the  metatarsals;  in  the  birds 
the  union  is  completed,  the  proximal  series  fusing  with  the  lower 
end  of  the  tibia  to  form  a  tibio-tarsus,  the  distal  with  the  metacar- 
pus to  form  the  tarso-metatarsus,  in  this  way  producing  the  inter- 
tarsal  joint  so  characteristic  of  birds. 

In  respect  to  the  vertebral  column,  it  only  needs  mention  that 
the  vertebrae  articulate  with  each  other  by  a  so-called  saddle-joint, 
that  (in  living  birds)  only  a  few  caudal  vertebrae  persist  behind  the 
pelvis,  that  these  are  partially  fused  to  a  single  bone,  the  pygo- 
style,  which  supports  the  tail  feathers,  and  that,  corresponding  to 
the  well-developed  neck,  there  are  many  cervical  vertebras,  among 
them  an  atlas  and  an  axis,  all  except  the  last  two  fused  with  the 
corresponding  cervical  ribs. 

The  skull  (fig.  637)  resembles  closely  that  of  the  lizards  in  the 
presence  of  a  single  occipital  condyle,  in  the  movable  condition 
of  the  quadrate  upon  the  cranium,  and  in  the  presence  of  a  slender 
columella.  On  the  other  hand  an  os  transversum  is  lacking.  The 
cranium,  as  a  result  of  the  increase  in  size  of  the  brain,  is  more 
spacious;  the  bones  of  its  walls  fusing  early  so  that  the  sutures 


Pal 


FIG.  637.— Skull  of  young  bustartt.  (From  Glaus.)  Als,  alisphenoid  ;  Ang,  angulare  ; 
Art,  articulare;  £),  dentary  ;  Et,  mesethmoid ;  Fr,  frontal ;  Jmx,  premaxillary  ; 
J,  jugal ;  L,  lachrymal ;  MX,  maxillary  ;  JV,  nasal ;  01,  exoccipital ;  Os,  supra- 
occipital ;  P<j,  parietal  ;  Pal,  palatine:  Pf,  pterygoid:  Q,  quadrate;  QJ,  quadrato- 
jugal ;  8m,  interorbital  septum  ;  Spb,  basi-  and  presphenoid. 

between  them  are  obliterated.  The  occipital  condyle  is  on  the 
under  surface,  so  that  the  skull  is  carried  at  nearly  right  angles 
to  the  axis  of  the  vertebral  column.  Teeth  are  lacking  in  living 
birds,  although  they  occurred  in  some  fossil  forms.  In  their  place 


608  CHORD  ATA. 

are  hard  horny  sheaths  covering  the  jaws  which  are  frequently  car- 
ried back  on  the  outside  into  a  softer  cere  (fig.  634,   WH). 

The  cranium  consists  of  four  occipitals,  a  basi-  and  a  presphenoid;  above, 
the  parietals  and  frontals  ;  and  on  the  sides  prootics,  alisphenoids  and 
orbitosphenoids,  while  the  broad  squamosals  also  enter  its  wall.  The  large 
mesethmoid  lies  in  the  interorbital  septum  ;  the  nasal  cavity  is  roofed  by 
the  nasals,  and  beside  them  are  the  lachrymals.  The  quadrate  articulates 
with  the  squamosal,  and  from  it  extend  forward  internally  the  pterygoid, 
palatine,  and  vomer ;  externally  a  zygomatic  arch  of  quadratojugal  and 
jugal  to  the  maxillaries  and  premaxillaries.  The  maxillaries  are  hinged 
in  the  ethmoidal  region,  so  that  in  opening  the  mouth  there  is  besides  the 
depression  of  the  lower  jaw  an  upward  motion  of  the  upper  jaw. 

The  pneumaticity  of  the  bones  is  an  important  feature  of  the 
skeleton.  In  place  of  marrow  and  bony  tissue,  the  inside  of  the 
bones  in  strong  flying  birds  is  more  or  less  completely  occupied  by 
air  spaces,  around  which,  as  a  sheath,  is  the  compact  bone.  This 
gives  the  greatest  possible  lightness  and  strength  to  the  skeleton. 
In  Buceros  and  Palamedea  all  of  the  bones  are  pneumatic;  in 
others  (Pelecanus,  Sula,  Tachy petes,  etc.)  only  the  phalanges  of 
the  toes  contain  marrow,  while  in  the  penguin  and  Apteryx,  as  in 
mammals,  air  spaces  occur  only  in  some  of  the  cranial  bones. 

The  air  spaces  of  the  bones  are  in  part  (skull)  connected  with 
the  nose  and  tympanum,  but  most  of  them,  by  means  of  the  air 
sacs,  communicate  with  the  lungs.  The  long  trachea  forks  at  its 
lower  end  into  two  bronchi.  At  its  upper  end  is  a  larynx,  as  in 
other  vertebrates,  but  this  is  not  vocal ;  the  notes  of  birds  are  pro- 
duced by  the  syrinx,  which  lies  at  the  division  of  trachea  into 
bronchi.  It  is  usually  formed  of  both  trachea  and  bronchi,  but 
more  rarely  of  either  trachea  or  bronchi  alone.  Its  vocal  cords 
are  regulated  by  special  muscles,  which  in  the  singing  birds  have 
a  complicated  arrangement.  The  relatively 
small  lungs  send  out  from  their  surface  air 
sacs,  especially  well  seen  in  embryos  (fig.  638, 
,  3  1-5).  These  later  become  large,  thin-walled 
spaces,  easily  torn  away  in  dissection,  leaving 
*  large  openings  on  the  surface  of  the  lungs 
(fig.  639,  1-5).  Usually  five  pairs  of  these 
air  sacs  are  present,  largely  in  the  coelom, 
(Aftger8alei°enkaa.)chictr;  but  extending  in  between  the  muscles  (breast 
trachea;  1-5, lung  sacs!  and  axii]ary  region),  and  also  into  the  bones. 

The  spongy  lungs  lie  on  either  side  of  the  vertebral  column  and  are 


IV.    VERTEBRATA:    AVES. 


609 


united  to  the  ribs.  On  entrance  to  the  lung  the  bronchus  (fig.  639,  br)  loses 
its  cartilage  supports  and  enlarges  into  a 
vestibule  (0)  and  extends  thence  as  a 
mesobronchus  (6m)  backwards,  termi- 
nating in  the  abdominal  air  sac  (.5).  A 
side  branch  supplies  the  hinder  sub- 
costal sac  (4).  Secondary  bronchi  arise 
from  the  vestibule  and  mesobronchus; 
of  these  there  are  three  to  five  euto- 
bronchi  (I-IV)  supplying  the  remaining 
air-sacs  and  six  or  more  ectobronchi. 
Arising  from  the  mesobronchi  and 
secondary  bronchi  are  tertiary  bronchi, 
or  air  pipes,  running  parallel  to  each 
other  and  anastomosing  frequently. 
Each  air  pipe  has  a  thick  spongy  wall 
(tig.  640)  composed  of  numerous  thin- 
walled  sacs,  the  lung  vesicles,  closely 
enveloped  by  capillaries,  and  connected 
with  the  central  air-conducting  tube, 
the  lumen  of  the  pipe. 

Inspiration  is  .effected  by  raising  the  framework  of  the  chest,  this 
causing  a  straightening  of  the  hinged  ribs  and  an  increase  of  the  sterno- 
vertebral  diameter  ;  expiration  by  the  reverse  motion.  By  this  the  lungs, 
attached  to  the  ribs,  are  alternately  enlarged  and  contracted  in  spite  of 
their  slight  elasticity.  This  is  also  true  of  the  lung  sacs,  which,  on  account 
of  their  poor  blood  supply,  are  not  respiratory  but  serve  as  accessory  air 


FIG.  639.— Right  lung  of  hen,  some- 
what diagrammatic.  A  window- 
shows  a  mesobronchus  with  its- 
branches,  a,  artery  ;  brn,  meso- 
bronchus, arising  from  the  vesti- 
bule :  br,  bronchus  swelling  to 
vestibule ;  eb,  ectobronchus  :  I, 
lung  pipes  ;  7-7F,  mesobronchi  -t 
1-5,  ducts  of  lung  sacs. 


FIG.  640.— Section  of  lung  pipe.    (After  Schulze.) 

pumps.  It  is  probable  that  in  flight  this  air-pump  action  occurs  espe- 
cially with  the  subpectoral  and  axillary  air  sacs,  drawing  air  through  the 
lungs  and  rendering  other  respiratory  motions  superfluous,  thus  enabling 
the  thorax  to  remain  quiet,  an  important  matter.  If  the  trachea  be 
closed  and  the  air  canal  in  the  humerus  opened,  the  bird  can  breathe 
through  the  latter. 


610 


CHORD  ATA. 


The  circulation  in  the  birds  has  arisen  from  that  of  the  reptiles 
by  complete  separation  of  systemic  and  pulmonary  systems.  Of 
the  three  great  arterial  trunks  present  there  (fig.  621),  the  pul- 
monary artery  and  the  right  aortic  arch,  arising  from  the  left  ven- 
tricle, are  retained,  the  left  venous  arch  being  lost.  The  septum 
between  the  ventricles  is  complete.  The  striking  features  of  the 
alimentary  canal  (fig.  60)  are  the  crop  (not  always  present),  a 
glandular  stomach  or  proventriculus  (c),  and  a  muscular  chewing 
stomach  or  gizzard  (d),  as  well  as  two  long,  rarely  rudimentary, 
caeca  {/c}  at  the  junction  of  small  and  large  intestine.  Liver  and 
gall  bladder  (e,  /),  pancreas  (g),  and  spleen  are  present.  A  blind 
sac  (the  bursa  Fabricii),  the  paired  ureters  (m),  and  the  sexual 
ducts  (n)  open  into  the  cloaca.  The  latter  show  the  peculiarity 
that  the  right  oviduct  and  ovary  are  degenerate,  while  those  of  the 
left  side  are  correspondingly  larger.  Since  copulation  occurs  the 
large  eggs  (the  'yolk')  are  fertilized  in  the  oviduct  (fig.  99).  As 
they  pass  slowly  through  the  duct,  they  become  enveloped  first 
with  a  thick  layer  of  albumen,  ' white'  (w),  then  with  a  double 
egg  membrane  (ism,  sw,)  the  two  parts  being  separate  and  enclos- 
ing an  air  chamber  at  the  larger  end  of  the  egg.  Lastly  comes  the 
shell.  All  of  these  accessory  structures  are  secreted  by  the  gland- 
ular walls  of  the  enlarged  oviducts.  During  the  passage  down  the 
oviduct  the  first  phenomena  of  development  (segmentation,  gastru- 
latiou)  occur,  and  after  oviposition  the  development  stops  and  again 
starts  when  the  necessary  warmth  is  supplied. 

The  care  for  the  young,  the  sexual  life  connected  with  copula- 
tion, and  the  complicated  conditions  of  ex- 
istence connected  with  flight  have  resulted  in 
an  intelligence  far  superior  to  that  of  the 
reptiles,  which  finds  its  expression  in  the  bet- 
ter development  of  sense  organs  and  brain. 
In  the  brain  (fig.  641)  the  cerebellum,  which 
is  the  central  organ  for  the  coordination  of  the 
action  of  parts,  is  strikingly  developed.  Cor- 
respondingly large  are  the  cerebral  hemi- 
Fio.  64i.-Brain  of  pig-  spheres,  the  frontal  lobes  of  which  begin  to 
;  cover  the  olfactory  lobes,  the  temporal  lobes 
in  like  manner  extending  back  over  the  'twixt 
c  brain  and  °Ptic  lobes-  Corresponding  to  the 
vocal  aPParatl1  s >  tne  ear  is  highly  organized, 
z,  pineaiis.  the  lagena  of  the  labyrinth  being  greatly  en- 
larged and  the  sound-conducting  apparatus  (columella,  tympanum, 


2  V.    VERTEBRA  TA  :   A  VES.  611 

etc.)  well  developed.  The  beginnings  of  an  external  ear  are  seen  in 
the  deeper  position  of  the  drum  membrane.  Since  the  power  of 
flight  necessitates  vision  at  great  distances,  most  birds  have  exceed- 
ingly sharp  sight,  and  the  eye  itself  (fig.  642)  is  in  general  con- 
re 


Op 

FIG.  642.— Eye  of  owl.  (From  Wiedersheim.)  Oi,  choroid;  CM,  ciliary  muscle;  Co, 
cornea;  Cv,  vitreous  body ;  Ir,  iris :  L,  lens;  Op,  optic  nerve;  OS,  sheath  of  nerve ; 
P,  pec  ten;  Rt,  retina;  Sc,  sclera;  VK,  anterior  chamber;  t,  sclerotic  bones. 

structed  for  distance.  Peculiarities  of  the  bird's  eye,  already 
weakly  developed  in  the  reptiles,  are  the  pecten  (P),  a  comb- 
shaped  growth  of  the  choroid  into  the  vitreous  body,  and  the 
scleral  ring,  a  circle  of  bones  developed  in  the  sclera  and  support- 
ing the  outer  part  of  the  eye. 

Among  birds  there  is  spirited  rivalry  for  the  females,  especially 
among  polygamous  species.  At  the  time  of  mating  the  males  seek  to  win 
the  favor  of  the  females  either  through  striking  motions  (dances),  by 
singing,  or  by  beauty  of  plumage.  All  of  these  peculiarities  are  confined 
to  the  male  and  frequently  lead  to  a  marked  sexual  dimorphism.  The  dis- 
tinction in  plumage  is  commonly  strengthened  at  this  time,  the  male 
receiving  the  brilliant  wedding  dress.  Thus  we  speak  of  the  spring  molt, 
although  there  is  only  a  color  change  and  only  exceptionally  a  renewal  of 
the  feathers.  The  return  to  every-day  clothes  only  occurs  with  a  molt,  and 
this  comes  at  the  close  of  the  reproductive  season. 

The  reason  for  the  dull  plumage  of  the  female  is  due  to  the  fact  that 
she  usually  sets  on  the  nest,  at  which  time  inconspicuous  colors  protect  her 
from  destruction  by  enemies.  In  only  a  few  instances  is  the  heat  neces- 
sary for  incubation  produced  by  other  causes,  such  as  the  heat  of  the  sun 
upon  the  sand  in  which  the  eggs  are  buried,  or  the  increase  of  temperature 
caused  by  fermentation  in  decaying  vegetation  (Megapodes).  The  rule  is 


612  CHORD  AT  A. 

that  both  sexes  build  the  nest,  which  with  the  weaver  birds  is  most  skil- 
fully constructed;  occasionally  among  social  species  the  nests  are  placed 
under  a  common  roof.  When  the  clutch  of  eggs  is  complete  the  female 
(rarely  the  male)  begins  the  incubation,  at  this  time  in  some  instances 
losing  the  feathers  from  certain  regions  the  better  to  warm  the  eggs. 
Many  birds,  like  hens  and  ducks,  are  so  far  advanced  when  they  leave  the 
nest  that  they  can  follow  the  mother  and  feed  themselves.  Such  birds 
are  called  Prsecoces — in  contrast  to  the  Altrices,  which  hatch  with  incomplete 
coat  of  feathers  and  therefore  need  the  warmth  of  the  nest  and  the  pro- 
tection and  care  of  the  parents. 

The  migrations  of  birds  possess  great  interest.  We  distinguish  among 
birds  permanent  residents  and  others  which,  in  order  to  obtain  food,  take 
long  journeys,  the  migratory  species.  At  the  approach  of  cold  weather 
these  seek  the  south,  following  regular  paths  in  their  travels.  They  can- 
not, like  reptiles  and  amphibians,  hibernate  at  the  period  when  insects 
and  fruit  are  scarce,  because  their  greater  intelligence  and  their  more  ener- 
getic vital  processes  demand  a  more  rapid  metabolism  and  a  continuous 
food  supply.  Hence  the  birds,  like  the  mammals,  in  contrast  to  the 
'cold-blooded  '  reptiles,  amphibia,  and  fishes,  maintain,  under  all  extremes 
of  external  temperature,  a  body  heat  of  38-40°  (44°  ?)  C.  (100-104°  F.). 

The  classification  of  birds  is  in  a  state  of  change.  The  older  system 
based  upon  adaptive  characters  is  not  in  harmony  with  the  results  of  care- 
ful anatomical  study,  which  would  divide  the  whole  class  into  many  small 
groups.  For  this  reason  it  has  been  thought  best  to  retain  the  older  sys- 
tem of  larger,  easily  recognized  divisions,  and  to  call  attention,  where 
necessary,  to  the  contradictions  with  later  results. 

Order  I.  Saururae. 

The  view  that  birds  are  closely  related  to  reptiles  has  received 
considerable  support  by  the  discovery  of  fossil  birds  with  teeth. 
The  most  reptilian  of  these  occur  in  the  Jurassic  of  Bavaria,  and 
only  two  specimens  have  been  found.  In  these  (Arch&opteryx 
lithographica)  the  carpals  and  metacarpals  have  not  fused,  the 
three  fingers  are  well  developed  and  clawed,  and  the  caudal  verte- 
brae, although  bearing  feathers,  form  a  long  slender  tail  like  that 
of  a  lizard  (fig.  2). 

Order  II.  Odontornithes. 

These  forms,  from  the  cretaceous  of  Kansas  and  Colorado,  also 
had  teeth.  In  the  ODONTOEM^:  (Ichthyomis)  there  was  a  keeled 
sternum  and  normal  pygostyle.  In  the  ODONTOHOLC^E  (Hesper- 
ornis)  the  wings  were  reduced  (only  the  humerus  persisting),  the 
sternum  was  without  a  keel,  and  the  caudal  vertebrae  formed  a 
broad  paddle. 

Order  III.  Ratitae. 

Here  are  included  several  families,  very  different  in  structure, 
which  agree  in  having  the  feathers  not  arranged  in  feather  tracts;, 


IV.    VERTEBRATA:  AVE8,  CARTNAT^E.  613 

and  in  that,  together  with  the  lack  of  flight,  many  structures 
normally  connected  with  it  are  absent.  The  bones  are  but  slightly 
pneumatic,  the  sternum  has  no  keel,  and  a  furcula  is  not  formed, 
the  clavicles  being  rudimentary  (Dromceus]  or  not  present  as  dis- 
tinct bones.  The  wings  are  small  and  lack  primaries  and  seconda- 
ries adapted  for  flight,  for  typical  contour  feathers  with  close 
vanes,  as  well  as  typical  down  feathers,  are  absent. 

Since  several  structures  apparently  adapted  for  flight  occur  here 
(fusion  of  hand  bones  and  often  of  caudal  vertebrae ;  arrangement 
of  wing  muscles),  it  is  probable  that  the  Ratites  have  descended 
from  carinate  forms  by  loss  of  power  of  flight.  The  anatomical 
distinctions  between  the  various  families  lead  one  to  believe  that 
they  have  arisen  from  different  groups  of  carinates  and  hence  do 
not  form  a  natural  assemblage. 

Section  I.  STRTJTHIONES,  with  long  humerus,  long  legs  and  neck. 
STRUTHIONIDJE,  two-toed  ostriches  of  Africa,  Struthio  camelus.  RHEID^E, 
South  American  three-toed  ostriches,  Rliea  americana,  nandu.  Section 
II.  CASUARINA  ;  three  toes,  humerus  short.  Dromceus,  emus;  Casu- 
arius,  cassowaries.  Section  III.  APTERYGES,  bill  long,  nostrils  near  the 
tip,  rudimentary  wing  skeleton;  four  toes.  Apteryx,  kiwi,  of  New  Zealand. 
The  DINORNITHID^E,  three  toes,  wing  skeleton  absent ;  giant  birds  (thirteen 
feet  high)  of  New  Zealand;  now  extinct,  but  apparently  contemporaneous 
with  man.  The  JEpiornis,  a  gigantic  bird  of  Madagascar,  possibly  belonged 
near  these.  Skeletons  and  eggs  holding  two  gallons  found  in  alluvium. 

Order  IV.  Carinatae. 

The  name  refers  to  the  presence  of  the  keel  to  the  sternum, 
which  is  correlated  with  the  powers  of  flight  possessed  by  most 
species.  Other  characters  of  the  class  are  the  presence  of  rectrices 
and  remiges  on  tail  and  wings,  and  the  fusion  of  clavicles  to  a 
furcula.  There  are  strong  fliers,  like  the  raptores  and  albatrosses, 
which  have  but  a  small  carina ;  in  many  poor  fliers  the  carina  may 
be  entirely  absent.  The  furcula  is  not  always  present,  the  clavicles 
not  uniting  (many  parrots  and  toucans)  or  being  absent  (Mesites). 
The  remiges  are  also  degenerate  in  some  carinates,  as  in  the  pen- 
guins (which  are  flightless,  although  they  have  a  strong  carina), 
where  they  take  the  shape  of  small  scales.  Thus  the  distinctions 
between  ratite  and  carinate  birds  vanish  in  places. 

Sub  Order  I.  GALLINACEA.  The  hen-like  birds  are  praecoces  with 
compact  bodies  and  well-developed  wings  and  legs,  so  that  they  run  and 
fly  well  without  excelling  in  either  direction.  The  feet  have  three  toes  in 
front,  usually  connected  by  a  membrane  at  the  base  (fig.  643,  c);  the  fourth 
toe  is  behind  and  at  a  higher  level.  Above  this  in  the  male  is  usually  the 


614 


CHORD  ATA. 


spur,  a  process  of  the  tarso-metatarsus,  covered  with  horn.  The  margins 
of  the  upper  jaw  overlap  the  lower;  the  beak  is  bent  downward  at  the  tip 
and  is  about  as  long  as  the  head.  Naked,  richly  vascular  lobes  form  comb 
and  wattles  which  are  specially  large  in  the  more  elegantly  plumaged 
males. 

The  PHASIANID^E  are  polygamous;  Phasianus,  with  many  species  of 
pheasants;  Gallus  bankiva  of  the  Sunda  Islands,  the  ancestors  of  domestic 


FIG. 643.— Foot  forms.  (From  Schmarda.)  a, semi-palmate,  wading  of  Ciconia  ;  ^perch- 
ing of  Turdus ;  c,  rasorial  of  Pliasiu-uua ;  d,  raptorial  of  Falco ;  e,  adherent  of 
Cypselus  ;  /,  cursorial  of  strut hio  ;  0,  zygodactyl  (scansorial)  of  Pious  ;  h,  lobate  of 
Pnrticcps  ;  i,  lobate  and  scalloped  of  Fulica  ;  fc,  palmate  of  Anas  ;  I,  totipalmate  of 
Phaethon. 

fowl.  Meleagris*  the  turkeys.  The  TETRAONID^E  are  partly  polygamous, 
partly  monogamous.  Coturnix,*  quail ;  Perdix*  partridge  ;  Bonasa,* 
grouse.  The  incubation  of  the  Megapodes  has  been  referred  to  (p.  611). 

Sub  Order  II.  COLUMBINE.  The  pigeons  are  distinguished  from 
the  Gallinacese  by  the  more  slender  bodies,  shorter  legs,  the  toes  free,  and 
the  longer  wings  capable  of  prolonged  flight.  They  are  altrical ;  the  crop 
produces  a  milky  secretion  used  in  feeding  the  young.  The  COLUMBID^E 
are  the  most  widely  distributed  and  are  represented  in  the  tropics  by 
numerous  beautifully  colored  species.  Columba.*  According  to  Darwin 
the  domestic  pigeons  come  from  C.  Uvia,  the  blue  rock  pigeon  ;  Ectopistes 
migratorius*  passenger  pigeon,  practically  exterminated.  Allied  was  the 
dodo,  Didus  ineptus,  of  Madagascar,  exterminated  in  the  eighteenth 
century. 

Sub  Order  III.  NATATORES.  A  number  of  families,  while  differing 
much  in  structure,  are  united  by  their  inclination  for  an  aquatic  life. 
They  are  called  swimming  birds  (Natatores)  because,  thanks  to  their 


IV.    VERTEBRATA:  AYES,   CARINAT^E. 


(U5 


webbed  feet,  they  are  excellent  swimmers  and  divers.  Either  all  four  toes 
are  connected  by  the  web  (totipalmate,  fig.  643,  Z),  or  only  the  three  anterior 
toes  are  webbed  (palmate,  fig.  643,  k),  or  the  three  toes  are  each  bordered 
with  a  swimming  membrane  (lobate,  fig.  643,  h).  Thus  the  foot  struc- 
ture gives  distinctions  which  forbid  a  closer  association  of  the  families,  and 
this  is  strengthened  by  differences  of  wing  and  beak.  On  the  other  hand 
palatal  structures  show  that  here,  as  in  the  Grallatores,  very  diverse  forms- 
are  associated. 

Section  I.  LAMELLIROSTRES  (Anseriformes),  feet  palmate;  the  beak  soft- 
skinned  up  to  the  hard  tip,  its  margins  with  transverse  horny  plates. 
Anas  boschas*  wild  duck,  source  of  domestic  breeds.  A.  mollissima,  eider; 
Anser*  goose  (domestic  derived  from  A.  ferus).  Cygnus*  swans.  Sec- 
tion II.  TUBINARES  (Longipennes),  predaceous  birds  with  strong  beak, 
tubular  nostrils,  palmate  feet,  and  long  wings  capable  of  rapid  and  pro- 
longed flight.  Diomedea,  albatross;  Larus,*  gulls  ;  Sterna,*  terns.  Sec- 
tion III.  URINATORES.  Birds  with  small  wings,  sometimes  reduced  to- 
flippers,  and  upright  position  owing  to  position  of  the  legs  far  back.  The 
ALCID^E  (Alca  impennis*  the  great  auk,  exterminated  in  the  nineteenth 
century),  which  are  northern  and  are  related  to  the  gulls,  and  the  antarctic 
IMPENNES  (Aptenodytes  —  fig.  644, 
penguin)  agree  in  having  palmate 
feet,  but  otherwise  differ  greatly  in 
structure.  Some  of  the  COLYMBID.E 
(Urinator*  loons)  have  palmate 
feet,  others  (Colymbus,* grebes)  have 
lobate  feet.  Section  IV.  STEGANO- 
PODES,  with  totipalmate  feet.  Pele- 
canus*  pelicans;  Phalarocorax,* 
cormorants;  Phaethon,*  tropic  birds. 

Sub  Order  IV.  GRALLATORES. 
The  wading  birds  affect  swampy 
lands  and  the  shores  of  the  sea, 
ponds  and  streams,  their  legs  being 
lengthened,  chiefly  by  elongation  of 
the  tarso-metatarsus,  the  feet  semi- 
palmate  (fig.  643,  a),  and  the  feath- 
ers only  on  the  upper  parts,  the 
lower  with  horny  plates,  all  feat- 
ures adapted  to  the  wading  life.  FlG-  &&•— Aptenodyt.es  patagonica,  penguin. 
Correlated  is  the  striking  length  of  (From  Brehm-> 

neck  and  beak.  These  features  have  appeared  in  groups  which  are  very 
different  in  anatomical  characters. 

Section  I.  CICONIFORMES.  Beak  with  a  strong  horny  coat.  Ardea* 
herons;  Ibis ;  Ciconia,  storks  ;  Phcenicopterus,*  flamingo.  Section  II. 
GRUIFORMES.  Beak  always  with  soft  skin  at  the  base,  often  extending  to 
the  tip.  Grus,*  cranes;  Rallus*  rails;  Otis,  bustards,  terrestrial.  Section 
III.  CHARADRIFORMES.  Allied  to  the  auks  and  gulls.  Scolopax*  woodcock  ; 
Charadrius,*  plover. 


616  CHORDATA. 

Sub  Order  V.  SCANSORES.  The  climbing  birds  are  readily  recog- 
nized by  their  zygodactyle  feet  (fig.  643,  gn,  in  which  two  toes  (2  and  3) 
are  directed  forwards,  the  other  two  (1  and  4)  backwards.  The  forms 
united  under  this  head  differ  much  in  structure  and  their  association  does 
not  rest  on  blood-relationship. 

Section  I.  CUCULIFORMES.  The  PSITTACI,  or  parrots,  are  brightly  colored 
mostly  tropical  birds  with  short,  high,  compressed,  and  strongly  bent  beak 
.and  fleshy  tongue.  But  one  species  (Conurus  carolinensis*)  in  the  United 
States.  Cacatua,  Plictolophus,  cockatoos;  Mdopsittacus,  Psittacus,  parrots. 
CUCULI,  bill  slightly  arched  or  straight ;  outer  toe  usually  versatile  ; 
Cuculus,  Coccygus,*  cuckoos.  Section  II.  PICARLE.  The  woodpeckers 
have  a  long,  straight,  conical  beak  and  long,  protrusible  tongue;  Picus* 
Nearly  allied  are  the  toucans  (Rhamphastos)  of  the  tropics. 

Sub  Order  VI.  PASSERES.  This  is  by  far  the  richest  in  species  of  all 
the  groups  of  birds.  They  are  altrices  of  moderate  size,  with  slender  feath- 
ered tarsi  and  strong,  horny  beak  without  cere.  Of  the  three  anterior 
toes  the  two  outer  are  either  united  or  separated  to  the  base  (fig.  643,  6), 
while  the  hind  toe  is  at  a  level  with  the  rest.  In  some,  which  are  usually 
"but  not  invariably  noticeable  for  the  powers  of  song  of  the  males,  there 
are  special  muscles  to  the  syrinx  which  are  lacking  in  other  birds.  These 
are  called  Oscines,  in  contrast  to  the  other  Passeres,  the  crying  birds,  or 
€lamatores.  These  groups  are  further  distinguished  by  a  large,  freely 
movable  hind  toe  in  the  Oscines,  while  in  the  Clamatores  it  is  restricted 
in  its  motions. 

Section  I.  OSCINES.  All  our  song  birds  belong  here:  FRINGILLID.E, 
finches;  Passer  domesticus*  English  sparrow;  Loxia*  crossbills ';  ICTER- 
ID.E  ;  Icterus,*  orioles ;  Dolichonyx*  bobolink;  ALAUDID.E,  Alatida*  sky- 
lark ;  SYLVICOLID^:,  Dendrceca,*  Helminthophaga*  warblers;  TURDED^B, 
Turdus*  thrushes;  Siala*  bluebirds;  HIRUNDINID.E,  Hirundo*  swallows; 
TROGLODYTID^S,  wrens;  CORVID^E,  Corvus*  crows;  Cyanocitta*  jays. 
The  PARADISEID^E,  or  birds  of  paradise,  with  marked  sexual  dimorphism,  are 
closely  related  to  the  crows  (fig.  15).  Section  II.  CLAMATORES.  Here  are 
frequently  included  a  few  groups  (COTINGID^E,  TYRANNID^E)  best  developed 
in  South  America  and  the  lyre  birds  (MENURID.E)  of  Australia.  Earlier 
other  forms  were  regarded  as  allied,  but  now  are  separated  as  Cypselo- 
morphre,  or  Coraciformes,  and  united  with  the  owls  and  Picarise.  CYPSELE- 
D^E  ;  Chcetura*  chimney  'swallow,'  with  adherent  feet  (fig.  643,  c). 
TROCHILID.E,  humming  birds,  best  developed  in  tropical  America;  Trochi- 
lus*  CAPRIMULGID.E,  night  hawks ;  Antrostomns  vociferus*  whippoor- 
will.  ALCEDINID.E,  kingfishers,  Ceryle*  BUCERONTID^E,  horn  bills,  tropical. 

Sub  Order  VII.  RAPTORES.  The  birds  of  prey  are  strong  birds  of 
considerable  size.  They  have  the  tarso-metatarsus  feathered  and  four 
strongly  clawed  toes  of  what  is  termed  the  raptatorial  type  (fig.  643,  d). 
The  beak  is  strong,  the  upper  half,  strongly  hooked  at  the  tip,  extending 
over  the  lower.  There  are  two  groups  recognized  which  probably  are 
not  closely  related. 

Section. I.  FALCONIFORMES.  Slender  birds  with  close  plumage  and 
extraordinary  sight;  related  structurally  to  the  herons.  CATHARTIDJE, 


IV.    VEETEBRATA:  MAMMALIA. 


617 


buzzards  ;  Cathartes  aura*  turkey  buzzard.  PANDIONID.E,  Pandion 
halmtus*  fish  hawk;  FALCONID^E  :  Aquila*  Halicetus,*  eagles  ;  Buteo* 
buzzards;  Falco*  falcons;  Accipiter*  hawks.  Section  II.  STRIGES, 
owls;  compact  birds  with  loose,  fluffy  plumage,  large  eyes  in  a  circle  of 
feathers;  more  closely  related  structurally  to  the  Caprimulgidae  than  to 
the  Falconiformes.  Bubo,*  horned  owls;  Scops,*  screech  owls;  Strix* 
gray  and  brown  owls  ;  Speotyto*  burrowing  owls. 

Class  III.  Mammalia. 

The  mammals  occupy  the  highest  place  among  the  vertebrates, 
and  consequently  in  the  animal  kingdom;  they  also  possess  a 
special  interest  for  us,  for  man,  in  structure  and  development, 
belongs  to  the  group,  although  separated  in  intelligence  from  the 
most  highly  organized  of  the  members  by  a  wide  gap. 

The  most  striking  characteristics  of  the  mammals  again  are 
furnished  by  the  skin.  In  fact  one  may,  with  Oken,  call  them 
hair-animals,  since  hair  is  as  diagnostic  as  feathers  are  for  birds. 
The  hairs  (fig.  645,  H)  are  cuticular  structures  which  are  seated 


FIG.  645.— Section  of  skin  of  man.  (From  Wiedersheim.)  Co,  derma  (corium);  D, 
oil  gland;  F,  fat;  (?,  blood-vessels;  GP,  vascular  papilla;  If,  hair;  JV,  nerves;  J!VP, 
nerve  papilla;  Sc,  stratum  corneum ;  £D,  SD\  sweat  gland  and  duct;  £M, 
stratum  Malpighii. 

on  papillae  of  the  derma,  and  are  nourished  by  blood-vessels  in 
these.  The  lower  end,  the  root  of  the  hair,  lies  in  a  pit  in  the 
epidermis,  the  hair  follicle,  and  is  surrounded  by  a  double  envelope, 
the  epithelial  root  sheath,  formed  by  an  inpushing  of  the  epidermis 
and  an  outer  connective-tissue  follicular  sheath.  Small  muscles 
attached  to  the  base  of  the  larger  hairs  serve  for  their  erection. 


618  CHORDATA. 

Since  side  branches  are  lacking,  the  structure  of  the  hair  is  more 
simple  than  that  of  feathers,  and  the  forms  fewer.  Wool  is  char- 
acterized by  its  spiral  turns;  then  there  is  straight  hair  which,  by 
increase  in  size,  forms  the  *  whiskers '  (vibrissse)  on  the  upper  lip 
of  many  mammals,  bristles  (swine),  and  lastly  the  spines  of  hedge- 
hogs and  porcupines.  In  the  pelts  of  many  animals  two  kinds  of 
hair  may  occur,  wool  below  and  straight  hair  outside.  Histolog- 
ically  hair  consists  of  cornified  cells,  often  arranged  in  medullary 
and  cortical  layers.  On  the  outside  there  may  be  another  layer 
recalling  the  pseudocuticula  of  reptiles.  In  most  mammals  there 
is  a  periodic  shedding  and  renewal  of  the  hair,  the  new  hair  aris- 
ing from  the  old  follicle  (?  from  the  old  papilla).  Ordinarily  this 
occurs  only  in  spring.  Besides  hair  some  mammals  have  true 
scales.  Constant  horny  structures  are  the  armatures  of  the  tips  of 
the  digits,  which,  according  to  form,  are  divided  into  claws  (ungues), 
hoofs  (ungulae),  and  nails  (lamnae). 

The  old  view  that  the  hair,  like  feathers,  corresponds  to  the  scales  of 
reptiles  has  recently  found  both  defenders  and  opponents,  the  latter  think- 
ing it  probable  that  the  hair  has  arisen  from  the  nerve-end  structures  of 
aquatic  vertebrates.  The  claws,  together  with  those  of  reptiles  and  birds, 
must  have  come  from  horny  scales,  which  indeed  occur  in  many  amphibia 
as  hollow  cones  capping  the  toes.  The  dorsal  part  of  this  scale,  the  claw 
plate,  becomes  especially  strong,  its  formation  taking  place  at  the  base, 
the  root,  from  whence  it  is  forced  forward  over  the  bed  (in  man  the  limit 
of  nail  formation  is  shown  by  the  lunule).  The  ventral  part  of  the  scale, 
the  subungua  or  solenhorn,  is  poorly  developed  in  true  claws  because  its 
region  is  restricted  by  the  arching  of  the  claw  plate  in  both  directions,  but 
is  more  evident  in  hoofs,  in  which  the  plate  is  curved  only  horizontally. 
In  the  horse  it  forms  the  *  sole,'  lying  between  the  frog  and  the  hoof.  It  is 
rudimentary  or  entirely  lost  in  the  nails  of  apes  and  man. 

The  skin  of  mammals  is  further  characterized  by  its  richness 
in  glands,  of  which,  with  few  exceptions,  there  are  two  kinds, 
sebaceous  and  sweat  glands.  The  first  are  acinose  glands,  and  usu- 
ally open  in  the  hair  follicles,  giving  the  hair  the  required  oiliness 
(fig.  645,  D}.  The  sweat  glands,  except  in  the  monotremes,  are 
entirely  independent  of  the  hairs,  and  are  simple  tubes,  coiled 
at  their  deeper  ends  (SD),  secreting  a  fluid  sweat  which  is  of  great 
value  in  the  preservation  of  a  constant  temperature,  its  evapora- 
tion cooling  the  body.  Under  the  influence  of  sexuality  the  glands 
in  certain  regions,  and  especially  the  sebaceous  glands,  develop 
great  activity  and  form  considerable  glandular  pouches  or  pockets : 
caudal  and  anal  glands  of  many  carnivores,  hoof  glands  and  sub- 
orbital  glands  of  ruminants,  musk  and  castor  glands  of  musk  deer 


IV.    VERTEBRATA:  MAMMALIA.  619 

and  beaver  (fig.  652,  a).  More  important  than  these  are  the  modi- 
fications of  dermal  glands  into  mammary  or  in  ilk  glands,  which, 
indeed  are  so  characteristic  that  they  have  given  rise  to  the  name 
mammalia.  These  are  almost  invariably  sebaceous  glands  (in  the 
monotremes  sweat  glands)  which  empty  in  great  numbers  upon  a 
restricted  area  of  the  skin,  which,  except  in  monotremes,  is  elevated 
into  true  nipple  (fig.  646,  A),  or  around  which  the  adjacent  skin 


B 
FIG.  646.—^!,  true,  B,  false  nipple.    (After  Gegenbaur.) 

becomes  elevated  in  tubular  form  (B)  as  in  the  cows.  The  mam- 
mae are  always  symmetrically  arranged  upon  the  ventral  surface, 
sometimes  in  the  breast  region,  but  more  frequently  in  the  inguinal 
region.  There  are  at  least  two,  usually  more  (22  in  Centetes).  In 
general  the  number  corresponds  to  the  maximal  number  of  young 
at  a  birth. 

A  dermal  skeleton  occurs  in  few  species  (e.g. ,  the  firm  bony 
plates  of  the  armadillos) ;  on  the  other  hand  the  axial  skeleton 
shows  many  features  not  occurring  elsewhere.  In  the  skull  many 
ol  the  bones  already  referred  to  are  evident  only  as  centres  of  ossi- 
fication, fusing  early  with  their  neighbors  to  form  larger  bones. 
As  the  temporal  bone  shows,  parts  of  diverse  origin  may  fuse — parts 
of  the  visceral  skeleton  and  parts  of  the  cranium;  membrane  and 
cartilage  bones — so  that  a  sharp  line  between  cranial  and  facial 
portions  cannot  be  drawn.  So  it  becomes  necessary  in  describing 
the  skull  not  to  follow  exactly  the  model  adopted  so  far,  but  to 
take  that  of  human  anatomy. 

In  the  hinder  region  of  the  skull  is  a  large  occipital  bone  (figs. 
561,  562),  jointed  to  the  atlas  by  double  occipital  condyles,  and 
arising  by  the  fusion  of  the  four  bones  of  the  occipital  region. 
Besides  it  includes  usually  a  membrane  bone,  the  interparietal, 
which  occurs  only  in  mammals.  This  is,  strictly  speaking,  a 
paired  bone,  arising  in  the  angle  between  the  parietal  and  the 
supraoccipital  and  fusing  with  the  latter.  In  front  of  it  lie  in  the 
rool  oi  the  cranium,  as  in  other  vertebrates,  the  parietals  (fused 
with  the  interparietals  in  many  ruminants  and  rodents),  the 
fiontals  and  nasals,  the  lachrymals  being  always  associated  with 


620  CHORDATA. 

them.  In  the  floor  of  the  cranium  the  sphenoid  bone  lies  in  front 
of  the  basioccipital  portion  of  the  occipital.  In  many  mammals 
this  consists  of  an  anterior  and  a  posterior  portion  throughout  life  ; 
in  man  this  condition  occurs  at  least  in  the  embryo.  Each  of 
these  parts  in  development  consists  of  three  elements,  the  posterior 
of  the  basisphenoid  as  the  body,  and  the  paired  al  [sphenoids  (great 
wings);  the  anterior  is  similarly  composed  of  the  presphenoid  and 
the  paired  orbitosphenoids  (lesser  wings)  (fig.  562,  Spb,  Ps,  Ah, 
Ors).  In  front  of  the  sphenoid  lies  the  ethmoid,  Eth,  likewise 
formed  from  three  parts,  the  mesethmoid,  which  forms  a  partition 
between  the  two  nasal  cavities,  and  the  paired  ectethmoids,  which 
form  the  lateral  walls  of  the  nasal  cavities.  These  last  have  com- 


-fia. 
os. 


Fio.  647.— Skull  of  embryo  Tatusia.  (After  Parker,  from  Wiedersheim.)  Cartilage 
dotted,  membrane  and  membrane  bones  lined,  o,  incus  (quadrate);  de,  dentary; 
/r,  frontal;  h,  (above)  membrane  over  anterior  fontanelle,  (below)  hyoid  bones; 


iw,  premaxillary:  ju,  jugal  (malar) ;  kb,  remnants  of  gill  arch;  to,  lachrymal; 
m/f,  Meckel's  cartilage;  mx,  maxillary;  n,  malleus  (articulare);  rw,  nasal:  <>,  oc- 
cipital cartilage;  os,  supraoccipital;  pa,  parietal ;  pe,  petrosal ;  sg,  squamosal ;  .sf, 
stapes;  tj/,  tympanic. 

plicated  folds  on  their  inner  surface,  the  superior  and  middle 
turbinated  bones,  which  support  the  olfactory  membrane,  thus 
greatly  increasing  its  surface.  With  these  is  associated  the  os  tur- 
binale,  a  distinct  bone,  the  inferior  turbinated  bone  of  human 
anatomy. 

The  temporal  bone,  which  is  intercalated  between  the  roof  and 
floor  of  the  skull,  can  only  be  understood  by  its  embryonic  rela- 
tions and  its  connexion  with  the  first  and  second  visceral  arches 
(fig.  647).  Its  centre  is  formed  by  the  petrosal  (pe),  developed 
in  the  walls  of  the  otic  capsule,  to  which,  as  elsewhere  in  the 
vertebrates,  are  attached:  (1)  the  cartilaginous  jaw  arches,  the 
quadrate  (a),  and  the  mandibular  (n  and  mlc} ;  (2)  the  cartilaginous 


IV.    VERTEBRATA:  MAMMALIA.  621 

hyoid  arch,  the  stapes  (in  part  equalling  the  hyomandibular,  st), 
and  the  hyoid  proper  (h)  (compare  with  the  visceral  skeleton  of 
the  selachian,  fig.  588).  To  these  are  added  the  membrane  bones, 
the  squamosal  (sq),  at  the  base  of  the  quadrate,  which  increases  as 
the  latter  loses  in  size,  and  below  the  squamosal  the  tympanic  (ty). 
With  ossification  of  the  cartilaginous  parts  several  centres  form 
the  petrosum,  which  fuses  with  the  squamosal,  and  frequently 
with  the  tympanic,  which  in  some  forms  enlarges  to  a  conspicuous 
bulla  ossea.  Petrosum  and  squamosal  on  the  one  side,  tympanic 
on  the  other,  enclose  a  space,  the  tympanic  cavity,  into  which  the 
upper  parts  of  both  visceral  arches  extend,  ossifying  into  the  ear 
bones,  the  quadrate  to  the  incus,  the  hyomandibular  possibly  to 
stapes  (fig.  577). 

The  tympanic  in  uniting  with  the  squamosal  (forming  Glaser's 
fissure)  encroaches  on  the  mandibular  cartilage  so  that  the  upper 
end  (n),  which  is  homologous  with  the  articulare  of  other  ver- 
tebrates, is  enclosed  in  the  tympanic  cavity  and,  along  with  a  sec- 
ond bone,  the  angulare,  ossifies  to  form  the  malleus,  while  the 
lower  portion,  Meckel's  cartilage  proper  (mk),  becomes  pinched 
off.  Meckel's  cartilage  gradually  disappears;  on  the  other  hand 
the  surrounding  membrane  bone,  the  dentary  (de)  increases  and 
alone  forms  the  lower  jaw,  which  now  forms  a  new  articulation 
with  the  squamosal.  It  will  be  noticed  that  the  old  articulation 
was  between  cartilage  bones,  the  new  between  membrane  bones 
developing  around  the  cartilages.  (There  is,  however,  some  evi- 
dence to  show  that  the  mammalian  lower  jaw  consists  of  several 
bones,  some  of  them  preformed  in  cartilage,  and  that  one  of  these 
forms  the  articulation  with  the  squamosal.) 

The  lower  part  of  the  hyoid  arch,  the  hyoid,  remains  outside  the 
ear  and  often  fuses  with  the  petrosal.  The  upper  end  (styloid 
process)  may  then  become  entirely  separate  from  the  lower,  which 
becomes  attached  to  the  copula  (body  of  hyoid)  as  the  anterior 
horn,  the  connecting  cartilage  being  reduced  to  a  stylohyoid  liga- 
ment. In  the  hyoid  of  mammals  there  is  also  included  a  remnant 
of  a  gill  arch  as  the  posterior  horn. 

As  the  quadrate,  by  its  modification  into  the  incus,  becomes 
strikingly  reduced,  the  rest  of  the  arch — vomer,  palatine,  and 
pterygoid — is  poorly  developed  in  contrast  to  the  large  maxillary 
bones.  Premaxillaries  and  maxillaries  (fused  in  man  to  a  single 
bone)  form  an  important  element  in  the  face,  and  send  backwards 
and  inwards  palatine  processes  into  the  roof  of  the  mouth.  These 
encroach  upon  the  bones  of  the  palatal  series;  the  vomers  of  the 


622  CHORDATA. 

two  sides  are  pressed  together  to  a  single  bone  lying  vertically 
entirely  within  the  nasal  partition;  the  palatine  and  pterygoid  are 
forced  backwards.  The  palatines  contribute  to  the  hard  palate, 
the  pterygoids  only  exceptionally  (Cetacea,  many  edentates);  the 
latter  usually  lose  their  independence  and  fuse  with  the  nearest 
bone  of  the  base  of  the  cranium,  the  basisphenoid  (more  accurately 
with  a  process  of  the  basisphenoid,  the  lamina  externa  of  the 
pterygoid  process,  the  pterygoid  forming  the  lamina  interna). 
Thus  the  hinder  sphenoid,  like  the  temporal,  contains  cranial  and 
visceral  elements. 

In  the  vertebral  column  the  cervical  and  the  rib-bearing 
thoracic  vertebrae  are  always  distinct,  and  the  same,  with  the  ex- 
ception of  the  Cetacea  and  Sirenia,  is  true  of  lumbar,  sacral,  and 
caudal  vertebrae.  Of  sacrals  there  is  one  in  all  embryos,  and 
throughout  life  in  the  marsupials,  elsewhere  from  two  to  five, 
rarely,  as  in  edentates,  as  many  as  thirteen.  The  number  of  ver- 
tebrae in  each  group  is  rather  restricted.  Thus,  except  in  Brady- 
pus  tridactylus  (9),  Cholcepus  hoffmanni  and  Manatus  (6),  the 
number  of  cervicals  is  always  seven. 

Of  the  appendicular  skeleton  the  girdles  are  most  interesting. 

The  coracoid,  which  in  mono- 
tremes  reaches  the  sternum,  is 
reduced  in  all  other  mammals  to 
a  small  coracoid  process  of  the 
scapula.  More  rarely  the  clavicle 
is  lacking  (rapid  runners);  in 
the  monotremes  it  extends  to  the 
episternum  (fig.  648,  Cl,  Ep)\ 
elsewhere  it  appears  to  articulate 
with  the  sternum,  in  reality  by 
the  intervention  of  interarticular 
cartilages  (once  regarded  as  a 

FIG.  648.— Sternum  and  shoulder  girdle 

of  Ornithorhynchus  paradoxm.   (From  rudimentary      episternum,        now 

Wiedersheim.)    Cl,  clavicle;   Co,   Co',  i      •    N        T 

coracoid;  Ep,  episternum;  G,  glenoid  Called    preclaviae).       In  the    pelvis 

fossa  for   humerus;    S,    scapula;    St,  ,,   ,-,              ,                               ,         *    . 

manubrium  sterni  (anterior  element  all  three   elements   are   lUSed   to  a 

of  sternum).  •       i           •               •                          -i  • 

single  os  mnommatum;  pubis  and 

ischium  unite  ventrally  with  each  other,  enclosing  between  them 
the  obturator  foramen  (fig.  655).  The  pubes  of  the  two  sides 
unite  by  a  symphysis  which  can  extend  back  to  the  ischia. 

Since  the  mammals  in  general  are  distinguished  from  other 
vertebrates  by  their  intelligence,  the  brain  is  characterized  by  the 
size  of  cerebrum  and  cerebellum  (fig.  649).  In  contrast  to  birds 


IV.    VERTEBRATA:  MAMMALIA. 


623 


and  fishes,  the  cerebellum  (IV)  is  differentiated  into  a  median 
verrnis  and  lateral  cerebellar  hemispheres.  In  the  cerebrum  the 
mantle  comes  first  into  consideration.  Its  frontal  lobes  grow  for- 
wards over  the  olfactory  lobes,  which  consequently  lie  farther  and 
farther  back  on  the  lower  surface.  The  temporal  lobes  extend 
right  and  left  over  the  optic  lobes  and  down  to  the  floor  of  the 
cranium ;  the  occipital  lobes  cover  successively  the  mid  brain,  cere- 
bellum, and  medulla  oblongata.  Since  the  greatest  increase 
of  intelligence  lies  within  the  mammals,  the  cerebra  may  be 
arranged  in  an  ascending  series.  In  the  monotremes,  marsupials, 
insectivora,  and  rodents  (fig.  649,  A)  the  olfactory  lobes  are 

A  B  C 

-to 
x.    Y-^a 


FIG.  649.—  A,  brain  of  rabbit  (after  Gegenbaur);  J3,  of  fish  otter.  <7,  of  pavian 
monkey  (after Leuret  and  Gratiolet).  I,  cerebrum;  III,  optic  lobes;  IT7,  cerebel- 
lum; V,  medulla  oblongata;  to,  olfactory  lobes. 

visible  in  front,  usually  the  mid  brain  behind  (/// ).  In  the  lemurs, 
carnivores  (fig.  649,  B),  and  ungulates  the  olfactory  lobes  are 
completely,  the  cerebellum  partly,  covered.  In  man  and  the 
anthropoid  apes,  on  removing  the  roof  of  the  skull,  only  the  two 
cerebral  hemispheres  are  visible,  all  other  parts  being  more  or  less 
completely  covered. 

Further,  it  is  to  be  noted  that  in  the  first  group  the  surface  of 
the  cerebrum  is  smooth,  while  in  the  others  the  cortex  is  increased 
by  infolding  and  the  formation  of  convolutions  (gyri  and  sulci) 
which  reach  their  greatest  complication  in  the  anthropoid  apes 
and  especially  in  man.  A  consequence  of  the  increase  in  size  of 
the  brain  is  the  great  development  of  the  connecting  nerve  tracts, 
which  become  more  and  more  prominent  as  parts  of  the  brain. 
Thus  the  two  halves  of  the  cerebrum  are  connected  by  a  large 
transverse  tract,  the  corpus  callosum;  two  solid  cords,  the  crura 
cerebri,  run  back  from  the  cerebrum  to  the  other  parts,  while  a 
transverse  commissure,  the  pons  Varolii,  passes  below,  connecting 


624  CHORDATA. 

the  two  sides  of  the  cerebellum.  These  connexions  in  the  other 
vertebrates  are  small,  and  even  in  the  lower  mammals,  like  mono- 
tremes  and  marsupials,  are  but  slightly  developed. 

The  increase  of  cerebrum  and  cerebellum,  which  occurs  chiefly  in  the 
dorsal  portion,  has  resulted  in  flexures  in  the  axis  of  the  brain,  already  in- 
dicated in  the  reptiles,  increased  in  the  birds,  and  reaching  their  maximum 
in  the  mammals.  Instead  of  continuing  in  the  course  of  the  spinal  cord, 
the  axis  of  the  brain  bends  ventrally  in  the  medullar  region  (cervical 
flexure),  then  in  the  region  of  the  pons  again  dorsally  (pontal  flexure),  an 
at  the  level  of  the  optic  lobes  again  ventrally  (cephalic  flexure).  By  its 
increase  in  size  the  brain  has  influenced  the  skull  in  an  interesting  way  ; 
for,  while  even  in  birds  the  brain  is  almost  entirely  confined  to  the  region 
behind  the  eyes,  in  the  higher  mammals  it  has  extended  forward  to  the 
olfactory  region.  Thus  there  comes  an  increase  of  the  cranium  at  the  ex- 
pense of  the  face.  The  relative  sizes  of  the  two  were  adopted  by  Camper 
as  an  index  of  intelligence,  and  were  measured  by  l  Camper's  angle,'  a 
method  which  has  since  undergone  considerable  improvements. 

Of  the  sense  organs  the  nose  is  characterized  by  three  features. 
An  outer  nose,  supported  by  cartilage  and  often  extended  as  a 
proboscis,  has  been  formed.  Its  cavity  has  been  increased,  since 
by  the  formation  of  hard  and  soft  palate  a  part  of  the  primitive 
mouth  cavity  has  been  included  in  it.  Its  upper  portion,  the 
olfactory  region,  has  been  complicated  by  the  formation  of  olfactory 
folds,  supported  by  the  turbinated  bones  already  referred  to  (p. 
620).  To  increase  the  mucous  surface  there  are  extensions  of  the 
nasal  cavity,  frontal,  maxillary,  and  sphenoidal  sinuses,  into  the 
corresponding  bones.  The  eye  has  the  upper  and  lower  lids,  besides 
the  nictitating  membrane  in  a  more  or  less  reduced  condition. 
The  ear,  except  in  monotremes,  Cetacea,  Sirenia,  and  some  seals, 
has  a  conch  supported  by  cartilage,  while  the  external  auditory 
meatus  is  always  present.  Internally  the  ear  is  much  modified, 
since  the  three  bones,  malleus,  incus,  and  stapes  (p.  544),  occur 
nowhere  else,  while  the  lagena  has  been  greatly  lengthened,  coiled 
into  a  spiral  with  two  to  four  turns  (figs.  80,  576),  while  inside 
the  wonderful  organ  of  Corti  has  been  developed. 

Of  digestive  structures,  the  teeth — which  are  restricted  to  max- 
illary, premaxillary,  and  dentary  bones — need  special  mention, 
because  of  the  distinctions  they  afford  from  all  other  vertebrates, 
and  because  of  their  importance  in  differentiating  the  various 
orders.  If  we  omit  the  monotremes,  edentates,  and  whales,  in 
which  there  is  marked  degeneration  in  the  dentition,  there  are 
four  particulars  which  show  the  dentition  of  mammals  more  de- 
veloped than  that  of  other  vertebrates.  (1)  The  number  of  teeth 


IV.    VERTEBRATA:   MAMMALIA. 


625 


is  constant  for  the  species,  usually  for  the  genus,  and  often  for 
the  family.  As  man  normally  has  thirty-two  teeth,  so  the  dog 
has  forty-two,  the  anthropoid  apes  thirty-two,  the  platyrhine  apes 
thirty-six,  etc.  (2)  The  teeth  are  firmer.  The  body  of  dentine 
is  divided,  by  a  slight  constriction,  into  a  crown  covered  with 
enamel,  and  a  root  enveloped  in  cement  (bony  tissue).  The  roots 
are  placed  in  separate  sockets  (alveoli)  in  the  jaws,  and  in  those 
cases  where  continuous  growth  is  necessary  the  pulp  persists  and 
the  teeth,  as  in  the  incisors  of  rodents  and  the  tusks  of  elephants 
and  pigs,  grow  indefinitely.  (3)  In  consequence  of  their  greater 
firmness  the  teeth  are  not  used  up  so  fast  and  do  not  require  rapid 
replacement.  There  occurs  only  one  change,  in  which  the  denti- 
tion present  at  birth  or  developed  soon  after — the  milk,  or  lacteal, 
dentition  or,  better,  first  dentition — is  replaced  by  the  second  or 
permanent  dentition  (diphyodont  mammals).  In  some  cases 
(monophyodont  mammals)  there  is  no  change,  the  first  dentition 
being  permanently  retained  (marsupials,  perhaps  toothed  whales), 
or  the  first  dentition  is  more  or  less  rudimentary  (edentates,  many 
rodents,  bats,  seals,  some  insectivores).  Besides  the  two  typical 
dentitions  traces  of  a  third  or  even  of  a  fourth  may  occur.  A 
prelacteal  dentition  of  calcified  germs  which  are  never  functional 
is  best  seen  in  marsupials,  and  is  rare  in  placental  mammals.  A 
dentition  following  the  permanent  one  is  outlined  in  many  placen- 
talia,  and  some  of  its  teeth  may  exceptionally  come  into  function. 
(4)  Among  the  teeth  a  division  of  labor  has  brought  about  change 
of  form  (heterodont  dentition).  The  teeth  of  the  premaxillaries  and 
their  antagonists  in  the  lower  jaw  are 
single-rooted  and  usually  have  more  or 
less  a  chisel  shape,  hence  they  are 
called  incisors  even  when,  as  in  in- 
sectivores, the  crowns  are  needle-like 
(fig.  661).  Behind  the  incisors  (in  the 
maxillary  bone  in  the"  upper  jaw)  is 
the  canine  tooth  (fig.  650,  c),  which  is 
single-rooted  and  has  usually  a  conical 
crown  (probably  a  modified  premolar). 
Following  the  canine  come  the  mo- 
lars, broad  teeth  mostly  with  two  roots  FIG.  650.  —  Permanent  and  milk 
,  J  _  _  ,.  dentitions  of  the  cat.  (From 

ana  tubercular  crowns.     Only  the  an-     Boas.)    c,  canines;  pa-p4,  pre- 

,  .,  .,,       _  molars;  m',  molar  (the  milk  den- 

terior    ones    appear    in    the    milk    den-      tition  darker   and    each   letter 

tition,  while  the  others  appear  only  in 

the  permanent  dentition  and  are  not  replaced. 


preceded  by  d). 


On  this  develop- 


626  CHORD  AT  A. 

mental  basis  the  molars  are  divided  into  premolars  (bicuspids  of 
dentists),  which  appear  in  both  dentitions,  and  the  true  molars, 
which  occur  only  in  the  last. 

From  the  foregoing  it  will  be  seen  that  every  species  of  mam- 
mal is  characterized  by  its  dentition,  and  these  features  may  be 
expressed  by  a  short  formula.  It  is  only  necessary  to  place  the 
number  of  each  of  the  four  kinds  of  teeth  mentioned  in  their  regular 
order,  those  of  the  upper  jaw  separated  from  those  of  the  lower  by 
a  horizontal  line,  to  express  this.  Since  the  two  sides  of  the  body 
are  symmetrical,  only  those  of  one  side  need  be  enumerated,  and 
in  case  that  one  kind  be  absent  the  deficiency  is  indicated  by  a 
zero.  The  dental  formula  of  man  would  thus  be  fff|;  of  the  rein- 
deer, in  which  in  the  upper  jaw  incisors  and  canines  are  absent, 
•JJf-f.  The  different  formulae,  by  comparison,  give  us  a  funda- 
mental formula  from  which  they  have  been  derived  by  reduction. 
This  was  probably 


The  molars  undergo,  according  to  the  food,  the  greatest  modification 
of  form.  As  a  starting  point  the  bunodont  tooth  may  be  taken  which 
occurs  in  omnivorous  mammals  and  which  has  the  crown  with  several 
blunt  projections  or  cones.  With  animal  food  (fig.  650,  657)  the  cones  be- 
come sharper  and  cutting  (secodont  dentition  of  carnivores  and  insec- 
tivores),  and  when  the  cutting  angle  becomes  very  sharp,  with  a  special 
prominence  on  the  inner  side,  it  is  spoken  of  as  a  flesh  or  carnasial  tooth. 
In  vegetable  feeders  the  cones  become  connected  by  crests  (lophs)  or  are 
half-moon-shaped  (lophodont  or  selenodont).  Since  the  cones  and  lophs 
become  in  part  worn  away  and  the  grooves  between  them  are  filled  with 
cement,  there  arise  broad  grinding  surfaces  strengthened  by  the  harder  and 
more  resistant  enamel  of  the  cones  and  lophs  ;  this  extends  inwards  as 
folds  from  the  outer  enamel  wall  of  the  tooth  ;  the  folds  may  become  cut 
off  and  form  islands  of  enamel  on  the  grinding  surface  (dentes  complicati 
of  ungulates).  When  the  folds  extend  in  regular  order  from  the  outside 
and  inside  and  meet  in  the  middle  they  form  numerous  successive  leaves, 
bound  together  by  cement  (compound  teeth  of  elephants,  fig.  667,  and 
many  rodents). 

Paleontological  investigation,  with  which  the  more  recent  erabryologi- 
cal  results  are  in  accord,  has  shown  that  a  great  regularity  prevails  in 
the  formation  of  the  cones  of  the  molars.  Triconodont  and  tritubercular 
teeth  are  recognized,  in  which  the  three  cones  are  either  arranged  in  a 
line  or  in  a  triangle,  as  well  as  multitubercular  teeth  with  more  numerous 
cones  irregularly  arranged.  The  triconodont  type  develops  farther  by  the 
formation  of  secondary  cones.  The  development  of  these  occurs  in  dif- 
ferent ways  in  molars  and  premolars.  Since  the  latter  are  the  more  sim- 
ple, their  distinction  from  the  molars  does  not  rest  alone  upon  the  existence 
of  a  milk  dentition,  but  upon  structure  as  well.  This  is  important,  because 
it  happens  that  there  are  premolars  which  are  not  replaced  (marsupials, 


IV.    VERTEBRATA:  MAMMALIA.  627 

many  insectivores  and  rodents)  and,  on  the  other  hand,  beneath  the  molars 
the  anlagen  of  replacing  teeth  may  be  found.  The  latter  fact  shows 
that  the  molars,  strictly  speaking,  belong  not  to  the  permanent  but  to  the 
milk  dentition.  They  are  late  in  formation  and  are  therefore  parts  of  the 
first  dentition  carried  over  into  the  second. 

The  mouth,  which  contains  tongue  and  teeth,  is  separated  from 
the  next  division  of  the  alimentary  tract,  the  pharynx,  by  the 
uvula.  The  pharynx  narrows  behind  into  the  oesophagus,  the  en- 
trance of  which  into  the  stomach  is  marked  by  a  constricting 
cardia.  At  its  other  end  the  stomach  has  a  similar  constrictor, 
the  pylorus,  separating  it  from  the  intestine.  In  the  latter  small 
and  large  intestines  (the  latter  consisting  of  colon  and  rectum) 
are  differentiated  by  the  diameter  of  the  lumen.  The  small  in- 
testine opens  laterally  into  the  colon  and  at  the  junction  arises  a 
blind  diverticulum,  the  caecum,  which  is  small  in  mammals  with 
animal  food,  but  in  herbivores  (especially  rodents)  is  always  large 
and  forms  a  conspicuous  part  of  the  alimentary  tract.  The  ver- 
miform appendix  (primates,  rodents)  is  a  narrower  part  of  the 
caecum.  Three  pairs  of  salivary  glands  empty  into  the  mouth, 
the  liver  and  pancreas  into  the  small  intestine  (duodenum). 

Most  important  of  respiratory  peculiarities  is  the  diaphragm, 
which  separates  the  body  cavity  into  thoracic  and  abdominal  cavi- 
ties. This  occurs  only  in  its  beginnings  in  other  vertebrates 
(perhaps  even  in  Amphibia).  In  the  thoracic  cavity  are  the 
O3sophagus,  heart  with  its  pericardium,  and  especially  the  trachea, 
bronchi,  and  lungs;  the  remaining  vegetative  organs  are  in  the 
abdominal  cavity.  The  diaphragm  is  a  muscular  dome,  its  con- 
vex side  towards  the  thoracic  cavity;  by  contraction  it  flattens  an 
increases  the  size  of  the  cavity,  in  consequence  of  which  air  is 
drawn  into  the  lungs  (inspiration).  On  relaxation  the  lungs  con- 
tract from  their  own  elasticity  and  force  out  a  part  of  the  air 
(expiration).  The  intercostal  muscles,  which  raise  and  lower  the 
framework  of  the  chest,  also  play  a  part,  as  in  birds.  The  respira- 
tory ducts  (fig.  579)  begin  with  the  larynx  (with  vocal  cords), 
which  can  be  closed  from  the  pharynx  by  the  epiglottis;  this  is 
followed  by  the  trachea,  which  divides  into  right  and  left  bronchi. 
Each  bronchus  divides  again  and  again,  and  the  finest  of  these 
divisions,  the  bronchioles,  are  continued  as  alveolar  ducts  to  small 
chambers,  the  infundibula,  both  these  and  the  alveolar  ducts  being 
lined  with  small  respiratory  pockets,  the  alveoli. 

The  heart,  with  two  auricles  and  two  ventricles,  is  completely 
separated  into  systemic  and  pulmonary  halves.  In  early  embryonic 


628 


CHORD  AT  A. 


life  the  arterial  trunk,  which  at  first  is  simple,  is  divided  into  a 
pulmonary  artery,  arising  from  the  right  half  of  the  heart  and 
carrying  venous  blood,  and  an  aorta  ascendens,  with  arterial  blood, 
connected  with  the  left  half.  In  contrast  with  the  reptiles,  the 
right  aortic  arch  is  entirely  lost,  the  left  persisting. 

The  urogenital  system  is  of  great  importance  in  the  separation 
of  the  group  into  smaller  divisions  (fig.  651).  In  both  sexes  this 
consists  of  practically  the  same  parts  in  early  embryonic  life. 
These  are  the  early  formed  Wolffian  body  ( W ) ;  the  permanent 
kidneys,  which  appear  later  and  are  not  shown  in  the  diagram ;. 


FIG.  651. 


FIG.  65i. 


FIG.  651.— Diagram  of  embryonic  mammalian  urogenital  system.  (From  Balfour,. 
after  Thompson.)  cl,  cloaca;  cj>,  genital  process:  go,  genital  cord;  i,  rectum; 
te,  ridge  for  formation  of  labia  or  scrotum ;  rn,  Mullerian  duct ;  ot,  gonad  ;  mi, 
urogenital  sinus ;  W,  Wolfflan  body ;  w,  Wolffian  duct;  3,  ureter;  A,  urinary  blad- 
der ;  5,  continuation  of  latter  to  allantois  (urachus). 

FIG.  652. — Urogenital  system  of  male  beaver.  (From  Blanchard.)  a,  castor eum 
sacs  ;  b,  openings  of  their  ducts  into  preputial  canal;  c,  tip  of  penis  ;  d,  preputial 
opening  ;  e,  anal  glands;  f,  their  ducts;  0,  anus  ;  7i,  base  of  tail;  i,  corpora  caver- 
nosa  ;  7c,  Cowper's  glands;  /,  seminal  vesicles;  7n,  vasa  deferentia;  n,  testes;  o, 
urinary  bladder  with  ureters. 

the  urinary  bladder  (4),  a  part  of  the  allantois  which  extends  (5) 
into  the  foetal  appendages;  the  three  ducts,  the  Mullerian  (m), 
the  Wolffian  (w),  and  the  ureter  (3).  These  ducts  no  longer 
empty  into  the  intestine,  but  into  the  allantoic  structures,  the 
ureters  into  the  base  of  the  urinary  bladder,  the  Wolffian  and 


IV.    VERTEBRATA:  MAMMALIA. 


629 


Miillerian  ducts  into  the  urogenital  sinus  (ug},  the  lower  continua- 
tion of  the  bladder.  The  gonad  is  connected  with  the  Wolffian 
duct.  In  the  anterior  wall  of  the  urogenital  sinus  is  a  mass  of 
highly  vascular  tissue  (cp),  from  which  and  a  surrounding  fold  the 
external  genitalia  are  developed.  Since  the  urogenital  sinus  opens 
from  in  front  into  the  intestine,  there  is  always  a  claoca  (cl)  in 
the  embryonic  stages,  which  persists  throughout  life  in  the  mono- 
tremes,  and  to  a  considerable  extent  in  the  female  marsupials;  in 
all  other  vertebrates  it  is  divided  by  a  partition,  the  perinaeum, 
into  a  urogenital  opening  in  front  and  an  anal  opening  behind. 

From  this  indifferent  condition  the  male  and  female  apparatus 
are  derived,  the  structures  being  closely  similar  in  most  males  (fig. 
652).  The  Miillerian  duct  vanishes,  while  the  Wolman  duct  be- 
comes the  vas  deferens  and  its  accessories,  serving  as  a  canal  for 
the  genital  products,  while  the  external  genitals  arise  from  the 
other  parts  mentioned,  these  forming  an  intromittent  organ 
(penis).  In  the  female  the  Wolffian  body  and  duct  degenerate, 
the  Miillerian  ducts  become  the  reproductive  canals.  The  modi- 
fications of  these  become  of  great  systematic  importance.  In  the 
monotremes  both  ducts  open  separately  and  become  differentiated 
into  two  parts  (fig.  653,  A),  anterior  oviducts  with  wide  openings 


A. 


FIG.  653.— Female  genitalia  of  (^4)  Echidna  aculeata;  (B)  of  Didelphys  dorsigera;  (O 
Phascolom us  wombat.  (B  and  C,  after  Wiedersheim.)  c/,  cloaca;  d,  rectum;  /*, 
urinary  bladder;  n,  kidney;  o,  ovary;  od,  oviduct;  pu,  month  of  ureters;  s?t,  uro- 
genital sinus;  f,  ostium  abdominale  tubae;  rt,  uterus;  it',  opening  into  vagina; 
wr,  ureter;  r,  vagina;  vb,  vaginal  blind  sac. 

into  the  body  cavity  (od,  t)  and  the  uterus  («).  The  ureters  open 
into  the  sinus  (and  not  into  the  bladder)  between  the  uterine 
openings.  In  the  marsupials  (B  and  C)  there  are  three  divisions, 
oviduct,  uterus,  and  vagina;  besides,  the  two  Miillerian  ducts  may 
approach,  near  the  uterus  (B},  or  fuse  in  this  region  (C')  in  some 


630  CHORD  ATA. 

species,  forming  an  unpaired  blind  sac  (vJ),  which  may  even  open 
into  the  urogenital  sinus  as  a  third  vagina.  This  partial  fusion  of 
the  vaginae  of  the  marsupials  is  completed  in  the  placental  mam- 
mals, the  single  vagina  and  the  sinus  forming  a  single  canal  (fig. 
654).  Here  the  uterine  portions  may  remain  distinct  (uterus 

A 


FIG.  651.— A,  uterus  duplex;   B,  uterus  bicornis;  C,  uterus  simplex.    (From  Gegen- 
baur.)    od,  oviduct;  i/,  uterus;  v,  vagina. 

duplex  of  rodents,  A),  or  they  may  fuse  partially  (uterus  bicornis 
of  insectivores,  whales,  ungulates,  and  carnivores,  J9),  or  they  may 
be  completely  fused  (uterus  simplex  of  apes  and  man,  C). 

Thus  there  are  three  different  types  of  the  female  genitalia,  in 
which  the  vagina  is  not  differentiated  (Ornithodelphia),  or  is 
double  (Marsupialia),  or  is  single  and  unpaired  (Monodelphia). 
To  these  correspond  three  types  of  development.  The  Ornitho- 
delphia are  oviparous,  the  others  viviparous,  but  are  distinguished 
by  the  duration  of  pregnancy.  The  eggs  of  the  viviparous  forms 
are  so  small  (about  .01  inch)  that  they  have  a  total,  nearly  equal 
segmentation.  Such  eggs  require  nourishment  from  the  mother 
in  order  to  produce  an  animal  with  the  complicated  structure  of  a 
mammal.  Since  in  the  Didelphia  the  uterine  nourishment  is 
usually  very  incomplete,  the  period  of  pregnancy  is  very  short, 
in  comparison  with  the  Monodelphia,  in  which  a  placenta,  a  com- 
plicated apparatus  for  the  nourishment  of  the  young,  appears; 
hence  the  marsupials,  with  their  small  imperfectly  formed  young, 
are  often  called  Aplacentalia;  the  Monodelphia,  Placentalia. 

All  mammals  care  for  the  young,  this  being  chiefly  or  wholly  done  by 
the  mother,  who  not  only  supplies  them  with  milk  but  protects  them  in 
warm  if  rude  nests.  Most  mammals  are  monogamous,  some  polygamous, 
while  in  others  there  is  no  permanent  association  of  the  sexes.  The  body 
temperature  is  constant  and  ranges  from  36°  to  41°  C.  (98°  to  106°  F.) ;  in 
Echidna  it  is  only  26°  to  34°  C.  (79°  to  83°  F.).  In  most,  continual  feeding 
is  necessary  for  existence;  from  this  rule  there  are  a  few  exceptions,  like 
the  bears,  marmots,  badgers,  etc.,  which  hibernate  during  the  winter^ 
taking  no  food.  At  this  time  there  is  a  fall  in  the  temperature  due  to  the 
diminished  metabolism. 


IV.    VERTEBRATA:  MAMMALIA,  MONOTREMATA.       631 


Sub  Class  I.   Monotremata  (Ornithodelphia,  Prototheria). 

A  few  mammals,  confined  to  Australia  and  New  Guinea, 
divided  among  the  genera  Echidna,  Proechidna,  and  Ornitho- 
rhynchus,  are  the  only  living  representatives  of  the  group.  They 
are  distinguished  from  all  other  mammals  by  laying  eggs  about 
half  an  inch  long,  rich  in  yolk  and  with  soft  shells.  These  undergo 
in  the  uterus  a  discoidal  (meroblastic)  segmentation  and  are  then 
incubated  by  Ornithorhynchus  in  a  nest,  by  Echidna  in  a  tempo- 
rary pouch  (marsupium)  on  the  ventral  surface  of  the  body.  On 
hatching  the  young  are  nourished  by  the  secretion  of  enormously 
enlarged  sweat  glands,  which  form  two  large  masses  to  the  right 
and  left  of  the  mid-ventral  surface,  and  which  must  not  be  con- 
founded with  the  milk  glands  (sebaceous)  of  other  mammals. 
Each  opens  on  a  special  region  of  the  ventral  surface,  which  is 
slit-like  in  OrnithorUynchus,  a  flattened  pocket  in  the  others. 

Other  distinctions  from  other  mammals,  which  are  also  points 
of  resemblance  to  reptiles  and  birds,  are  the  strong  development 
of  the  episternum  and  the  extension  of  the  coracoid  to  the  ster- 
num (fig.  648),  the  termination  of  the  ureters  in  the  urogenital 
sinus  and  not  in  the  fundus  of  the  bladder  (fig.  653),  the  exist- 
ence of  a  cloaca  in  both  sexes,  and  the  specifically  bird-like  char- 
acter of  the  female  sexual  organs,  in  which  the  large  left  ovary  is 
alone  functional,  and  uterus  and 
vagina  are  not  differentiated.  But 
with  all  this  it  must  not  be  forgot- 
ten that  the  monotremes  have  the 
hair,  the  skull,  the  urogenital  sinus 
of  true  mammals,  and  in  the  pres- 
ence of  marsupial  bones  (fig.  655, 
Om)  show  a  close  relationship  with 
the  marsupials.  The  upper  end  of 
the  hyoid  is  connected  directly  or  by 
a  ligament  with  the  cartilaginous 
auditory  opening,  while  a  scarcely  /* 

visible  external  ear  occurs.    The  jaws 

,         i    •      i  FIG.  655.— Pelvis  (left  side)  of  Ornith. 

are  toothless  and  enclosed  in  horny      orhynchus  paradox™.   (From  Wie- 
sheaths,  yet  in  the  young  of  Orni-     S*T Ilium;'  r^SEIfem'SS 
thorhynchus  there  are  in   each  jaw     ^rsnpiai  bone;  p,osPui 
three  pairs  of  multitubercular  molars,  which  are  later  replaced 
by  four  broad  horny  plates. 


632  CHORDATA. 

ECHIDNID.E.  The  spiny  ant-eaters  have  the  body  covered  with  bristles, 
snout  with  a  worm-shaped  tongue  used  in  catching  insects;  Echidna 
aculeata  of  Australia,  feet  five-toed,  with  digging  claws  ;  Proechidna 
(Acanthoglossus)  of  New  Guinea,  three-toed.  ORNITHORHYNCHID.E.  The 
duckbills  are  toothless,  close-haired  animals  with  horny  jaws  which 
resemble  those  of  a  duck;  the  five-toed  feet  with  a  swimming  web  especially 
well  developed  on  the  fore  feet.  Ornithorhynchus  paradoxus  of  Australia. 


Fio.  656.— Ornithorhynchus  paradoxus,  duckbill.    (From  Schmarda.) 

The  male  has  a  spine  with  a  gland  on  the  hind  feet  which  fits  in  a  corre- 
sponding pit  on  the  thigh  of  the  female  and  apparently  plays  a  role  in 
copulation. 

The  oldest  fossil  mammals  are  possibly  to  be  regarded  as  belonging  to 
the  monotremes.  These  appear  in  the  trias  and  form  a  group,  MULTITU- 
BERCULATA  (Allotheria),  which  is  but  imperfectly  known  (Tritylodon, 
MicrolesteSj  Plagiaulax).  Their  multitubercular  teeth  resemble  the  tempo- 
rary ones  of  Ornithorhynchus,  while  there  are  indications  that  the  cora- 
coid  existed  as  a  distinct  bone.  Less  certain  are  the  PROTODONTA  (Droma- 
therium,  Microconodon)  of  the  American  Jurassic,  of  which  only  the  lower 
jaws  are  known. 

Sub  Class  II.   Marsupialia  (Didelphia). 

These,  like  the  remaining  mammals,  are  viviparous.  They 
have  small  eggs  which  undergo  a  total  segmentation  in  most  species, 
and  develop  in  the  maternal  uterus,  being  nourished  by  a  secre- 
tion from  its  walls.  In  a  few  species  there  is  a  placenta  which,  in 
Perameles,  is  allantoic  in  origin,  in  Dasyurus  viverrinus  possibly 
also  from  the  yolk  sac.  In  most  species  there  is  no  placenta.  In 
all  there  is  insufficient  nourishment  and  the  young  are  born  in  a  very 
immature  condition.  They  are  therefore  carried  a  long  time  by 
the  mother  in  the  marsupium,  a  pouch  formed  by  a  fold  of  skin 
on  the  posterior  ventral  surface,  into  which  the  nipples  open. 
The  ventral  surface  is  supported  by  the  marsupial  bones,  slender 
rods  articulated,  right  and  left,  at  the  pubic  symphysis.  Other 
characteristics  of  the  marsupial  skeleton  are  the  inflected  posterior 
angle  of  the  lower  jaw  (fig.  657,  a)  and  the  rudimentary  replace- 
ment of  teeth.  The  milk  teeth  and  molars  (first  dentition)  are 
as  a  whole  retained,  only  premolar  3  being  replaced  by  another 


IV.    VERTEBRATA:   MAMMALIA,   MARSUP1ALIA.        633 

tooth;  but  it  is  in  question  whether  this  belongs  to  the  second  den- 
tition or  is  a  belated  member  of  the  first.  The  sexual  apparatus 
has  already  been  described  (p.  630). 

Marsupials  are  known  from  the  secondary  (Jurassic)  and  tertiary  strata 
of  Europe  and  both  Americas.  They  were  apparently  then  spread  over 
the  whole  earth,  but  were  crowded  out  by  the  placental  mammals  and 
persisted  only  as  remnants  (Ccenolestes  and  the  opossums)  in  America,  but 
as  a  richly  developed  fauna  in  Australia.  In  the  latter  region  they  con- 


FIG.  657.— Lower  jaw  of  Thylacinus  cynocephalus   (from  Flower),  showing  (a)  the 
inflected  angle  chaiacteristic  of  marsupials;  cd,  articular  surface. 

tinued  because  here,  on  account  of  the  early  separation  of  this  continent 
from  the  rest  of  the  world,  no  development  of  Placentalia  occurred.  The 
placentals  are  entirely  lacking  in  Australia  with  the  exception  of  those 
introduced  by  man  and  such  (mice,  bats,  seals)  as  easily  pass  from  island 
to  island.  In  their  present  habitat,  in  adaptation  to  similar  conditions 
they  have  undergone  a  development  analogous  to  that  of  the  placentals  in 
other  parts  of  the  earth,  so  that  they  present  groups  parallel  with  the 
carnivores,  rodents,  insectivores,  and  ungulates. 

Order  I.  Polyprotodonta  (Zoophaga). 

Many  marsupials,  among  them  the  oldest,  have  a  dentition 
adapted  to  animal  food.  They  have  numerous  incisors  (up  to  five 
in  each  half-jaw),  strong  canines,  and  sharp-pointed  molars  (fig. 
657).  Some  in  teeth,  as  well  as  in  body  form,  resemble  the  Insec- 
tivora,  others  the  carnivores. 

The  Dasyuridse  are  carnivorous:  Dasyurus ;  Sarcophilus  ursinus,  the 
Tasmanian  'devil,'  dangerous  to  larger  mammals;  Thytaeinus,  pouched 
wolf.  The  PERAMELID.E  are  insectivorous  ;  Perameles,  bandicoot.  The 
DIDELPHYID^E,  or  opossums,  which  are  confined  to  America  (chiefly  South) 
are  more  carnivorous  in  dentition  and  recall  the  apes  with  their  opposable 
thumb.  Didelphys  virginiana* 

Order  II.  Diprotodonta  (Phytophaga). 

The  herbivorous  habits  are  correlated  with  the  degeneration 
of  canines,  which  usually  are  lacking  in  the  lower  jaw  and  are  at 
least  very  small  in  the  upper.  There  are  also  only  two  incisors, 
of  large  size,  in  the  lower  jaw,  while  the  middle  two  of  the  upper 
are  much  larger  than  the  one  or  two  lateral  which  may  be  present. 

The  PHASCOLOMYID.E  are  the  rodents  of  the  marsupials  with  one  chisel- 
like  incisor  in  each  half  of  each  ja\v.  Phascalomys,  wombat.  The  MACRO- 


63J: 


CHORD  AT  A. 


PODID.E,  or  kangaroos,  resemble  the  ungulates  in  their  large  herds  on  the 
grassy  places.  The  fore  legs  being  very  small,  the  animals  leap  with  the 
strong  hind  legs  and  tail.  Macropus  giganteus.  The  PHALANGISTID.E 
have  very  variable  teeth.  They  resemble  in  habits  the  squirrels,  Petaurus 
having  the  same  parachute  folds  as  does  our  flying  squirrel.  The  Dipro- 
todonta  contain  many  fossil  forms  in  Australia  and  a  few  in  South 
America.  Some  of  the  Australian  fossils  were  very  large,  Diprotodon 
australis  larger  than  a  rhinoceros. 

Sub.  Class  III.  Placentalia  (Monodelpliia). 

The  first  reason  for  associating  the  mammals  of  the  Old  World 
and  most  of  those  of  the  New  together  as  Placentalia  is  an  embry- 
ological  one,  the  presence  of  a  placenta.  When  serosa,  amnion, 
and  allantois  (p,  553)  have  developed  in  the  embryo,  the  vessels 
of  the  allantois  spread  out  beneath  the  serosa  and  form  with  this 
the  chorion,  which  sends  small  processes  or  villi  into  the  now 

highly  vascular  mucous  mem- 
brane of  the  uterus  in  order  to 
obtain  nourishment  somewhat  as 
a  tree  obtains  food  by  its  roots. 
These  villi  may  be  distributed 
over  the  greater  part  of  the  sur- 
face (fig.  658),  producing  the 
chorion  frondosum,  or  diffuse 
placenta,  which  occurs  in  Cetacea, 
perissodactyles  and  many  artio- 
dactyles  (swine).  On  the  other 
hand  the  vilH  may  be  restricted 
to  certain  places,  becoming  very 

FIG.  658.— Diagram  of  mammalian    em-     .  .,  _,,,  . 

biTO  with  chorion  frondosum;afo,amni-  Strong  there.       JLhlS    glVCS    rise  to 
otic  cavity;  aZ,  allantois;  am,  amnion;        ,    -i    j  ->•        -j    i 

as,  umbilical  cord;  c/i,  chorion;  c/,z,  cotyledonary,  discoidal,  or  zonary 

chorion ic  villi ;  dg,  yolk  stalk  ;   ds,  yolk     -,  m       , -i  T 

sac ;  r,  space  (extra-embryonic  coelom),  placentae,       lo    these    Correspond 
between  chorion  and  ammon;*,  serosa.  porti(mg     rf    ^    ^^    ];ning 

which  are  distinguished  from  the  rest  by  becoming  extremely  vas- 
cular (uterine  placenta).  The  cotyledonary  placenta  (fig.  659) 
consists  of  many  small  placentar  patches,  the  cotyledons  (most 
ruminants).  In  the  zonary  placenta  the  villous  area  takes  the 
shape  of  a  girdle  or  barrel  (carnivores,  Sirenia),  while  the  discoidal 
(other  mammals)  is,  as  its  name  indicates,  disc-like. 

Besides  the  placental  structures  the  higher  mammals  are  char- 
acterized by  the  disappearance  of  the  cloaca,  the  unpaired  vagina, 
and  absence  of  marsupial  bones  and  inflected  angle  of  the  jaw.  The 


eh 


IV.    VERTEBRATA:   MAMMALIA,  EDENTATA.  635 

dentition,  on  the  other  hand,  has  undergone  a  progressive,  diver- 
gent development,  so  that  the  distinctions  are  much  more  pro- 
nounced than  in  the  marsupials,  and  hence  of  importance  in 
differentiating  the  orders. 

Order  I.  Edentata. 

A  few  families,  poor  in  species,  are  united  under  the  name 
Edentata  because  teeth  are  absent  or,  as  is  more  usually  the  case, 
are  markedly  degenerate.  Persistent  functional  incisors  are  lack- 


FIG.  659.— Cotyledonary  placenta  and  embryo  of  cow.  (From  Balfour,  after  Colin.) 
C1,  cotyledons  of  uterine,  C2,  of  foetal  placenta;  Ch,  chorion ;  U,  uterus;  V, 
vagina. 

ing,  canines  but  rarely  occur  (Brady pus)',  molars  may  be  present, 
sometimes  in  great  numbers  (Priodongigas,  the  large  armadillo,  has 
about  a  hundred  molars),  but  they  are  poorly  rooted,  prismatic, 
without  enamel,  and  usually  monophyodont.  Since  the  aardvark 
(Orycteropus)  and  Tatusia  have  a  heterodont  milk  dentition  in 
embryonic  life  in  which  incisors  occur,  and  fossil  edentates 
(Entelops)  with  complete  dentition  are  known,  the  absence  of  a 
replacement  of  the  teeth  is  to  be  explained  by  degeneration,  which 
may  affect  other  parts,  and  is  to  a  certain  extent  the  reason  for  the 
low  position  accorded  these  forms.  The  great  number  of  sacral 
vertebrae  is  striking,  being  as  many  as  thirteen  in  some  armadillos. 
The  placenta  is  very  variable,  being  diffuse,  discoidal,  or  zonary  in 
different  species.  The  group  is  essentially  tropical,  but  one  species 


636  CHORDATA. 

entering  the  United  States.     The  oldest  fossils  occur  in  the  Santa 
Cruz  beds  of  Patagonia  (eocene  or  oligocene). 

Sub  Order  I.  NOMARTHRA.  Old  World  edentates.  FODIENTIA. 
Animals  with  strong  digging  claws,  long  tail,  and  long,  vermiform,  sticky 
tongue  used  in  catching  ants  and  other  insects.  Orycteropus  capensis, 
aardvark,  with  long  snout,  sparse  bristly  hair,  five  small  molars,  and 
rudimentary  milk  dentition.  SQUAMATA.  Toothless,  body  covered  with 
overlapping  scales.  Manis,  pangolins  of  Asia  and  Africa  (fig.  660). 


FIG.  660.— Manis  longicaiidata,  pangolin.    (From  Monteiro.) 

Sub  Order  II.  XENARTHRA.  Edentates  of  the  New  World.  VER- 
MILINGrUIA,  ant  eaters.  Resemble  manids  in  toothless  jaws,  long  ant- 
catching  tongues,  and  strong  digging  claws,  but  are  hairy  and  lack  scales. 
Myrmecophaga.  TARDIG-RADA,  sloths.  Hairy,  head  short,  rudimentary 
tail,  and  few  teeth,  long  strong  claws  by  which  they  hang  back  down- 
wards from  limbs  of  trees.  Bradypus  tridactylus,  nine  cervical  verte- 
brae ;  Cholcepus,  six  cervicals.  Fossils  allied  are  Megatherium,  as  large 
as  an  elephant,  Mylodon,  Megalonyx,  these  two  extending  north  to  Penn- 
sylvania. LORICATA,  armadillos.  Body  with  armor  of  bony  plates, 
molars  numerous  ;  insectivorous.  In  the  extinct  GLYPTODONTID^E  of  South 
America  the  plates  fused  to  a  continuous  armor.  One  species  twelve  feet 
long.  One  species  may  have  occurred  in  Europe.  DASYPODID.E  ;  dermal 
armor  in  three  or  more  movable  transverse  plates  ;  nocturnal.  Genera 
based  upon  the  number  of  bands:  Dasypus,  Xenurus ;  Tacusia  novem- 
cincta  *  enters  United  States. 

Order  II.  Insectivora. 

These  primitive  forms  have  a  complete  dentition,  all  the  differ- 
ent kinds  of  teeth  being  present,  although  they  vary  in  number. 
The  roots  are  developed  early  and  consequently  the  teeth  are  small. 
Since  they  end  with  sharp  cusps,  adapted  for  eating  insects,  they 
resemble  the  carnivores,  from  which  they  may  be  distinguished  by 


IV.    VERTEBRATA:  MAMMALIA,   CHIROPTERA.          637 


the  rudimentary  condition  or  occasional  absence  of  the  canines 
(Talpa  fjii.  miny  shrews  ffff).  There  is  great  variability  in 
the  matter  of  replacement  of  teeth;  in  the  shrews,  for  instance, 
the  milk  dentition  is  suppressed  and 
the  second  only  is  functional,  while  in 
the  hedgehog  one  incisor  and  one  pre- 
molar  in  each  jaw,  a  second  premolar 
and  the  canine  of  the  lower  jaw  func- 
tion in  both  dentitions.  In  many  re-  FIG.  66i.-skuii  of  Sorex.  (From 

,,  .  ,.  Ludwig-Leunis.) 

spects    the     insectivores     resemble    the 

rodents :  a  clavicle  is  present ;  there  are  usually  five  toes  furnished 
with  claws;  there  is  a  uterus  bicornis,  often  divided  its  whole 
length,  and  discoidal  placenta. 

Aside  from  the  proboscis-like  snout  the  insectivores  resemble  the 
rodents  in  appearance,  forming  parallel  groups  to  those  of  that  order. 
The  ERINACID.E,  or  hedgehogs,  of  the  Old  World  are  spined  like  the  porcu- 
pines ;  the  SORICID.E,  or  shrews  (Sorex,*  Blarina*),  are  mouse-like,  as  are 
the  allied  TALPID.E,  or  moles  (Scalops,*  Condylura,*  star-nosed  mole), 
which  burrow  in  the  earth  and  have  the  eyes  more  or  less  rudimentary. 
Some  authors  place  here  Galeopithecus  of  the  East  Indies,  which  has  a 
similar  membrane  and  similar  sailing  powers  as  the  flying  squirrels.  It 
also  presents  resemblance  to  the  bats  and  to  the  lemurs.  The  earliest 
known  insectivores  date  from  the  eocene. 


FIG.  662.— Skeleton  of  bat.    (After  Brehm.) 

Order  III.  Chiroptera. 

The  bats  are  the  only  mammals  which  actually  fly,  and  this  at 
once  characterizes  them.  The  flying  membrane  (patagium)  is  a 
thin  fold  of  skin,  richly  supplied  with  nerves,  which  begins  at  the 


638  CHORDATA. 

tail,  includes  the  lower  extremities  to  the  foot,  and  extends  thence 
to  the  fingers,  leaving  the  thumb  free.  Fingers  2-5  are  enormously 
elongated  and  support  the  membrane.  Since  flight  requires 
strong  muscles,  the  sternum  develops  a  small  keel,  recalling  that 
of  birds,  for  the  attachment  of  the  large  pectoral  muscle.  In  con- 
nexion with  the  flying  powers  the  clavicle  is  strong.  The  patagium 
is  the  seat  of  a  very  acute  tactile  sense,  by  means  of  which  blinded 
bats  can  fly  among  all  kinds  of  obstacles  without  disturbing  them. 
The  enormous  ear  conchs  and  a  noticeable  nose  leaf,  widely  dis- 
tributed through  the  group,  also  have  marked  tactile  powers.  In 
the  pectoral  position  of  the  mammary  glands  and  in  the  discoidal 
placenta  these  animals  resemble  the  primates.  In  temperate 
regions  bats  hibernate  during  the  winter.  The  dentition  is  vari- 
able, often  fllf .  Fossils  occur  in  the  eocene. 

Sub  Order  I.  MICROCHIROPTERA,  with  insectivorous  dentition, 
only  the  thumb  of  the  fore  limbs  clawed.  VESPERTILIONID^,  tail  long,  no 
nose  leaf  ;  Vesperugo*  Atalapha*  PHYLLOSTOMID^E,  with  nose  leaf,  trop- 
ical America  ;  Desmodus,  the  blood-sucking  or  vampyre  bat. 

Sub  Order  II.  MACHROCHI  ROPIER  A  (Frugivora),  with  smooth- 
crowned  molars,  claws  on  thumb  and  first  two  fingers.  Includes  the  flying 
foxes,  Pteropus,  of  the  East  Indies. 

Order  IV.  Rodentia. 

The  rodents  unite  great  similarity  in  appearance  with  a  char- 
acteristic dentition.  The  canines  are  absent,  and  the  molars  are 
separated  by  a  large  gap  (diastema)  from  the  incisors  (fig.  663). 

The  latter  are  strong,  chisel-like, 
have  persistent  pulps  and  grow  at 
the  lower  end  as  they  are  worn 
away  at  the  cutting  edge.  Since 
only  the  front  surface  has  enamel, 
wear  keeps  them  constantly  sharp. 
Usually  there  is  but  a  single  in- 
cisor, and  only  in  the  Duplici- 
dentata  is  a  second  present  in  the 
upper  jaw.  The  molars  are  cus- 
FIG.  663.— skull  of  porcupine.  (From  pidate  or  have  enamel  folds  and 

Schmarda.)    /,  frontal;  im,  premaxil-    * 

lary;  k,  temporal  fossa  continuous  in    frequently       Continue       to        ffl'OW 
front  with  orbit;  o,  infraorbital  fora-  J  .  ° 

men,  enormous  on  account  of  the  por-    throughout  llie.       Their  number  IS 
tion  of   the   masseter   muscle   which    „  , ,  ,          ,      . ,        _ 

passes  through  it.  frequently  reduced,  the  formulae 

varying  between  f-°-f  |  and  -J-J-J-f.  Many  species  have  an  inflected 
angle  of  the  jaw  like  that  of  marsupials.  The  infraorbital  canal 
is  a  striking  feature  in  Muridae  and  Hystricidae  (fig.  663,  0),  a 


IV.    VERTEBRATA:  MAMMALIA,  UNGULATA.  639 

large  opening  in  front  of  the  orbit  in  which  a  part  of  the  masseter 
muscle  is  attached. 

The  rodents  are  distinguished  from  the  ungulates,  which,  like 
them,  are  herbivorous,  by  the  usually  smaller  size,  the  possession 
of  claws,  five  toes  (sometimes  reduced  to  three),  the  occurrence 
usually  of  a  clavicle,  and  a  discoid  placenta.  The  mammae  are 
inguinal  in  position  and,  corresponding  to  the  great  fertility,  are 
very  numerous.  The  occurrence  of  glands  with  a  strong-smelling 
secretion,  which  open  near  the  anus,  is  common.  About  nine 
hundred  living  species  are  known,  occurring  in  all  regions  except 
the  Australian.  The  order  appears  in  the  eocene. 

Sub  Order  I.  DUPLICIDENTATA  (Lagomorpha),  two  upper  incisors, 
includes  the  hares,  Lepus,*  and  the  picas,  Lagomys.* 

Sub  Order  II.  SCIUROMORPHA.  The  squirrels,  SCIURID.E,  are  distin- 
guished by  the  soft  fur  and  bushy  tail.  Sciurus,*  squirrels ;  Cynomys,* 
prairie  dogs  ;  Sciuropterus,*  flying  squirrels.  The  CASTORID^E  have  soft  fur 
and  scaly  tail.  Castor  fiber  *  beaver  of  Europe  and  America. 

Sub  Order  III.  MYOMORPHA,  rats  and  mice.  Mus  musculus,* 
common  mouse;  Mus  rattus,*  house  rat,  once  abundant  but  now  replaced 
by  the  gray  rat,  M.  decumanus,*  an  immigrant  from  Asia.  White  rats  are 
albinos  of  M.  rattus.  Fiber  zibethicus*  musk  rat;  Arvieola*  field  mice. 

Sub  Order  III.  HYSTRICOMORPHA.  The  porcupines  (HYSTRICID^E) 
have  spines;  the  Old  World  forms,  Hystrix,  are  terrestrial,  ours  (Erethyzon) 
arboreal.  The  CAVHD.E  of  South  America  have  hoof-like  claws.  Cavia 
cobaya,  guinea  pig.  Hydrochosrtis,  capybara,  the  largest  existing  rodent. 

Order  V.  Ungulata. 

Under  the  heading  of  Ungulata,  or  hoofed  animals,  are  here 
included  two  groups  of  living  animals  in  which  the  body  weight 
is  supported  on  hoofs  on  the  tips  of  the  toes,  and  which  are  sharply 
marked  off  from  other  forms.  If,  however,  the  fossils  are  in- 
cluded, the  limits  of  the  group  must  be  extended  so  that  it  includes 
the  elephants  and  conies  of  the  existing  fauna  as  well  as  several 
extinct  forms,  for  these  so  interlock  and  intergrade  that  sharp 
lines  cannot  be  drawn. 

The  ungulates,  which  arise  from  common  ancestors,  the  Oon- 
dylarthra,  the  representatives  of  which  occur  in  the  eocene  of 
America  (Phenacodon),  are  preeminently  herbivorous;  the  canines 
are  rarely  well  developed,  the  molars  numerous  and  adapted  to 
grinding  the  food,  more  or  less  flattened  and  frequently  with 
folded  enamel.  The  mammae  are  inguinal,  the  uterus  bicornuate, 
and  the  placenta  either  diffuse  or  (most  ruminants)  cotyledonary 
(fig.  659).  The  legs  are  exclusively  locomotor  structures  and,  to 


640 


CHORD  ATA. 


permit  freer  motion,  the  clavicles  are  absent ;  the  feet  touch  but 
the  tips  of  the  toes,  enclosed  in  hoofs,  to  the  ground  (unguligrade). 
Since  the  metacarpals  and  metatarsals  are  greatly  elongate,  the 
wrist  and  ankle  are  raised  high  from  the  ground  so  that  they  are 
frequently  confounded  with  elbow  and  knee.  With  this  exclu- 
sively supporting  character  of  the  limbs  there  is  the  same  tendency 
to  reduction  and  fusion  of  bones  which  was  noticed  in  birds  (p. 
606).  There  is  a  constant  increase  in  the  development  of  radius 
and  tibia  to  the  chief  supports  of  the  body,  the  fibula  becoming 
rudimentary,  the  ulna  being  developed  sometimes  throughout  its 
whole  extent,  sometimes  only  in  its  upper  part,  which  serves  for  the 
attachment  of  muscles  (olecranon),  and  is  more  or  less  fused  with 
the  radius.  The  same  tendency  to  simplification  prevails  in  the 
feet,  but  is  expressed  differently  in  the  odd-toed  (perissodactyle) 
and  even-toed  (artiodactyle)  forms.  In  the  Perissodactyla  the 


\ 


FIG.  664.— Fore  feet  of  ungulates.  (After  Flower.)  A-C,  perissodactyle ;  D-F,  artio- 
dactyle. A,  tapir;  R,  rhinoceros;  C,  horse;  D,  pig;  E,  deer;  F,  camel,  c,  trique- 
trum  (ulnare);  /,  lunatum  (intermedium);  rn,  capitatum;  w2-w5,  rudiments  of 
metacarpals  II  and  V;  p,  pisiforme;  R,  radius;  ,s,  scaphoid  (radiale);  td,  trapezoid; 
tm,  trapezium;  U,  ulna;  w,  hamatum;  II- V,  digits. 

axis  of  pressure  passes  through  the  middle  toe  (fig.  664,  A—C, 
III),  while  the  other  toes  disappear  symmetrically  around  this. 
Since  the  first  toe  is  early  lost,  toe  V  is  next  to  disappear  (J?),  and 
then  toes  II  and  IV  (C),  so  that  at  last  there  remain  only  the 
skeleton  and  hoof  of  the  middle  toe  (horse),  the  rudiments  of  toes 
II  and  IV  persisting  as  the  small  splint  bones. 

In  the  Artiodactyla  the  axis  of  pressure  falls  between  toes  III 
and  IV  (fig.  664,  /)),  which  both  unite  in  supporting  the  body 
and  are  equally  developed  and  frequently  fuse,  at  least  so  far  as 
the  metacarpals  are  concerned  (E,  F}.  The  figures  D-F  show 


IV.    VERTEBRATA:  MAMMALIA,    UNO  UL  AT  A.  641 

how  the  other  digits  disappear,  digit  I  being  lost  still  earlier. 
Since  the  weight  of  the  body  rests  more  upon  the  hind  legs  than 
upon  the  front  ones,  the  former  are  the  first  to  become  modified. 
Since  we  are  able,  by  using  abundant  paleontological  material,  to 
follow  in  detail  the  lines  of  descent  of  both  artiodactyles  and  peris- 
sodactyles,  the  conclusion  is  certain  that  these  form  diverging 
series,  distinct  from  the  beginning.  In  each  series  most  of  the 
common  characters  enumerated  above  have  been  independently 
acquired  so  that  the  uniformity  in  appearance  of  the  various 
groups  of  ungulates  is  in  great  part  the  result  of  convergence. 
The  discussion  of  the  fossils  will  be  given  under  a  separate  head. 

Sub  Order  I.  PERISSODACTYLA  (Solidungula).  The  dentition  is 
peculiar  in  having  molars  and  premolars  (with  more  or  less  pronounced 
enamel  folds)  of  equal  size;  a  second  character  is  the  predominant  devel- 
opment of  the  middle  toe,  the  others  in  the  three  existing  families  reduced 
to  different  degrees.  TAPIRID^E,  fore  feet  four-toed,  hind  feet  three-toed; 
teeth  i^||;  nose  elongate  into  a  proboscis.  Tapirus,  tapirs,  tropical  Amer- 
ica and  India.  RHINOCEROTIDJE,  three  toes  on  all  feet,  teeth  f££f;  one  or 
two  horns  on  the  nasal  bones,  these  without  skeleton;  skin  thick,  hairless, 
hence  these  were  formerly  united  with  elephants  as  Pachydermata. 
Rhinoceros,  a  single  horn,  India;  Ceratorhmus  (Asia),  Atelodus  (Africa), 
have  two  horns.  EQUID.E,  a  single  functional  toe,  toes  II  and  IV  forming 
splint  bones  (fig.  664,  c);  teeth  |{|| ;  Equus  cdballus*  horse,  a  native  of 
Asia;  E.  asinus,  ass;  E.  zebra.  Hybrids  between  jackass  and  mare  are 
called  mules;  between  stallion  and  she-ass,  hinnies. 

Sub  Order  II.  ARTIODACTYLA.  Besides  the  features  of  the  feet, 
these  forms  have  the  premolars,  three  or  four  in  number,  smaller  than  the 
molars.  The  species  are  much  more  numerous  than  the  perissodactyles 
and  may  be  divided  into  three  sections.  Section  I,  NON-RUMINANTIA 
(Bunodontia);  omnivorous  and  have  correspondingly  a  bunodont  dentition, 

Jj|,  the    canines    frequently    developed   into    tusks;  the   stomach  is 

usually  simple,  but  is  occasionally  divided  into  three  chambers  (Dicotyles, 
Hippopotamus),  although  rumination  does  not  occur.  The  leg  skeleton  is 
little  modified  (fig.  664,  D),  ulna  and  fibula  not  being  reduced,  and  meta- 
carpals  and  metatarsals  separate.  HIPPOPOTAMUS  ;  all  four  toes  reach  the 
ground;  skin  thick  ('  pachyderm'),  body  heavy;  living  species  all  African. 
Hippopotamus.  SUID^E;  two  functional  toes,  skin  with  bristles,  snout 
proboscis-like.  Sus  scrofa,  swine ;  Dicotyles*  peccaries  of  warmer 
America. 

Section  II.  RUMINANTIA  (Pecora);  teeth  and  stomach  are  adapted  to 
the  exclusively  herbivorous  diet.  The  stomach  (fig.  665)  is  divided  into 
two  portions,  each  again  subdivided.  The  first  of  these,  the  rumen,  or 
paunch  (ru),  receives  the  food  as  it  is  eaten;  then  at  a  time  of  quiet  it  is 
regurgitated  into  the  mouth  and  ground  by  the  molars  ('  chewing  the  cud '). 
It  then  passes  back,  this  time  into  the  second  division,  the  honeycomb,  or 


642 


CHORD  AT  A. 


reticulum  (re),  thence  to  the  many  plies  or  omasum  (o),  and  lastly  to  the 
abomasum,  or  true  stomach  (a).  Usually  not  only  the  canines  but  the  in- 
cisors of  the  upper  jaw  are  degenerate,  while  the  incisors  of  the  lower  jaw 
are  strong  and  the  canines  have  taken  the  form  and  position  of  incisors. 
The  molars  are  seleriodont  (have  crescent-shaped  cusps).  With  few  excep- 
tions they  are  of  large  size  and  many  bear  horns  on  the  frontal  bones.  These 
are  larger  in  the  males  and  may  occur  exclusively  in  that  sex.  In  the  sim- 
plest case  (giraffes)  these  are  cones  of  horn  free  from  the  frontals  and  cov- 
ered with  skin.  In  others  (Cavicornia)  the  horn  cores  fuse  secondarily  with 


FIG.  665.— Stomach  of  sheep.    (After  Cams  and  Otto.)  a,  abomasum  (true  stomach); 
o,  omasum  (manyplies) ;  re,  reticulum  (honeycomb) ;  rw,  rumen  (paunch). 

the  frontals  and  are  covered  with  a  firm  sheath  of  horn.  Lastly,  the  horns 
are  outgrowths  of  the  frontal  bone,  in  which  usually  the  outer  coats  are 
lost  and  only  the  bone  projects  freely  (antlers).  These  are  shed  yearly, 
the  new  antler  which  takes  its  place  being  larger  and  consisting  of  a  larger 
number  of  branches  or  tines,  thus  constituting  an  index  of  age  (Cervicornia). 
CAMELOPARDALID^E  (Devexa),  giraffes,  long-legged  forms  (two  genera)  from 
Africa  with  persistent  horns;  teeth -$$ff,  Giraffa.  CERVID^E,  deer,  with 
deciduous  horns  in  the  male.  Cervus,*  common  deer;  Alces,*  moose; 
Rangifer*  reindeer;  MOSCHID^E,  horns  lacking,  males  with  enlarged  upper 
canines  and  with  a  musk  gland  (the  source  of  the  familiar  perfume)  on  the 
ventral  surface;  Moschus,  central  Asia.  The  TRAGULHWE,  primitive  Asiatic 
and  African  forms,  includes  the  chevrotain,  Tragulus  javanicus,  the  small- 
est living  ungulate.  The  CAVICORNIA  include  a  large  number  of  forms, 
some  of  great  economic  importance;  teeth  §fff.  BOVID^E:  Bos  taurus, 
domestic  cattle,  probably  descended  from  three  distinct  stocks  (B.  primi- 
genius,  the  aurochs,  B.  longifrons  and  B.  frontosus);  Bison,*  including 
B.  europeus,  the  bison  proper,  and  B.  americanus*  our  *  buffalo,'  so  near 
extinction;  Bubalus,  the  true  buffalo  of  the  Old  World.  OVHXE:  Ovisaries, 
sheep;  0.  montana*  big  horn;  Capra  hircus,  goat;  Ovibos  moschatus,* 
musk  ox.  ANTILOPID^E:  including  a  host  of  Old  World  forms  (Antilope, 
Oazella,  Rupicapra  tragus,  the  chamois,  etc.)  and  Antilocapra  americana,* 


IV.    VERTEBRATA:  MAMMALIA,  PROBOSCIDIA.          643 


the  prong  horn,  which  sheds  its  horns,  and  Hoploceras  montanus*  the 
Kocky  mountain  sheep. 

Section  III.  TYLOPODA,  stomach  without  manyplies,  no  frontal 
horns,  diffuse  placenta.  Camelus,  the  camels  of  the  Old  World;  C.  drome- 
darius,  one  hump;  C.  bactrianus,  two  humps.  Auchenia  lama,  A.  alpaca 
of  South  America. 

Paleontology  of  the  Ungulata. 

Extensive  paleontological  material,  especially  from  the  tertiary  rocks 
of  our  western  states,  has  cleared  up  many  lines  of  ungulate  descent 
and  has  rendered  it  probable  that  the  CONDYLARTHRA  of  the  eocene, 
with  five-toed  plantigrade  feet,  well-developed  ulna  and  fibula,  and  an 
omnivorous  dentition,  formed  the  stock  from  which  descended  the  artio- 
dactyles  and  perissodactyles,  and  possibly  carnivores  and  primates  as  well, 
the  ungulate  line  extending  through  the  Amblypoda.  From  one  group  of 
these  (the  PHENACODONTID^E)  the  lines  of  rhinoceros  and  tapir  have  come, 
and  in  an  almost  complete  series  we  know  the  ancestry  of  the  horse. 
Hyracotherium  (Eohippus)  and  Orohippus  of  the  eocene  had  the  fore  feet 
four-toed  (fig.  666.  1) ;  Pafaotherium  and  Mesohippus  (2}  of  the  lower 


FIG.  666.— Evolution  of  fore  foot  of  horse.  (From  Wiedersheim.)  1,  Orohippus 
(eocene):  2,  Mesohippus  (lower  miocene);  3,  Miohippus  (miocene);  It,  Protohippun 
(upper  pliocene);  5,  Pliohippus  (pleistocene);  0,  Equus. 

miocene  and  Miohippus  of  the  later  miocene  were  three-toed,  while  Mery- 
hippus  and  Hipparion  (Pliohippus,  4)  of  the  pliocene  were  near  the  horse 
in  tooth  structure.  The  single-toed  horses  appeared  in  the  pleistocene 
with  Pliohippus  (5)  and  then  Equus  itself  (6).  It  is  a  peculiar  fact  that 
the  horse  entirely  died  out  in  America,  although  the  chief  part  of  its  his- 
tory was  enacted  here. 

The  AMBLYPODA,  mentioned  above,  were  semi-plantigrade  penta- 
dactyle  forms,  appearing  in  the  lowest  eocene,  and  reaching,  in  Uinta- 
therium  (Dinocerus)  an  elephantine  size.  The  TOXODONTIA  of  the 
South  American  tertiaries  combined  perissodactyle,  rodent,  hyracoid,  and 
proboscidian  features,  while  the  TILLODONTIA  of  the  eocene  recall  both 
carnivores  and  rodents. 

Order  VI.  Proboscidia. 

The  elephants  and  their  allies,  with  their  hoofs  and  herbivorous 
•dentition,  are  closely  related  to  the  ungulates.  They  are  charac- 
terized by  their  thick  skin  (« pachyderm '),  the  large,  massive, 
five-toed  legs,  and  especially  by  the  nose  drawn  out  into  a 


644  CHORD  ATA. 

long  proboscis  with  a  finger-like  process  at  the  tip,  lastly  by 
the  dentition.  Canines  are  entirely  lacking,  but  the  incisors  of 
the  upper  jaw  have  pulps  and  therefore  continue  to  grow 
throughout  life,  forming  the  well-known  tusks.  In  the  living 
elephants  there  are  but  a  single  pair  of  tusks,  but  in  some  extinct 
Mastodons  there  were  a  second  smaller  pair  in  the  lower  jaw,  while 

in  Dinotherium  only  the  lower  in- 
cisors were  developed,  these  pro- 
jecting downwards.     The  molars 
(in   Mastodon   and    Dinotherium 
with     normal     replacement    and 
cusps)  consist  of  numerous  plates 
of    enamel    and   dentine    united 
FiG.667.-inside~71e7t  lower  jaw  of  bJ  cement,  and  undergo  a  lateral 
Sff  owenri,  fuLtfon^mXr;6!;  displacement.     Of  the  three  large 
its  successor.  molars  and   premolars   only   one 

at  a  time  is  functional  (fig.  667,  ./);  when  worn  out  the  next  one 
behind  (2)  takes  its  place.  Further  features  are  a  uterus  bicornis, 
a  zonary  placenta,  and  two  pectoral  mammas. 

ELEPHANTINE  :  Elephas  indicus,  small  ears ;  E.  africanus,  large 
ears.  E.  primigenius,  mammoth,  in  the  pleistocene ;  specimens  found 
frozen  in  ice  in  Siberia  have  close  woolly  hair,  in  some  places  three  feet 
long.  Mastodon,  with  tuberculate  teeth,  range  from  miocene  through  the 
pliocene.  DINOTHERIDJE,  only  lower  incisors  ;  Dinotherium,  Old  World 
miocene. 

Order  VII.  Hyracoidea. 

The  single  genus  Hyrax,  including  species  from  western  Asia 
and  Africa,  with  four-toed  front  feet,  hind  feet  with  three  toes, 
the  digits  with  nails,  the  placenta  zonary,  and  the  dentition  |~§£f , 
forms  this  group,  no  fossils  being  known.  Hyrax  syriacus  is  sup- 
posed to  be  the  '  coney '  of  the  Bible. 

Order  VIII.  Sirenia. 

This  order  consists  of  a  few  aquatic  mammals  which  are  whale- 
like  in  form,  with  the  fore  limbs  fin-like,  the  hind  legs  lacking,  and 
a  horizontal  caudal  fin.  They  live  in  shallow  seas  or  in  the 
mouths  of  rivers,  where  they  feed  on  the  tang,  which  they  chew 
with  jaws  covered  with  horny  plates.  The  teeth  (in  the  fossil 
Prorostomus  f  yf-f )  are  reduced  or  entirely  lacking.  The  fore  legs 
are  pentadactyle  and  often  have  rudimentary  nails  and  always  a 
flexible  elbow.  The  two  pectoral  mammae  have  possibly  caused 
these  animals  to  furnish  the  germ  of  truth  in  the  mermaid  myth. 
Manatus  americanus,*  the  manatee,  six  cervical  vertebrae,  eight  to 


IV.    VERTEBRATA:   MAMMALIA,   CETACEA.  645 

ten  molars;  Halicore  dugong,  Indo-Pacific;  Rhytina  stelhri  of  the 
northern  Pacific,  exterminated  in  1768. 

Order  IX.  Cetacea. 

In  external  form  the  whales  resemble  the  sirenians,  a  result  of 
an  aquatic  life,  but  the  resemblance  ends  here.     The  whales  are 


FIG.  668.— Restoration  of  skeleton  of  HaUtherium,  an  extinct  sirenian.    (After  Miss 

Woodward.) 

so  fish-like  that  they  are  commonly  included  by  the  laity  in  that 
group,  and  every  one  speaks  of  the  whale  fishery.  Head  and  trunk 
are  scarcely  distinguished,  the  cervical  vertebrae  being  very  short 
and  more  or  less  completely  fused.  The  hinder  limbs  are  absent, 
and  of  the  pelvic  girdle  only  a  small  ilium  remains,  and  no  sacral 
vertebras  are  developed.  The  caudal  fin  is  two-lobed  and  differs 
from  that  of  a  fish  in  being  horizontally  flattened ;  the  skin  is  thick 
and  is  sparsely  haired  or  completely  naked,  in  some  hair  being 
lacking  even  in  the  embryo.  Most  of  the  species  inhabit  the  high 
seas,  Inia  botiviensis  and  Platanista  gangetica  occur  in  rivers. 

The  fore  limbs  are  modified  into  flippers,  the  bones  of  which  are  of 
nearly  equal  size  and  are  jointed  only  at  the  shoulder.  A  dorsal  fin  ( '  fin 
backs ' )  occurs  in  some.  The  lack  of  hair  is  compensated  by  the  thick 
layer  of  subcutaneous  fat  (blubber)  which,  like  the  fat  penetrating  the 
spongy  bones,  tends  to  lessen  the  specific  gravity.  In  order  that  the  ani- 
mals may  breathe  while  feeding,  the  larynx  is  prolonged  into  a  tube 
which  extends  up  through  the  pharynx  to  the  choanaB,  from  which  the 
nostrils  extend  directly  upwards  to  the  single  (Denticetes)  or  paired  (Mys- 
ticetes)  external  opening.  Since  the  air  driven  out  with  great  force  con- 
tains much  moisture  and  this  is  condensed  on  contact  with  the  cooler 
external  air,  the  impression  was  natural  that  the  animals  in  'blowing' 
spouted  water.  Since  the  olfactory  membrane  is  degenerate  and  the 
olfactory  lobes  are  reduced,  the  nose  is  an  organ  of  respiration  only. 

The  eyes  are  small,  external  ears  are  lacking,  the  mammas  are  close  to 
the  sexual  opening.  Tho  teeth  are  either  present  in  large  numbers,  similar 
and  conical,  and,  since  the  second  dentition  is  rudimentary,  are  mono- 
phyodont  (Denticetas)  or  they  are  outlined  early  and  then  resorbed  and 
replaced  by  plates  of  baleen  (Mysticetaa).  This  is  composed  of  large  horny 
plates  (whalebone)  in  large  animals  a  dozen  feet  long  (fig.  669,  ?/;),  of 
which  several  hundred  are  arranged  in  close  succession  extending  inward 
to  the  tongue.  They  correspond  to  the  transverse  palatal  folds  which 


646 


CHORD  AT  A. 


occur  in  other  mammals.  As  they  are  fringed  on  the  inner  edges  they 
form  a  strainer  which  retains  the  small  marine  animals  (plankton,  Ceto- 
cliilus  septentrionalis,  a  copepod,  and  Clione  borealis,  a  pteropod)  on  which 
these  whales  feed.  The  oesophagus  is  too  narrow  for  the  passage  of 
much  larger  animals. 

The  origin  of  the  whales  is  one  of  the  unsolved  problems.     That  they 
came  from  some  terrestrial,  quadrupedal  forms  is  beyond  question,  and  the 


FIG.  669.— Section  through  jaws  of  whalebone  whale.  (After  Delage.)  c,  septum  of 
nose;  m,  mouth  cavity;  ma;,  maxillary  bone;  p,  premaxillary  (hinder  end);  t% 
vomer ;  tt>,  baleen. 

little  evidence  would  seem  to  point  to  an  ungulate  or  a  carnivore  ancestry. 
It  is  possible  that  the  toothed  and  whalebone  whales  may  have  had  differ- 
ent ancestries,  and  their  resemblances  may  be  the  result  of  convergence. 

Sub  Order  I.  ZEUGLODONTA.    Extinct  (eocene)  forms  with  hetero 
dont  dentition,  the  posterior  teeth  two-rooted. 

Sub  Order  II.  DENTICETJE,  toothed  whales,  carnivorous,  some  hav- 
ing but  two  teeth.  Delphinus,  dolphins  ;  GloMocephalus*  black  fish  ; 
Monodon,  narwal,  with,  in  the  male,  a  long  maxillary  tusk  (possible  origin 
of  the  'unicorn').  Physeter  macrocephalus,  sperm  whale,  pursued  for 
the  spermaceti,  an  oily  mass  situated  in  the  'chair'  between  the  cranium 
and  the  snout,  as  well  as  for  ambergris,  formed  in  the  intestines. 

Sub  Order  III.  MYSTACETI,  whalebone  whales,  with  baleen.  Bala- 
noptera*  rorquals  and  fin  backs.  B.  sibbaldi,*  the  largest  whale,  reach- 
ing a  length  of  eighty-five  feet.  Balcena,  right  whale. 

Order  X.  Carnivora. 

The  carnivores  live  chiefly  on  the  flesh  and  blood  of  other  ver- 
tebrates, which  they  catch  by  craft,  by  coursing,  or  by  pouncing 
upon  them,  overpowering  their  prey  by  their  sharp  claws  and 
cutting  teeth.  With  this  mode  of  life  correspond  the  high  devel- 
opment of  the  brain  (fig.  649,  B)  and  sense  organs,  as  well  as 


IV.    VERTEBEATA:  MAMMALIA,   CARNIVOEA.  647 

structure  of  teeth  and  claws.  Since  this  predaceous  character  in- 
creases within  the  order  from  the  bears  to  the  cats,  and  again  tends 
to  disappear  in  the  aquatic  species,  there  are  few  constant  charac- 
ters, but  a  great  variation  in  structure.  In  interest  of  greater 
mobility  the  clavicle  is  reduced  or  lost,  ulna  and  radius  well  de- 
veloped. In  the  structure  of  the  feet  there  is  a  gradual  transition 
from  the  plantigrade  bears,  in  which  the  whole  sole  of  hand  and 
foot  rest  upon  the  ground,  to  the  digitigrade  cats,  which  tread  on 
the  tips  of  the  toes.  In  the  latter  the  claws,  which  occur  in  all 
carnivores,  are  kept  from  injury,  when  not  in  use,  by  being  re- 
tracted by  an  elastic  ligament  into  pockets  on  the  penult  joint, 
from  which  they  are  extended  by  strong  muscles.  In  dentition 
(fig.  650)  the  striking  features  are  the  almost  constantly  three 
incisors,  and  the  great  size  of  the  canines;  the  molars,  on  the  other 
hand,  vary  with  the  different  families,  the  cusps  assuming  more  of 
the  shearing  character  (secodont  teeth).  The  last  premolar  of  the 
upper  jaw  and  the  first  molar  of  the  lower  jaw  become  carnassial 
teeth  (sectorial  teeth),  and  acquire  a  dominating  position  in  the 
jaw,  while  the  others  become  smaller  and  tend  to  disappear  at 
either  end  of  the  series.  Further  characters  are  the  possession  of 
a  penis  bone  in  the  males,  the  abdominal  position  of  the  milk 
glands  and  the  uterus  bicornis  in  the  females;  the  placenta  is 
zonary.  Anal  glands,  furnishing  a  strong,  even  offensive  smelling 
secretion,  are  of  wide  occurrence. 

Sub  Order  I.  FISSIPEDIA.  These  are  the  typical  members  of  the 
order  and  are  preeminently  terrestrial  animals  with  well-developed  toes 
usually  cleft  to  the  base.  The  number  of  digits  is  frequently  five  on  all 
feet,  but  is  often  reduced  to  four  on  the  hind  feet  (Felidse,  Canida?),  rarely 
on  the  fore  feet  (Hyeenidse);  but  in  these  cases,  as  in  the  domestic  dog,  the 
reduced  first  toe  may  bear  a  claw.  URSID^E,  plantigrade;  Ursus,*  bears; 
Procyon  lotor*  raccoon.  MUSTELINE;  many  species  ofMustela  *  and  Puto- 
rius*  which  include  minks,  martens,  sable,  ermines,  and  weasels,  are 
valuable  for  their  fur;  Lutra,*  otter ;  Enhydris,*  sea  otter;  Mephitis,* 
skunk;  Taxidea*  badger ;  Grido*  glutton  ;  anal  glands  common  in  this 
family.  Fossils  (Arctotheriwn,  etc.)  connect  the  bears  and  the  CANID^E 
with  five  toes  in  front,  four  behind,  claws  not  retractile ;  which  includes 
in  the  genus  Canis*  dogs,  foxes,  and  wolves.  The  FELIDJE  have  toes  as  in 
the  dogs,  but  with  retractile  claws.  Felis  domestica,  our  domestic  cat. 
F.  leo,  lion;  F.  tigris,  tiger;  F.  concolor,*  puma  or  cougar.  HT^BNIDJE, 
all  feet  four-toed;  Hycena  of  Africa. 

Sub  Order  II.  PINNIPEDIA.  These  are  aquatic  carnivores  with  the 
limbs  flattened  to  broad  flippers,  the  five  toes  long  and  webbed,  the  nails 
frequently  rudimentary  ;  the  dentition  differs  from  that  of  the  terrestrial 
forms  in  the  similarity  of  molars  and  premolars  (absence  of  carnassial)  ; 


648 


CHORD  AT  A. 


the  milk  dentition  degenerates  early,  without  being  functional.  PHOCID.E. 
seals,  without  external  ears ;  Phoca  vitulina,*  harbor  seal.  OTARIID^E, 
with  external  ears  ;  Otaria,*  sea  lions  ;  Callorhinus  ursinus,  fur  seal  of 
Alaska.  TRICHECHID^E  ;  incisors  reduced,  upper  canines  developed  into 
large  tusks ;  Trichechus,  walrus. 

The  first  carnivores  appear  in  the  eocene  in  the  order  CREODONTA, 
plantigrade  forms  with  slightly  differentiated  dentition   (no  carnassial)  ; 


FIG.  670.— Phoca  vitulina,  harbor  seal.    (After  Elliott.) 

they  present  marked  resemblances  to  marsupials,  insectivores,  as  well  as  to 
the  Condylarthra,  the  ancestral  ungulates.  True  carnivores  appear  in 
the  upper  eocene  and  become  abundant  in  the  miocene. 

Order  XI.  Prosimiae. 

Linne  united  with  the  true  apes  a  small  group  of  animals 
known  as  lemurs  (from  India  and  the  adjacent  islands,  and 
especially  from  Africa),  because  of  similarity  in  body  form  and 
climbing  habits,  because  they  had  grasping  hands  and  feet  (oppos- 
able  thumb  and  great  toe),  and  at  least  frequently  nails  on  some 
of  the  toes.  To-day  many  set  them  aside  as  a  separate  order  on 
account  of  their  lower  organization.  They  have  a  less-developed 
cerebrum,  uterus  bicornis,  and  a  diffuse  placenta.  Further 
peculiarities  are  the  peculiar  and  variable  dentition  (Chiromys 
•}{)-if,  Lemur  f|||)  and  the  presence  of  claws,  which  always  occur 
on  the  second  and  frequently  on  the  third  finger  of  the  hind  feet, 
and  in  Chiromys  replace  the  nails  on  all  the  digits  of  all  the  feet 
except  the  great  toe.  Their  nocturnal  habits  have  resulted  in 


IV.    VERTEBRATA:  MAMMALIA,  PRIMATES.  649 

large  eyes,  which  give  these  animals  a  most  striking  appearance. 
A  distinction  from  the  primates  is  the  connexion  of  orbital  and 
temporal  cavities  beneath  the  osseous  postorbital  ring.  Usually 
there  are  a  pair  of  pectoral  mammae,  to  which  are  added  in  many 


Fio.  671.— Stenops  gracilis,  slender  loris.     (From  Brehm.) 

species  a  pair  in  the  abdominal  or  inguinal  region,  the  latter  alone 
occurring  in  Chiromys. 

CHIROMYID^E,  digits  long,  all  except  the  great  toe  with  claws  ;  Chiromys 
madagascar  ensis,  aye-aye.  TARSIID^E,  second  and  third  hind  toes 
clawed.  Tarsius  spectrum  of  the  East  Indies  differs  from  all  Prosimite 
in  having  the  orbits  closed  and  a  discoidal  placenta  like  that  of  man. 
LEMURID^E,  second  hind  toe  alone  clawed.  Lemur;  Stenops,  loris.  The 
old  tertiary  PACHYLEMURID.E  and  ANAPTOMORPHID^E  are  close  to  the  most 
primitive  mammals  and  to  the  creodonts  and  insectivores.  The  GALEO- 
PITHECID^E  (p.  637)  are  often  referred  here. 

Order  XII.  Primates. 

The  most  highly  organized  mammals,  the  monkeys,  apes,  and 
man,  are  united  in  a  single  order  because  among  them  there  is  a 
great  agreement  in  features  of  classificatory  value.  If  we  here,  as 
elsewhere,  ignore  grades  of  intelligence  and  regard  alone  greater 
or  lesser  anatomical  resemblances,  we  are  forced  to  the  conclusion 
that  the  anthropoid  apes  are  much  closer  to  man  than  to  the  lower 
monkeys. 

The  primates  have  in  common  nails  on  all  the  fingers  and  toes 
(except  the  Hapalidae),  orbits  separated  from  the  temporal  fossae 
by  a  bony  wall,  and  a  cerebrum  which  covers  the  other  parts  of 


650 


CHORD  AT  A. 


the  brain  (fig.  649,  c).  They  have  a  single  pair  of  pectoral 
mammae,  uterus  simplex,  and  a  discoidal  placenta.  The  dentition 
is  essentially  the  same  throughout;  in  the  Platyrrhinae  f  Jf |,  in  the 
Ilapalidse  ffff ,  in  the  Catarrhinse  and  in  man  fff|.  Yet  there  is 
a  tendency  to  variation,  since  in  the  chimpanzee  and  in  man  the 
third  molar  (wisdom  tooth)  is  in  process  of  degeneration,  while  in 
the  orang  a  fourth  molar  often  occurs.  In  all  the  molars  are 
bunodont. 

The  skeleton  of  the  hand  and  foot  has  played  an  important 
role  in  classification.  As  in  the  lemurs  and  opossums,  the  thumb 
and  great  toe  can  be  opposed  to  the  other  digits,  so  that  an  ape  can 
grasp  objects  with  either  hand  or  foot.  In  man  this  opposability 
of  the  thumb  is  increased,  but  that  of  the  great  toe,  in  consequence 
of  the  upright  position,  is  only  retained  to  a  slight  degree  by  chil- 
dren and  primitive  people.  On  this  peculiarity  rest  the  names  often 
given  of  Bimana,  for  man,  and  Quadrumana,  for  the  apes  and 
monkeys.  In  contradiction  of  this  it  must  be  emphasized  that  the 
apes  do  not  have  a  hand,  but  rather  a  grasping  foot,  on  the  hinder 
extremities.  In  the  grasping  foot  (fig.  672)  are  the  same  bones, 


Fio.  672.— Hand  and  foot  of  gorilla,  c,  capitatum;  ca,  calcaneus;  CM,  cuboid;  7i,  ha- 
matum  ;  i,  lunatum  ;  me,  metacarpals  ;  ??»t,  metatarsals  ;  n,  iiaviculare ;  p,  pisi- 
forrne;  ph,  phalanges  ;  s,  scaphoid;  t,  triquetrum;  <a,  talus  ;  td,  trapezoid ;  tr,  tra- 
pezium ;  /-  F,  digits ;  1-3,  cuneiformia. 

similarly  arranged  and  of  about  the  same  shape  as  in  the  foot  of 
man,  while  the  musculature  is  essentially  the  same.     On  the  other 


INDEX. 


Aard  vark,  636 
Abalone,  379 
Abdominal  cavity,  546 
Abdominales,  562 
Abdominal  fin,  562 
Abdominalia,  426 
Abducens  nerve,  536 
Abomasum,  642 
Acantharia,  196 
Acanthia,  489 
Acanthias,  569,  571 
Acanthiidae.  489 
Acanthin,  195 
Acanthobdella,  318 
Acanthoderus,  48 
Acanthobothrium,  286 
Acanthocephala,  304 
Acanthocotyle,  274 
Acanthodes,  572 
Acanthodidoe,  572 
Acanthoglossus,  632 
Acanthometra,  194 
Acanthophracta,  196 
Acanthopteri,  57^,  577 
Acanthopterygii,  577 
Acaridse,  454 
Acarina,  453 
Accessory  nerve,  536 
Accessory  tissue,  99 
Accessory  yolk,  80 
Accipiter,  617 
Acephala,  358 
Acerata,  442 
Acetabula,  280 
Achseta,  317 
Achatina,  383 
Achoreutes,  477 
Achromatin,  65 
Achtheres,  36,  422 


Aciculum,  308 
Acineta,  212 
Acinetaria,  212 
Acinous  glands,  77 
Acipenser,  573 
Acipenseridae,  573 
Acmaea,  378 
Acmaeidae,  378 
Accela,  271 
Acontia,  253 
Acorn  barnacle,  423 
Acrania,  502 
Acraspedia,  246 
Acraspedota,  246 
Acridiidae,  481 
Acridium,  481 
Acris,  588 
Acrodont  teeth,  599 
Actinaria,  259 
Actinian,  section  of,  136 
Actinophrys,  191,  192 
Actinopoda,  349 
Actinosphaerium,  190,  192 
Actinotrichia,  527 
Actinotrocha,  325 
Actinozoa,  251 
Aculeata,  486  , 

Aculeus,  472,  476 
Acustic  nerve,  536 
Adambulacral  plate,  335 
Adamsia,  254 
Adhesive  cells,  264 
Adipose  fin,  576 
Adoral  band,  209 
Adradius,  246 
&ga,  441,  442 
^gina,  242 
JEotidx,  382 
^olidia,  382 


657 


658 


INDEX. 


^piornis,  613 

./Equoria,  240,  242 

yEschna,  479 

^Esthetes,  357 

^thalium,  199 

Aftershaft,  603 

Agalmia,  244 

Agamidae,  599 

Agassiz,  21 

Agelacrinoidea,  342 

Agelacrinus,  342 

Agkistrodon,  60 1 

Aglaophenia,  242 

Aglossa,  588 

Aglypha,  601 

Agnatha,  555 

Agrion,  479 

Air  bladder,  567 

Air  pipes,  609 

Air  sacs  of  birds,  608 

Alae,  466 

Alae  cordis,  470 

Alauda,  616 

Alaudidse,  616 

Albatross,  615 

Albertus  Magnus,  9 

Alca,  615 

Alcedinidae,  616 

Alces,  642 

Alcidae,  615 

Alciopidae,  313 

Alcippe,  424 

Alcyonaria,  258 

Alcyonidae,  259 

Alcyonidium,  324 

Alcyonium,  255,  259 

Aletia,  495 

Alima,  429 

Alimentary  tract  of  vertebrates,  546 

Alisphenoid  bone,  522 

Allantoidea,  553 

Allantois,  553 

Alligator,  602 

Alligator  turtle,  596 

Allobophiora,  316 

Alloposus,  395 

Allotheria,  632 

Alosophila,  494 


Alpaca,  643 

Alpheus,  434 

Alternation  of  generations,   144,  5 12 

Altrices,  612 

Alula,  604 

Alveolar  duct,  548 

Alveoli,  625 

Alveolus,  548 

Alytes,  585 

Amaroucium,  510 

Ambergris,  646 

Amblyopsidse,  576 

Amblypoda,  643 

Amblystoma,  587 

Amblystoma,  larva  of,  36 

Ambulacra,  331 

Ambulacral  grooves,  335 

Ambulacral  plates,  335 

Ambulacral  system,  122,  330 

Ambulacral  vessels,  331 

Ametabolous,  473 

Amia,  574 

Amia,  tail  of,  41 

Amicula,  357 

Amiidse,  574 

Ammocoetes,  557 

Ammonitidae,  394 

Amnion,  472,  553 

Amniota,  553,  588 

Amoeba,  61.  62,  187,  189 

Amoebina,  189 

Amoeboid  motion,  187 

Amphacanthe,  574 

Amphiaster,  70 

Amphibia,  580 

Amphibiotica,  479 

Amphiccele,  518 

Amphicoelias,  596 

Amphidiscs,  227 

Amphigony,  142 

Amphilina,  286 

Amphineura,  356 

Amphioxus,  502,  504 

Amphioxus.  cleavage  of,  151 

Amphioxus,  gastrula  of,  156 

Amphiphorus,   291 

Amphipoda,  438 

Amphisbaena,  599 


INDEX. 


659 


Amphistomum.  278 
Amphithoe,  428 
Amphitrite,  312,  313 
Amphiuma,  585,  587 
Amphiura,  338 
Amphoridae,  342 
Ampullaridae.  380 
Ampullae,  223,  331,  542 
Amyda,  596 
Anabrus,  481 
Anacanthini,  577 
Anaconda,  601 
Anal  glands.  106 
Anal  fin,  526,  562 
Anallantoidia,  553,  555 
Analogy,  14,  100 
Anamnia,  553,  555 
Anaplocephalus,  287 
Anaptomorphidge,  649 
Anas,  615 
Anaspides,  437 
Anatomy,  57,  58 
Anatomy,  comparative,  2 
Anaxial  form,  135 
Androctonus,  400 
Anelasma,  423,  424 
Angiostoma,  601 
Anguillidae,  576 
Anguillula,  300 
Anguillulidae,  300 
Anguis,  599 
Angulare,  582 
Animal  morphology,  57 
Animal  organs,  101.  121 
Animal  pole,  147,  151 
Animals  and  plants,  171 
Anisomyaria,  367 
Anisopoda,  442 
Annelida,  305 
Annulata.  599 
Anodonta,  361,  367 
Anolis,  599 
Anomodontia,  594 
Anopheles,  217,  492 
Anser,  615 
Anseriformes,  615 
Antarctic  province.  178 
Ant  eaters,  636 


Antedon,  339,  342 
Antenna,  401,  410,  430,  463 
Antennal  gland,  411 
Antennulae,  430 
Antheomorpha,  251 
Anthomastus,  259 
Anthomedusse,  239,  241 
Anthomyia,  492 
Anthozoa,  251 
Anthropinse,  651 
Anthropoidae,  651 
Anthropopithecus,  652 
Antilocapra,  642 
Antilope,  642 
Antilopidae,  642 
Antimeres,  137 
Antipatharia,  259 
Antipathes,  259 
Antlers,  642 
Ant  lion,  481,  483 
Antrostomus,  616 
Antrum  of  Highmore,  539, 
Ants,  487 
Ants,  white,  478 
Anura,  588 
Anurida,  477 
Aorta,  548 

Aorta  ascendens,  504,  549 
Aorta,  descendens,  504 
Apes,  651 
Aphaniptera,  493 
Aphidae,  490 
Aphrodite,  313 
Apiariae,  487 
Apical  plate,  306 
Apis,  487 
Aplacophora,  358 
Aplysia,  381 
Aplysilla,  226 
Aplysina.  224,  226 
Apoda,  349,  425,  587 
Apodidae,  416 
Apophysis,  517 
Aporosa,  260 
Appendicularia,  506 
Aprophora,  489 
Aptenodytes,  615 
Aptera,  491 


660 


INDEX. 


Apteria,  604 
Apteryges,  613 
Apterygota,  477 
Apteryx,  613 
Apus,  416 
Aquatic  faunae,  179 
Aqueduct  of  Sylvius,  534 
Aqueous  humor,  131 
Aquila,  617 
Arachnida,  444 
Araneina,  451 
Arbacia,  345 
Arcella,  198 
Archaean  era,  180 
Archaeopteryx,  33,  612 
Archegony,  139 
Archenteron,  103,  104,  156 
Archianellidse,  313 
Archigetis,  286 
Archiptera,  477 
Archipterygium,  529 
Architeuthes,  384,  395 
Arcidae,  367 
Arcifera.  588 
Arctic  province,  178 
Arctogaea,  177 
Arctotherium,  647 
Arcyria,  199 
Ardea,  615 
Arenicolidae,  313 
Areolar  connective  tissue,  85 
Argas,  454 
Argina,  367 
.Argiope,  327,  453 
Argonauta,  392,  395 
Argonautidae,  395 
Argulidae,  422 
Argulus,  421,  422 
Ariolimax,  383 
Arion,  383 
Arista,  493 
Aristotle,  7 

Aristotle's  lantern,  345 
Armadillidium,  442 
Armadillo,  636 
Armata,  317 
Army  worm,  495 
Artemia,  416 


Arterial  arches,  549 

Arteries,  112 

Arthrodira,  579 

Arthrogastrida,  447 

Arthropoda,  398 

Arthrostraca,  438 

Articular  bone,  525 

Articular  process,  519 

Articulata,  342,  398 

Artificial  impregnation,  147 

Artificial  selection,  43 

Artiodactyla,  640,  641 

Arvicola,  639 

Ascalabotae,  598 

Ascaridae,  301 

Ascaris,  145,  299 

Ascaris,  fertilization  in,  150 

Ascidiseformes,  508 

Ascidians,  505 

Ascones,  222,  225 

Ascyssa,  224 

Asellidae,  442 

Asellus,  440 

Asexual  reproduction,  140,  143 

Asilidas,  493 

Asp,  60 1 

Aspergillum,  368 

Aspidobranchia,  378 

Aspidonectes,  596 

Aspidotus,  490 

Ass,  641 

Assimilation,  organs  of,  102 

Astacidae,  435 

Astacoidea,  435 

Astacus,  431,  43 2>  435 

Astarte,  367 

Astartidse,  368 

Asterias,  337 

Asterias,  early  development,  146,  148 

Asteridae,  337 

Asterinidae,  337 

Asteriscus,  335,  337,  564 

Asteroidea,  333 

Astrsea,  261 

Astrangia.-  260,  261 

Astroides,  261 

Astrophyton,  338 

Asymmetrical  form,  135 


INDEX. 


661 


Asymmetron,  504 
Atalapha,  638 
Atax,  454 
Ateles,  651 
Atelodus,  641 
Atheca,  595 
Atlantidse,  380 
Atlas,  581,  590 
Atoke,  310 
Atoll,  258 
Atolla,  250 

Atrium,  in,  506,  508 
Atrypa,  328 
Attus,  453 
Atypus,  453 
Auchenia,  643 
Auditory  meatus,  545 
Auditory  nerve,  536 
Auditory  organs,    127 
Auk,  615 
Aulacantha,  196 
Aulophorus,  315 
Aulosphsera,  196 
Aurelia,  245,  250 
Auricle,  ill,  548 
Auricularia,  332 
Aurochs,  642 
Australian  region,  176 
Autoflagellata,  200 
Autoinfection,  215 
Autolytus,  313,  314 
Aves,  603 
Avicularia,  323 
Aviculidae,  367 
Aye-Aye,  649 
Axial  skeleton,  526 
Axiothea,  313 
Axis,  590 
Axis  cylinder,  96 
Axolotl,  587 
Axons,  94 
Azoic  era,  1 80 
Azygobranchia,  379 

Baboons,  651 
von  Baer,  17 
Bsetisca,  479 
Badger,  647 


Balsena,  646 

Balaenoptera,  646 

Balancers,  491 

Balaninus,  485 

Balanoglossus,  513 

Balantidium,  209,  210 

Balanus,  423 

Baleen,  645 

Bandicoot,  633 

Barbs   603 

Barbules,  603 

Barnacles,  423 

Basalia,  330,  340,  527 

Bascanion,  601 

Basioccipital  bone,  522 

Basiopodite,  410 

Basisphenoid  bone,  522 

Bass,  black^  577 

Bassomatophora,  383 

Bath  sponges,  227 

Bats,  637 

Bdellodrilus,  315 

Bdellostoma,  557 

Bdelloura,  271 

Bears,  647 

Beaver,  639 

Bedbug,  489 

Bee,  larva  of,  105 

Bees,  487 

Beetles,  483 

Bela?  380 

Belemnites,  388 

Bellovacensis,  9 

Bell's  law,  536 

Belosepia,  388 

Belostoma,  489 

Belostomidse,  489 

Beroe,  264 

Beroidge.  264 

Bestiarius,  9 

Bicidium,  259 

Bicoseca,  201 

Bicuspid  teeth,  626 

Big  horn,  642 

Bilateral  symmetry,  131 

Bilharzia,  277 

Bimana,  650 

Biogenesis,  fundamental  law  of,  34. 


662 


INDEX. 


Biology,  4,  57 
Bipalium,  271 
Bipinnaria,  332 
Biradial  symmetry,  136 
Bird  lice,  479 
Birds,  603 

Birds  of  paradise,  50,  616 
Birgus,  432,  436 
Bison,  642 
Bittacus,  483 
Bivium,  334 
Black  bass,  577 
Black  fish,  646 
Black  flies,  493 
Black  snake,  601 
Bladder,  urinary,  552 
Bladder  worm,  278,  284 
Blarina,  637 
Blastoderm,  153 
Blastodermic  vesicle,  155 
Blastoidea,  342 
Blastomeres,  151 
Blastopore,  156 
Blastostyle,  242 
Blastula,  151,  155 
Blatta,  480 
Blattidse,  480 
Blind  fish,  576 
Blissus,  489 
Blister  beetle,  484 
Blood,  88,  in 
Blood  corpuscles,  88 
Bloodvessels,  no,  ill 
Blow  flies,  493 
Blue  birds,  6iC 
Boa,  60 1 
Bobolink,  616 
Body  cavity,  109 
Bojanus,  organ  of,  363 
Bolina,  264 
Bombycina,  495 
Bombyx,  495 
Bonasa,  614 
Bone,  86 

Bonellia,  317,  318 
Book  lice,  479 
Bopyridse,  442 
Bopyrus,  442 


Bos,  642 
Bosmina,  417 
Botall's  duct,  550 
Bot  flies,  493 
Bothriocephalidae,  287 
Bothriocephalus,  281,  283,  287 
Bothrops,  60 1 
Botryllus,  510 
Bougainvillea,  144,  241 
Bovidae,  642 
Bow  fin,  574 
Box  turtle,  596 
Brachialia,  340 
Brachiolaria,  332 
Brachiopoda,  325 
Brachycera,  493 
Brachyura,  437 
Braconidse,  486 
Bradypus,  636 
Brain  coral,  261 
Branchial  arch,  524 
Branchial  chamber,  352,  506 
Branchial  clefts,  501 
Branchial  heart,  391 
Branchial  tree,  348 
Branchiata,  408 
Branchiomerism,  523 
Branchiopoda,  416 
Branchiostegal  membrame,  56 
Branchiostegal  rays,  562 
Branchiostegite,  431 
Branchipidae,  416 
Branchipus,  416 
Branchiura,  422 
Braula,  493 
Breast  bone,  518 
Brevilinguia,  599 
Brissus,  346 
Bristles,  311 
Bristles,  tactile,  126 
Bristle  tails,  477 
Brittle  stars,  337 
Bronchiole,  548 
Bronchus.  547 
Bryozoa,  321 
Bubalus,  642 
Bubo,  617 
Buccal  cavity,  106 


INDEX. 


663 


Buccal  ganglion,  390 

Buccinidae,  380 

Buccinum,  379 

Bucerontidae,  616 

Budding,  141 

Budding  and  germ  layers,  159 

Buffalo,  642 

Buffalo  leaf  hopper,  490 

Buffon,  21 

Bufo,  588 

Bufonidae,  588 

Bugs,  489 

Bugula,  324 

Bulbus  arteriosus,  568 

Bulbus  olfactorius,  534 

Bulimus,  383 

Bulla,  381 

Bulla  ossea,  621 

Bunodes,  259 

Bunodontia,  641 

Bunodont  teeth,  626 

Burbot,  578 

Bursa,  331,  338 

Bursa  copulatrix,  471 

Bustard,  615 

Buteo,  617 

Buthus,  448 

Butrinus,  575 

Butterflies,  494,  495 

Butterflies,  leaf,  47 

Buzzard,  617 

Byssus,  363 

Cabbage  worm,  496 
Cacatua,  616 
Cacospongia,  227 
Caddis  flies,  483 
Csecidotea,  442 
Caecilia,  587 
Caecum,  106,  461,  627 
Csenolestes,  633 
Caenosarc,  231 
Calamoichthys,  573 
Calandra,  485 
Calanidae,  421 
Calappa,  437 
Calcispongiae,  225 
Caligidae,  422 


Caligus,  422 

Callianira,  263 

Calliphora,  493 

Callorhinus,  648 

Calosoma,  484 

Calycella,  242 

Calyconectae,  244 

Calycophorse,  244 

Catycozoa,  250 

Calyptoblastea,  242 

Camarasaurus,  596 

Cambarus,  435 

Cambrian,  180 

Camelopardalidae,  642 

Camels,  643 

Camelus,  643 

Campanella,  242 

Campanula  Halleri.  564 

Campanularia,  232,  233 

Campanulariae,  239,  242 

Camper's  angle,  624 

Campodea,  400,  477 

Canal,  radial,  331 

Canal,  ring,  331 
Canal,  semi-circular,  128 
Canals  of  sponges,  223 
Cancer,  437 
Cancridae,  437 
Candona,  423 
Canidae,  647 
Canine  teeth,  625 
Canis,  647 
Canker  worms,  494 
Cannostomae,  250 
Cantharidae,  484 
Canthocamptus,  421 
Capillaries,  in 
Capillitium,  199 
Capra,  642 
Caprella,  440 
Caprimulgidae,  616 
Capsule,  central,  193 
Capybara,  639 
Carabidae,  484 

arapace,  410,  594 

arboniferous,    180 

archarinus,  571 

archarodon,  571 


G64 


INDEX. 


Carchesium,  210,  211 
Cardiidse.  367 
Cardinal  teeth,  359 
Cardinal  vein,  549 
Cardium,  367 
Cardoj  463 
Caridea,  434 
Carina,  424 
Carina  sterni,  605 
Carinaria,  380 
Carinariidae,  380 
Carinella,  291 
Carinatae,  613 
Carnassial  teeth,  626 
Carnivora,  646 
Carotid  artery,  549 
Carp,  576 
Carpal  bones,  529 
Carpocapsa,  494 
Cartilage,  86 
Cartilage  bone,  519 
Cartilaginous  cranium,  519,  520 
Caryogamy,  184 
Caryophyllaeus,  285,  286 
Caryophyllaeidae,  285 
Caryophyllia,  257,  260 
Cassowary,  613 
Castor,  639 
Castoridae,  639 
Casuarina,  613 
Casuarius,  613 
Cataclysm  theory,  2O 
Catallacta,  220 
Catarrhinae,  651 
Caterpillars,  494,  495 
Catfish,  576 
Cathartes,  617 
Cathartidae,  616 
Catocala,  495 
Catometopa,  437 
Catostomidae,  576 
Cats,  647 
Cattle,  642 
Caudal  fin,  526,  562 
Caudina,  347,  349 

Causal  foundation  of  theory  of  evolu- 
tion, 43 
Cavia,  639 


Caviare,  573 

Cavicornia,  642 

Caviidae,  639 

Cavolinidae,  382 

Cebidae,  651 

Cebus,  651 

Cecidomyia,  492 

Cell,  58 

Cell  complexes,  71 

Cell  division,  68 

Cell,  nature  of,  60 

Cell  organs,  183 

Cell-reticulum.  61 

Cell  theory,  17,  58 

Cells,  adhesive,  264 

Cells,  blood,  88 

Cells,  contractile  fibre,  92 

Cells,  division  of,  68 

Cell,  egg,  80 

Cells,  ganglion,  94 

Cells,  gland,  76 

Cells,  goblet,  77 

Cells,  muscle,  92    . 

Cells,  multiplication  of,  68 

Cells,  nettle,  229 

Cells,  sexual.  143 

Cells,  somatic,  143 

Cells,  supporting,  83 

Cells,  thread,  229 

Cells,  yellow,  195 

Cellular  connective  tissue,  84 

Cellularia,  324 

Cellulose,  172,  505 

Cenozoic  era,  181 

Centipedes,  459,  461 

Central  capsule,  153 

Central  nervous  system,  122 

Centrifugal  nerve  tracts,  94 

Centripetal  nerve  tracts,  94 

Centrodorsal,  338 

Centrolecithal  eggs,  152,  153 

Centrosome,  190 

Centrum,  518 

Centrurus,  448 

Cephalaspis,  557 

Cephalochordia,  502 

Cephalodiscus,  514 

Cephalopoda,  384 


INDEX. 


665 


Cephalothorax,  399 
Cephalothrix,  291 
Cerambycidse,  485 
Ceraospongias,  227 
Ceratium,  203 
Ceratodus,  579 
Ceratorhinus,  641 
Cercaria,  268,  276 
Cercomonas,  202 
Cercopidae,  489 
Cercus,  477 
Cere,  604 

Cerebellar  hemispheres,  535 
Cerebellum,  535 
Cerebral  flexures,  624 
Cerebral  ganglion,  123 
Cerebral  hemispheres,  534 
Cerebratulus,  292 
Cerebrum,  534 
Ceresa,  490 
Cereus,  253 
Cerianthus,  255 
Cervicornia,  642 
Cervidse,  642 
Cervus,  642 
Ceryle,  616 
Cestidse,  264 
Cestoda,  278 
Cestum,  264 
Cetacea,  645 
Cetochilus,  421 
Chaelura,  439 
Chsetse,  311 
Chsetiferi,  317 
Chaetoderma,  358 
Chsetognathi,  296 
Chaetonotus,  295 
Chaetopoda,  306 
Chaetura,  616 
Chain  salps,  512 
Chalcis,  486 
Chalcididae,  486 
Chalina,  227 
Chameleon,  599 
Chamois,  642 
Charadriformes,  615 
Charadrius.  615 
Charybdea,  250 


Chelicera,  445 
Chelifer,  450 
Chelone,  596 
Chelonia,  594 
Chelonidse,  596 
Chelydra,  596 
Chelydridae,  596 
Chermes,  450 
Chevron  bones,  518 
Chevrotain,  642 
Chiastoneury,  373 
Chigoe,  494 
Chilognatha,  496 
Chilomonas,  201 
Chilomycterus,  578 
Chilopoda,  459,  460 
Chilostomata,  324 
Chimaera,  572 
Chimney  swallow,  616 
Chimpanzee,  651 
Chinch  bug,  489 
Chiromys,  649 
Chiroptera,  637 
Chirotes,  599 
Chitin,  398 
Chiton,  357 
Chitonidse,  356 
Chlamydosaurus,  599 
Chlamydoselachus,  570 
Chloragogue  cells,  116 
Choanoflagellata,  202 
Choloepus,  636 
Chondrilla,  224 
Chondrin,  86 
Chondrioderma,  199 
Chondrocranium,  520 
Chondropterygii,  569 
Chondrostei,  573 
Chone.  313 
Chorda  dorsalis,  501 
Chordata,  501 
Chordodes,  304 
Chordotonal  sense  organs,  406 
Chorion,  148,  634 
Choroid,  540 
Choroidea,  13  I 
Choroid  coat,  130,  131 
Choroid  gland.  564 


666 


INDEX. 


Chromatin,  65 

Chromatophores,  387 

Chromomonadina,  202 

Chrysalis,  494 

Chrysomelidae,  485 

Chrysomitra,  244 

Chrysopa,  483 

Chyle,  550 

Chyle  vessels,  114 

Cicada,  488,  489,  490 

Cicadidse,  489 

Cicindelidae,  484 

Ciconia,  615 

Ciconiformes,  615 

Cidaridea,  345 

Ciliata,  204 

Ciliated  epithelium,  75 

Cilioflagellata,  203 

Cimbex,  486. 

Cinclides,  253 

Ciona,  507 

Circulatory  apparatus.  109 

Circulatory  organs  of  vertebrates,  548 

Cirolana,  442 

Cirri,  312 

Cirripedia,  423 

Cirrus,  120,  272,  308 

Cirrus  pouch,  272 

Cistenides,  314 

Cistudo,  596 

Citigrada,  453 

Cladocera,  417 

Cladocora,  260,  261 

Cladoselache,  572 

Clamatores,  616 

Clams,  368 

Class,  10 

Classification,  difficulties  in,  30 

Clathrulina,  191,  192 

Clava,  241 

Clavellinidse,  510 

Clavicle,  528 

Claws,  618 

Clear  wings,  495 

Cleavage  cavity,  155 

Cleavage  planes,  151 

Cleavage  of  eggs,  149,  151 

Cleavage  process,  151 


Cleavage,  types  of,  153 
Cleon,  479 
Clepsidrina,  213,  215 
Clepsinidse,  321 
Clibanarius,  436 
Clidastes,  600 
Climbing  birds,  616 
Clione,  382 
Clisiocampa,  495 
Clitellio,  315 
Clitellum,  315 
Cloaca,  106,  223,  506,  546 
Clothes  moth,  494 
Clupeidse,  576 
Clymene,  313 
Clypeaster,  343,  346 
Clypeastroidea,  346 
Clypeus,  462 
Clytia,  242 
Cnemidophorus,  599 
Cnidse,  229 
Cnidaria,  228 
Cnidocil,  229 
Cobra,  601 
Coccidae,  490 
Coccidiae,  215 
Coccidium,  215,  216 
Coccinellidse,  485 
Coccus,  490 
Coccygus,  616 
Cochineal,  490 
Cochlea,  128,  543 
Cockatoos,  616 
Cockroach,  480 
Cod,  577,  578 
Codlin  moth,  494 
Codosiga,  201,  202 
Coelenterata.  228 
Coelhelminthes,  295 
Ccelom,  109,  158 
Coelom  of  Vertebrates,  545 
Ccelomic  pouches,  158 
Ccelenteron.  228 
Coelodendron,  196 
Coeloplana,  264 
Coelopleurus,  343,  345 
Coeloria,  260,  261 
Coenurus,  285 


INDEX. 


667 


Coiter,  13 

Cold  rigor,  63 

Cold-blooded  animals,  114 

Coleoptera,  483 

Collaterals,  94 

Collembola,  477 

Collozoum,  194 

Coloborhombus,  49 

Colon,  461 

Colony,  164 

Coloration,  sympathetic,  46 

Colossendeis,  456 

Colossochelys,  596 

Colubriformia.  601 

Columba,  614 

Columbidas,  614 

Columella,  199,  256,  370,  525,  544,  598 

Columns  of  cord,  533 

Colymbidse,  615 

Colymbus,  615 

Comatrichia,  199 

Comatulidse,  342 

Commissures,  123 

Complemental  males,  424,  442 

Compound  eye,  403,  404 

Conchiolin,  352 

Conch  of  ear,  545 

Condylarthra,  639,  643 

Condylura,  637 

Cones  of  eye,  540 

Coney,  644 

Conidae,  380 

Conjugation,  184,  206 

Connective  tissues,  83 

Conocephalus,  481 

Conocladium,  200,  202 

Conotrachelus,  485 

Contractile  fibre  cells,  92 

Contractile  vacuole,  183 

Conurus,  616 

Conus,  380 

Conus  arteriosus,  567 

Convergent  development,  169 

Cope,  24 

Copelatae,  506 

Copepoda,  417 

Copperhead,  601 

Copula,  524 


Copulation,  147 

Coraciformes,  616 

Coracoid,  528 

Coral,  brain,  260,  261 

Coral,  deer's  horn.  261 

Coral,  organ  pipe,  259 

Coral,  precious,  259 

Coral,  red,  256 

Coral  reefs,  258 

Coral  snake,  601 

Corallium,  256,  259 

Cordyiophora,  239 

Coregonus,  576 

Corium,  514 

Cornea,  130,  131,  541 

Cornacuspongia,  227 

Coronula,  423,  424 

Corpora  bigemina,  534 

Corpora  quadrigemina,  534 

Corpus  callosum,  623 

Corpus  striatum,  534 

Corpuscles,  Miescher's,  218 

Corpuscles,  Meissner's,  126 

Corpuscles,  Rainey's,  218 

Corpuscles,  Vater-Pacinian,  126 

Correlation  of  parts,  14 

Corrodentia,  478 

Corti,  organ  of,  543 

Corvus,  616 

Corvidoe,  616 

Corycaeidse,  421 

Corydalis,  482 

Corymorpha,  241 

Costae,  256 

Costal  plates,  594 

Cotingidae,  616 

Cotton  worm,  495 

Cottus,  577 

Cotyledonary  placenta,  634 

Coturnix,  614 

Cougar,  647 

Covering  scale,  243 

Coverts,  604 

Cowries,  380 

Coxa,  463 

Coxal  glands,  445 

Crab  louse,  491 

Crabs,  437 


668 


INDEX. 


Crab  stones,  433 

Crangon,  434,  435 

Crane  flies,  492 

Cranes,  615 

Crania,  328 

Cranial  nerves,-  536 

Craspedon,  235 

Craspedota,  235 

Crassatella,  359 

Crassilinguia,  599 

Crayfish,  435 

Creodonta,  648 

Crepidula,  379 

Crepidula,  cleavage  of  egg,  154 

Cretaceous,  180 

Cribillina,  324 

Cribrellum,  452 

Crickets,  481 

Crinoidea,  338 

Crisia,  324 

Crista  acustica,  127,  542 

Crista  sterni,  605 

Crocodilia,  601 

Crocodilus,  602 

Crop,  106,  467 

Crossbill,  616 

Crosses,  27 

Crossopterygii,  573 

Crotalidae,  601 

Crotalus,  60 1 

Croton  bug,  480 

Crows,  616 

Crura  cerebri,  623 

Crustacea,  408 

Cryptobranchus.  587 

Cryptocephala,  313 

Cryptochiton,  357 

Cryptodira,  596 

Cryptoniscus,  442 

Cryptopentamera,  484 

Crystalline  cone,  405 

Crystalline  style,  364 

Ctenidia,  353 

Cteniza,   453 

Ctenobranchia,  379 

Ctenodiscus,  337 

Ctenoid  scale,  558 

Ctenolabrus,  576 


Ctenophora,  261 
Ctenoplana,  264 
Ctenostomata,  324 
Cubomedusae,  250 
Cuckoos,  616 
Cuculiformes,  616 
Cuculus,  616 
Cucumaria.  349 
Culcita.  334,  337 
Culicidae,  492 
Cumacea,  437 
Cunina,  242 
Cunocantha,  239,  242 
Curculio,  485 
Currant  worm,  494 
Cursorial  foot,  614 
Cursoria,  480 
Cuspidaria,  368 
Cutaneous  artery,  585 
Cuticle,  75 
Cutis,  514 

Cuttle  bone,  388,  395 
Cuttle  fish,  395 
Cuvier,  14,  15 
Cuvierian  ducts,  548,  567 
Cuvierian  organs,  348 
Cyamus,  440 
Cyanea,  250 
Cyanocitta,  616 
Cycladidse,  368 
Cyclas,  368 
Cycloid  scale,  558 
Cyclometopa,  437 
Cyclostomata,  324,  555 
Cyclostomidge,  380 
Cyclopidse,  421 
Cyclops,  37,  421 
Cydippidae,  264 
Cygnus,  615 
Cymbulidae,  382 
Cymothoa,  440,  442 
Cynipidse,  486 
Cynocephalus,  651 
Cynomorphee,  651 
Cynomys,  639 
Cynthia,  510 
Cynthiidse,  510 
Cyprseidse,  380 


INDEX. 


669 


Cyprididae,  423 
Cypridina,  423 
Cypridinidae,  423 
Cyprinidae,  576 
Cypris,  423 
Cypselidae,  616 
Cypselomorphae,  616 
Cyrtidse,  196 
Cyrtophilus,  481 
Cysticercoid.  285 
Cysticercus,  278,  284 
Cystid,  322 
Cystidea,  342 
Cystoflagellata,  203 
Cystonectse,  244 
Cytoblast,  58 
Cytopharynx,  183 
Cytopyge,  183 
Cytosporidse,  213 
Cytostome,  183 

Dactylethra,  588 
Daddy  long-legs,  450 
Daphnia,  417,  418 
Daphnidoe,  417 
Dart  sac,  376 
Darwin,  23 
Darwinian  theory,  25 
Dasyatis,  572 
Dasypodidse,  636 
Dasypus,  636 
Dasyuridse,  633 
Dasyurus,  633 
Datames,  450 
Decapoda,  394,  429 
Deer,  642 
Degeneration,  167 
Delamination,  157 
Delphinus,  646 
Demibranch,  566 
Demodex,  454 
Dendrites,  94 
Dendrocoalum,  271 
Dendrceca,  616 
Dendronotus,  382 
Dental  formula,  626 
Dentalium,  369 
Dentary  bone,  525,  582 


Denticetae,  645,  646 
Dentine,  515 
Derma.  514 
Dermal  teeth,  5 15 
Dermanyssus,  454 
Dermatobia,  493 
Dermatoptera,  480 
Dermochelys,  595 
Dero,  315 
Derotrema,  587 
Desmodont  hinge,  359 
Desmodus,  638 
Desor's  larva,  291 
Deutocerebrum,  462,  468 
Deutomerite,  214 
Deutoplasm,  80 
Devexa,  642 
Devonian,  180 
Diaphragm,  546 
Diapophysis,  518 
Diaptomus,  421 
Diastema,  638 
Diastictis,  494 
Diapheromera,  480 
Diastylis,  437 
Dibranchia,  394 
Dicotyle,  641 
Dicyemida,  220 
Didelphia,  632 
Didelphys,  633 
Didus,  614 

Differentiation  of  tissues,  71 
Difflugia,  198 

Diffuse  nervous  system,  122 
Diffuse  placenta,  634 
Digger  wasps,  486 
Digenea,  274 
Digestive  tract,  103 
Digiti  grade,  647 
Dimorphodon,  602 
Dimyaria,  367 
Dinichthys,  579 
Dinobryon,  200,  202 
Diomedia,  615 
Dinoflagellata,  203 
Dinoceras,  643 
Dinornithidae,  613 
Dinosauria,  596 


670 


INDEX. 


Dinotheridae,  644 

Dinotherium,  644 

Dioecious,  118 

Diopatra,  313 

Diotocardia,  378 

Diphasia,  242  * 

Diphycercal  fin,  41,  562 

Diphyes,  244,  245 

Diphyodont,  625 

Diploblastica,  230 

Diplocardia,  316 

Diplopoda,  459,  496 

Diploria,  261 

Diplospondyli,  570 

Diplozoon,  273 

Diplozoon,  development  of,  165 

Dipneumones,  453 

Dipneumonia,  579 

Dipneusti,  579 

Dipnoi,  579 

Diporpa.  165,  274 

Diprotodon,  634 

Diprotodonta,  633 

Diptera,  491 

Dipurena,  240,  242 

Dipylidium,  287,  289 

Direct  development,  160 

Directive  corpuscles,  140 

Directive  spindle,  146 

Directives,  254 

Discina,  328 

Discodermia,  226 

Discodrilidae,  315 

Discoidal  placenta,  634 

Discomedusae,  250 

Disconanthe,  245 

Discophori,  318 

Dispermy,  148 

Distaplia,  510 

Distichalia,  340 

Distichopus,  315 

Distomise,  274 

Distomum,  116,  272,  275,  276 

Distribution,  40 

Disuse,  55 

Division  of  labor,  72,  165 

Dobsons,  482 

Docoglossa,  378 


Docophorus,  479 
Dodo,  614 

Dog-day  harvest  fly,  489 
Dogfish,  569,  571 
Dog,  prairie,  639 
Dog  sharks,  571 
Dogs,  647 
Dolichonyx,  616 
Doliolum,  512 
Dolomedes,  453 
Dolphins,  646 
Dondersia,  358 
Doridiidse,  382 
Doris,  382 
Doryphora,  485 
Dorsal  aorta,  548 
Dorsal  fin,  526,  562 
Dorsal  organ,  341 
Draco,  599 
Dragon  flies,  479 
Dranculus,  303 
Dreissenia,  367 
Drepanidotaenia,  289 
Drills,  651 
Dromseus,  613 
Dromatherium,  632 
Drum  of  ear,  544 
Duck,  615 
Duckbill,  632 
Ducts,  genital,  120 
Ductus  Botallii,  550 
Ductus  choledochus,  546 
Ductus  cochlearis,  543 
Ductus  ejaculatorius,  120 
Dugong,  645 
Duplicidentata,  639 
Dynastes,  484 
Dysmorphosa,  241 
Dysodont  hinge,  359 
Dytiscidae,  484 

Eagles,  617 

Ear  bones,  525 

Ear  of  vertebrates,  542 

Earth  worms,  315 

Ear  wig,  480 

Ecardines,  328 

Ecdysis,  399 


INDEX. 


671 


Echeneis,  577 
Echidna,  631,  632 
Echidnidae,  632 
Echinarachnius.  345,  346 
Echinobothrium,  286 
Echinocardium,  34*? 
Echinococcus,  288 
Echinoderidse,  295 
Echinoderma,  329 
Echinoidea,  343 
Echinorhynchus,  304 
Echinosphaerite3,  342 
Echiuroidea,  317 
Echiurus,  317 
Eciton,  488 
Ecology,  4 

Ectethmoid  bone,  522 
Ectochondrostoses,  519 
Ectocyst,  322 
Ectoderm,  103,  156 
Ectoparasites,  169 
Ectopistes,  614 
Ectoprocta,  322 
Ectosarc,  189 
Edentata,  635 
Edriophthalmata,  438 
Edrioasteroidea,  342 
Edwardsia,  253,  255,  259 
Edwardsiella,  259 
Egg  cell,  80 
Egg  of  bird,  153 
Egg,  cleavage  of,  149,  151 
Egg,  fertilization  of,  147 
Egg,  maturation  of,  146 
Egg  nucleus,  146,  149 
Egg,  segmentation  of,  149,  151 
Egg  tooth,  593 
Eichhorn,  13 
Eiderduck,  615 
Eimeria,  213 
Elaps,  601 
Elasipoda,  349 
Elasmobranchii,  569 
Elastic  cartilage,  86 
Elastic  tissue,  85 
Elastica  externa,  5 16 
Elastica  interna,  516 
Elastin,  85 


Elater,  199 
Electric  catfish,  576 
Electric  eel,  576 
Electric  organs,  563 
Elephantiasis,  304 
Elephantidge,  644 
Elephants,  643 
Elephas,  644 
Elytra,  312,  466 
Embiotocidse,  577 
Embryo,  1 60 
Embryology,  3,  139,  160 
Emu,  613 
Enamel,  515 
Enchylema,  62 
Enchytraeidse,  315 
Encyrtidium,  193,  196 
Encystment,  184 
Endite,  410 
Endocyst,  322 
Endolymph,  127,  543 
Endolymphatic  duct,  542 
Endopodite,  410 
Endostyle,  506 
English  sparrow,  616 
Enhydris,  647 
Enopla,  290 
Ensatella,  368 
Entalis,  369 
Entelops,  635 
Enteroccele,  109 
Enteroponeusta,  512 
Entoconcha,  349 
Entochondrostoses,   519- 
Entoderm,  103,  156 
Entomobrya,  477 
Entomostraca,  414 
Entoniscus,  441 
Entoniscidae,  441,  442 
Entoparasites,  169 
Entophaga,  486 
Entopldstron,  594 
Entoprocta,  321 
Entosarc,  189 
Entovalva,  349 
Environment,  54. 
Eocene,  181 
Eohippus,  643 


672 


INDEX. 


Eozoon,  1 80,  198 
Epaxial  muscles,  518 
Epeira,  451,  453 
Ependyma,  124,  532 
Ephemera,  479 
Ephemerida,  479 
Ephippium,  417 
Ephydatia,  227 
Ephyra,  246,  248,  249 
Epiblast,  156 
Epibdella,  274 
Epididymis,  320,  552 
Epigenesis,  16 
Epiglottis,  547 
Epimerite,  214 
Epiotic  bone,  522 
Epipharynx,  463 
Epiphragm,  372 
Epiphysis,  535 
Epiplastron,  595 
Epipleural  bones,  574 
Epipodite,  410 
Epipodium,  369 
Epipterygoid  bone,  598 
Episternum,  528 
Epistropheus,  590 
Epistylis,  208,  211 
Epitheca,  256 
Epithelial  tissues,  73 
Epithelium,  germinal,  118 
Epitoke,  311 
Epizoanthus,  170,  259 
Equatorial  furrow,  151 
Equatorial  plate,  69 
Equidse,  641 

Equilibration,  organs  of,  128 
Equus,  641,  643 
Erax,  493 
Erethyzon,  639 
Eretmochelys,  596 
Erichthus,  429,  430 
Erigone,  453 
Erinacidse,  637 
Eristalis,  493 
Ermine,  647 
Errantia,  313 
Erythroblasts,  89 
Erythroneura,  490 


Eschara,  324 

Esocidae,  576 

Essence  of  pearl,  558 

Estheria,  417 

Estheriidae,  417 

Ethiopian  region,  176,  178 

Ethmoidalia,  521 

Ethmoid  bone,  523,  620 

Euchilota,  240 

Eucopepoda,  421 

Eucratea,  324 

Eucrinoidea,  342 

Eudendrium,  232,  242 

Eudoxia,  166,  244 

Euflagellata,  202 

Euglena,  200,  202 

Euglenidse,  202 

Euglypha,  198 

Euisopoda,  442 

Eumeces,  599 

Eunectes,  601 

Eunice,  311 

Eunicidae,  313 

Eupagurus,  170,  435,  436 

Euphausia,  429 

Euplectella,  226 

Euplexoptera,  480 

Eupolia,  292 

Euryalidse,  338 

Eurypauropus,  497 

Eurypterida,  444 

Eurypterus,  444 

Euselachii,  571 

Euspongia,  221,  227 

Eustachian  tube,  544 

Eustachius,  12 

Eusuchia,  602 

Eutainia,  601 

Eutima,  240 

Evadne,  417,  419 

Everyx,  495 

Evolution,  16 

Evolution,  Theory  of,  19,  25 

Evolution  vs.  Creation,  22 

Excreta,  73,  103 

Excretory  organs,  115 

Excretory  organs  of  Vertebrates,  550 

Exite,  410 


INDEX. 


673 


Exoccipital  bone,  522 
Exocoetidse,  576 
Exoccetus,  576 
Exopodite,  410 
Eugyra,  510 
Extracapsulum,  193 
Exumbrella,  235 
Eyes,  129 

Eyes  of  Vertebrates,  539 
Eye  spot,  183 

Fabricius,  13 

Face,  bones  of,  525 

Faceted  eye,  403,  404 

Facial  nerve,  536 

Factors  of  evolution,  44 

Fairy  shrimp,  417 

Falciform  spores,  215 

Falco,  617 

Falcons,  617 

Falconiformes,  616 

Fasciolaria,  276 

Fat  body,  407,  468 

Faunal  provinces,  175 

Fa  via,  260,  261 

Favositidse,  259 

Feather  tracts,  603 

Feathers,  603 

Feathers,  molting  of,  6li 

Fecundation,  147 

Felidse,  647 

Felis,  647 

Femoral  pores,  589 

Femur,  463,  529 

Fenestra  oval  is,  544 

Fenestra  rotunda,  544,  593 

Fertility  of  hybrids,  29 

Fertilization  of  eggs,  147,  148 

Fertilization  in  Protozoa,  206 

Fibre,  639 

Fibrec,  nerve,  94 

Fibula,  529 

Fibulare,  529 

Fiddler  crab,  437 

Field  mice,  639 

Fierasfer,  349 

Filaments,  mesenterial,  252 

Filar  substance,  61 


Filaria,  303 
Filibranch,  362 
Filibranchiata,  365 
Fin  backs,  646 
Finches,  616 
Fins,  526.  562 
Fireflies,  484 
Firmisternia,  588 
Fish  hawk,  617 
Fishes,  557 
Fishes,  tails  of,  41 
Fishes,  circulation  in,  1 12 
Fissilinguia,  599 
Fissipedia,  647 
Fissurella,  378 
Fissurellidse,  379 
Fissures  of  the  cord,  532 
Flabellum,  410 
Flagellata,  200 
Flagellate  epithelium,  75 
Flagellum,  376 
Flame  cell,  116,  280 
Flamingo,  615 
Flat  worms,  267 
Flea,  snow,  477 
Fleas,  493 
Flesh  flies,  493 
Flies,  491 
Flies,  black,  493 
Flies,  blow,  493 
Flies,  bot,  493 
Flies,  caddis,  483 
Flies,  crane,  492 
Flies,  dragon,  479 
Flies,  fire,  484 
Flies,  flesh,  493 
Flies,  gall,  486 
Flies,  harvest,  489 
Flies,  Hessian,  492 
Flies,  horse,  493 
Flies,  house,  492,  493 
Flies,  May,  479 
Plies,  robber,  493 
Flies,  saw,  485,  486 
Flies,  Spanish,  '484 
Flies,  stone,  479 
Fluke,  276 
Flustra,  324 


674 


INDEX. 


Flustrella,  324 

Flying  fish,  576 

Flying  foxes,  638 

Flying  squirrel,  639 

Fodientia.  636 

Fontanelles,  521 

Food  vacuole,  183 

Food  yolk,  80 

Foot,  351 

Foramen  magnum,  522 

Foramen  Panizzse,  592 

Foraminifera,  196 

Fore  brain,  533 

Fore  gut,  104 

Forficula,  480 

Formative  yolk,  80 

Formicarige,  487 

Fossa  rhomboidalis,  535 

Fossores,  486 

Fowl,  613 

Fowl,  digestive  tract  of,  105 

Fowl,  egg  of,  153 

Foxes,  647 

Frenulum  494 

Fringillidae,  616 

Fritillaria,  506 

Frogs,  588 

Frons,  462 

Frontal  bone,  523 

Frontal  sinus,  624 

Frontoparietal  bone,  581 

Frugivora,  638 

Fulcra,  573 

Function,  change  of,  100 

Function,  community  of,  165 

Fungia,  261 

Fungiacea,  261 

Funiculus,  322 

Furca,  420 

Furcula,  605 

Fur  seal,  648 

Gadidae,  578 
Gadus,  577,  578 
Galea,  463 
Galeidse,  571 
Galen,  12 
Galeodes,  450 


Galeopithecidae,  649 
Galeopithecus,  637,  649 
Galeus,  571 
Gall  flies,  486 
Gallinacea,  613 
.:Galls,  486 
Callus,  614 
Gamasidse,  454 
Gamasus,  4O°r  454 
Gammarina,  439 
Gammarus,  439 
Ganglion,  buccal,  390 
Ganglion  cells,  94 
Ganglion,  cerebral,  123 
Ganglion,  optic,  129 
Ganglion,  stellate,  390 
Ganglion,  supraoesophageal,  123 
Ganglionic  nervous  system,  122 
Ganoid  scale.  572 
Ganoidei,  558 
Ganoin,  558 
Gapes,  302 
Garpike,  574 
Garter  snake,  601 
Gasteropoda,  369 
Gasterosteus,  577 
Gastral  tentacles,  246 
Gastrochaena,  368 
Gastropliilus,  493 
Gastrotricha,  295 
Gastrovascular  space.  228 
Gastrovascular  system,  109 
Gastrula,  156 
Gastrulation,  156 
Gavialis,  602 
Gazella,  642 
Gecarcinus,  437 
Gecko,  598 
Gegenbaur.,  18 
Gelasimus,  437 
Gemmaria,  241 
Gemmellaria,  324 
Gemmulae,  227 
Gemmularia,  241 
Gena,  414,  462 
Generation,  asexual,  140 
Generation  by  parents,  140 
Generation,  sexual,  142 


IXDEX. 


675 


Generation,  spontaneous,  139 

Generations,  alternation  of,  144 

Genital  ducts,  120 

Genital  plates,  344 

Genus,  10 

Geocores,  489 

Geodia,  227 

Geographical  distribution,  174 

Geological  distribution,  180 

Geometrina,  494 

Geonemertes,  291 

Geophilidae,  461 

Geophilus,  461 

Gephyraea,  316 

Gerardia.  259 

Germinal  disc,  152 

Germinal  epithelium,  118 

Germinal  vesicle,  81,  146 

Germ  layer  theory,  17 

Germ  layers  and  budding,  159 

Germ  layers,  formation  of,  156 

Geryonia,  242 

Geryonid,  delamination  in,  157 

Geryonid,  germ  layers,  157 

Giant  cells,  71 

Gibbons,  651 

Gigantostraca,  443 

Gila  monster,  599 

Gill  arch,  524 

Gill  arteries,  504,  548 

Gill  clefts,  501,  547 

Gill  leaves,  361 

Gill  slits,  501,  547 

Gill,  tracheal,  469 

Gills,  108 

Gills  of  fishes,  565 

Gills  of  vertebrates,  547 

Gipsy  moth,  119,  495 

Giraffa,  642 

Girdles,  527 

Gizzard,  106,  461 

Glabelia,  414 

Gland  cells,  76 

Glands,  77 

Glands,  castor,  618 

Gland,  choroid,  564 

Glands,  germinal,  118 

Gland,  Harder's,  542 


Glands,  hoof,  618 
Gland,  lachrymal,  542 
Glands,  lymph,  550 
Gland,  lymphoid,  331 
Glands,  mammary,  619 
Glands,  milk,  619 
Glands,  musk,  618 
Gland,  nidamental,  392 
Gland,  ovoid,  331 
Gland,  paraxon,  331 
Gland,  parotid,  584 
Glands,  sexual,  80,  117 
Glands,  sweat,  618 
Gland,  subneural,  509 
Glands,  suborbital,  618 
Gland,  thymus,  547 
Gland,  thyroid,  547 
Glandular  epithelium,  73,  76 
Glaser's  fissure,  621 
Glass  crab,  436 
Glass  snake,  599 
von  Gleichen,  13 
Globiceps,  241 
Globigerina,  197,  198 
Globiocephalus,  646 
Glochidium,  364 
Glomeridae,  497 
Glomerulus,  117,  552 
Glossae,  464 

Glossopharyngeal  nerve,  536 
Glottis,  547 
Glugea,  218 
Glutin,  85 
Glutton,  647 
Glyptodontidse,  636 
Gnathobdellidse,  321 
Gnathochilarium,  496 
Goat,  642 
Goblet  cells,  77 
Goblet  organs,  307 
Goethe,  14,  21 
Goeze,  13 
Gomphus,  479 
Gonads,  117 
Goniatites,  394 
Goniodes,  479 
Gonochorism,  118 
Gonodactylus,  429 


676 


INDEX. 


Gonophore,  238 
Gonotheca,  242 
Gonys,  604 
Goose  barnacle,  425 
Gopher  turtle,  596 
Gordiacea,  304 
Gordius,  304 
Gorgonidae,  259 
Gorilla,  651 
Gradientia,  587 
Grallatores,  615 
Grantia,  225 
Grasshoppers,  48,  481 
Gray  matter,  124,  532 
Grebes,  615 
Green  gland,  411 
Green  turtle,  596 
Gregarina,  213,  215 
Gressoria,  480 
Gribble,  442 
Gromia,  62,  198 
Ground  substance,  62 
Grouse,  614  . 
Gruiformes,  615 
Grus,  615 
Gryllidse,  481 
Gryllotalpa,  481 
Gryllus,  481 
Guanin,  558 
Guard,  389 
Guinea  pig,  639 
Guinea  worm,  303 
Gula,  462 
Gulls.  615 
Gulo,  647 
Gunda,  271 
Gymnoblastea,  241 
Gymnodonti,  578 
Gymnolsemata,  323 
Gymnonoti,  576 
Gymnophiona,  587 
Gymnosomata,  382 
Gynsecophoral  canal,  119 
Gynandromorphism,  277 
Gyri,  535 
Gyrodactylus,  273,  274 

Habrocentrum,  453 


Haddock,  578 
Hadenoecus,  481 
Haeckel,  18,  24 
Haemadipsa,  321 
Haemal  arch,  516 
Haemal  ribs,  518 
Haemal  spine,  517 
Haemapophysis,  517 
Hsemoccele,  109,  113 
Haemoglobin,  89 
Haemosporida,  216 
Haemuntaria,  321 
Hagfishes,  555 
Hair,  617 
Hair  necks,  302 
Hair  worm,  304 
Hairs,  auditory,  127 
Hairs,  tactile,  126 
Halcampa,  259 
Haleremita,  241 
Haliaetus,  617 
Halibut,  578 
Halicore,  645 
Halicryptus,  317 
Haliomma,  135 
Haliommidae,  196 
Haliotidae,  379 
Haliotes,  379 
Halisarca,  226 
Halitherium,  645 
von  Haller,  17 
Halowises,  239 
Halteres,  491 
Halyclystus,  250 
Hammerhead  shark,  571 
Hapale,  651 
Hapalidae,  651 
Harder's  gland,  542 
Hares,  639 
Harpactidae,  421 
Hatteria,  596 
Haustellum,  465 
Haversian  canals,  87 
Haversian  lamellae,  87 
Hawks,  617 
Head  kidney,  310,  550 
Head,  segments  of.  536 
Hearing,  organs  of,  127 


INDEX. 


677 


Heart,  in 
Heart  shells,  367 
Heat  rigor,  63 
Hectocotylus,  393 
Hedgehogs,  637 
Heliaster,  337 
Helicidse,  383 
Helioporse,  259 
Heliozoa,  190 
Helix,  383 
Hell-bender,  587 
Hellgrammite,  482 
Helminthes,  169 
Helminthophaga,  616 
Heloderma,  599 
Helodermatidae,  599 
Hemelytra,  489 
Hemerobiidse,  482 
Hemibranchii,  575.  577 
Hemichordia,  512 
Hemimetabolous,  473 
Hemiptera,  489 
Hemitripterus,  577 
Hen.  613 
Hen  clam,  368 
Hepatopancreas,  106,  411 
Hepatus,  437 
Heptanchus,  570 
Heredity,  67,  150 
Hermaphroditism,  118 
Hermit  crabs,  436 
Herons,  615 
Herring,  576 
Hesperornis,  612 
Hessian  fly,  492 
Heterakis,  301 
Heteraxial  symmetry,  136 
Heterocercal  tail,  41,  562 
Heteroconchiae,  367 
Heterocotylea,  273 
Heterodera,  300 
Heterodont  dentition,  625 
Heterodont  hinge,  359 
Heterogony,  144,  145,  486 
Heteromera,  484 
Heteromyaria,  367 
Heteronemertini,  292 
Heteronereis,  3 1 1 


j  Heteronomy,  138 
Heteropleuron,  504 
Heteropoda,  380 
Heteroptera,  489 
Heterosyllis,  311 
Heterotricha,  209 
Hexacoralla,  259 
Hexactinellidae,  226 
Hexamita,  201 
Hexanchus,  570 
Hexapoda,  461 
Hind  brain,  533 
Hind  gut,  104 
Hinge,  358 
Hinny,  641 
Hipparion,  643 
Hippasterias,  337 
Hippidse,  437 
Hippocampus,  578 
Hippocrates,  12 
Hippocrene,  241 
Hippoglossus,  578 
Hippolyte,  434 
Hippopotamidae,  641 
Hippopotamus,  641 
Hippospongia,  227 
Hirudinei,  318 
Hirudo,  321 
Hirundinidse,  616 
Hirundo,  616 
Holoblastic  cleavage,  153 
Holoblastic  eggs,  152,  153 
Holocephali,  572 
Holocystites,  342 
Holometabolous,  473 
Holostei,  573 
Holostomate,  371 
Holothuria,  349 
Holothuria,  gastrula  of,  158 
Holothuridea,  346 
Holotricha,  209 
Homarus,  435 
Homaxial  animals,  135 
Homo,  651 

Homocercal  tail,  41,  563 
Homoiothermous,  1 15 
Homology,  14,  100 
Homonomy,  138 


678 


INDEX. 


Homoptera,  489 
Honey  ant,  488 
Honeycomb,  641 
Hoofs,  618 
Hooker,  24 
Hoploceras,  643 
Hoplorhynchus,  213 
Hop  worm,  495 
Hormea,  324 
Hormiphora,  262,  264 
Horn  bills,  616 
Horns  of  cord,  533 
Horned  toad,  599 
Horn  tails,  486 
Horse  flies,  493 
Horse  mackerel,  577 
Horses,  641 
Horseshoe  crab,  444 
House  fly,  492,  493 
Human  embryo,  35 
Humerus,  529 
Humming  birds,  616 
Huxley,  18,  24 
Hyaena,  647 
Hyaenidse,  647 
Hyalea,  381 
Hyaleidse,  382 
Hyaline  cartilage,  86 
Hyalonema,  226 
Hyalopus,  198 
Hyalospongia,  226 
Hyas,  437 
Hyatt,  24 
Hybrids,  28 
Hydnophyton,  488 
Hydra,  230,  240 
Hydra,  section  of,  141 
Hydrachna,  454 
Hydrachnidse,  454 
Hydractinia,  241 
Hydranth,  231 
Hydraria,  239,  240 
Hydrichthys,  240,  242 
Hydrobatidoe,  489 
Hydrocaulus,  231 
Hydrochoerus,  639 
Hydrocorallina,  239,  241 
Hydrocores,  489 


Hydroides,  313 
Hydromedusae,  230 
Hydrophilidse,  484 
Hydropolyp,  230 
Hydropsyche,  483 
Hydrorhiza,  231 
Hydrosauria,  594 
Hydrotheca,  233 
Hydrozoa,  230 
Hyla,  588 
Hylesinus,  485 
Hylidse,  588 
Hylobates,  651 
Hylodes,  586 
Hymenolepis,  287,  288 
Hymenoptera,  485 
Hyocrinus,  340 
Hyoid  arch,  524 
Hyoid  bone,  524 
Hyoid  cartilage,  524 
Hyomandibular,  524,  525 
Hypsena,  495 
Hypaxial  muscles,  518 
Hyperia,  439 
Hyperina,  439 
Hyperoartia,  557 
Hyperotretia,  557 
Hypoblast,  156 
Hypobranchial  groove,  503 
Hypoderma,  493 
Hypodermis,  398 
Hypogeophis,  587 
Hypoglossal  nerve,  536 
Hypopharynx,  463 
Hypophysis,  535 
Hypoplastron,  595 
Hyporachis,  603 
Hypotricha,  21 1 
Hyracoidea,  644 
Hyracotherium,  643 
Hyrax,  644 
Hystricidse,  639 
Hystricomorpha,  639 
Hystrix,  639 

lapyx,  477 
Ibis,  615 
Ichneumonidse,  486 


INDEX. 


679 


Ichthydium,  295 
Ichthyobdella.  321 
Ichthyodolurites,  570 
Ichthyophis,  585,  587 
Ichthyopsida,  555 
Ichthyosauria,  594 
Ichthyotomi,  572 
Icteridse,  616 
Icterus,  616 
Idiothermous,  115 
Idotea,  441,  442 
Idoteidse,  442 
Idyia,  264 
Iguanidse,  599 
Ilium,  528 
Ilyanassa,  379 
Imaginal  discs,  476 
Impennes,  615 
Impregnation,  147 
Inbreeding,  29 
Incisor  teeth,  625 
Incus,  525,  544 
Indirect  cell  division,  68 
Indirect  development,  160 
Inermes,  317 
Infrabasalia,  340 
Infundibulum,  534 
Ingluvies.  106,  467 
Inia,  645 

Inorganic  bodies,  133 
Inquilines,  486 
Insecta,  458 
Insectivora,  637 
Insects,  cleavage  of  egg,  155 
Integripalliata,  367 
Interambulacral  plate,  335 
Intercalaria,  516 
Interfilar  substance,  62 
Interhyal  bone,  561 
Intermaxillary  bone,  525 
Intermedium,  529 
Interorbital  septum,  560 
Interparietal  bone,  619 
Interradius,  246 
Intervertebral  ligament,  519 
Intestine,  106 
Invagination,  156 
Inversion  of  retina,  541 


Iris,  130,  131 

Irritability,  62 

Ischial  callosities,  651 

Ischium,  528 

Isinglass,  573 

Isis,  259 

Isodont  hinge,  359 

Isopoda,  440 

Isoptera,  478 

Itch,  454 

Iter,  534 

lulidse,  497 

lulus,  497 

Ixodes,  454 

Ixodidae,  454 

Jacobson's  organ,  539 
Jassidae,  490 
Jays,  616 
Jigger,  494 
Jugal  arch,  526 
Jugal  bone,  526 
Jugulares,  562 
Jugular  vein,  549 
June  bug,  484 
Jurassic,  180 

Kallima,  47 
Kangaroos,  634 
Karyokinesis,  68 
Katydid.  481 
Keyhole  limpets,  379 
Kidneys,  116,  550 
Kielmeyer,  15 
Kinetoskias,  324 
King  crab,  444 
King  fishers,  616 
Kinosternon,  596 
Kiwi,  613 
Koenenia,  449 
Kolliker,  18 
Kowalewskia,  506 
Kowalewsky,  18 

Labial  cartilage,  534 
Labial  palpi,  362 
Labidura,  480 
Labium,  463,  464 


680 


INDEX. 


Labor,  division  of,  165 
Labridse,  576 
Labrum,  463 
Labyrinth,  128,  542 
Labyrinthodonta.  586 
Lac,  490 
Lacerta,  599 
Lacertilia,  598 
Lacertilidse.  599 
Lace  wings,  482 
Lachrymal  bone,  590 
Lachrymal  gland,  542 
Lacinia,  473 
Lacteal  dentition,  625 
Lacuna,  379 

Lacunar  blood  system,  1 13 
Ladder  nervous  system,  124 
Lady  bird,  485 
Lady  crab,  436 
Laemodipoda,  439 
Lagena,  543 
Lagomys  639 
Lama,  643 
Lamarck,  14,  22 
Lamarckism,  53 
Lamblia,  202 
Lamellae,  Haversian,  87 
Lamellae  bone,  87 
Lamellibranchiata,  358 
Lamellicornia,  484 
Lamellirostres,  615 
Lamna,  571 
Lamnae,  618 
Lamprey  eels,  555,  557 
Lampyridee,  484 
Land  crab,  437 
Lanistes,  372 
Lantern  of  Aristotle,  345 
Larus,  615 
Larva,  160 
Larval  organs,  161 
Laryngeal  cartilages,  524 
Lateralia,  424 
Lateral  line,  537.  564 
Lateral  teeth,  359 
Latrodectes,  451 
Laurer's  canal,  273 
Leaf  butterflies,  47 


Leaf  hoppers,  489 
Leatherback  tortoise,  595 
Leather  turtle,  596 
Leda,  367 
Leeches,  318 
Leeuwenhoek,  13 
Lemniscus,  304 
Lemuridae,  649 
Lemurs,  648,  649 
Lens  of  eye,  130,  131,  541 
Lepadidae,  425 
Lepas,  172,  425 
Lepidonotus,  313 
Lepidoptera,  494 
Lepidosauria,  594,  597 
Lepidosiren,  579 
Lepidosteidae,  574 
Lepidosteus,  574 
Lepidurus,  416 
Lepisma,  477 
Lepralia,  324 
Leptalis,  48 
Leptasterias,  337 
Leptocardii,  502 
Leptocephalus,  575 
Leptochela,  441,  442 
Leptoclinum,  510 
Leptodiscus,  204 
Leptodora,  417 
Leptomedusae,  239,  242 
Leptoplana,  no,  271 
Leptostraca,  427 
Lepus,  639 
Lernaea,  422 
Lernaeidae,  422 
Lernaeocera,  421,  422 
Lernaeopodidae,  422 
Leucania,  495 
Leucetta,  225 
Leuckart,  18 
Leucocytes,  88 
Leucon,  223 
Leucones,  226 
Leucortis,  226 
Leucosoidea,  437 
Leucosolenia,  225 
Libel lula,  479 
Libellulidae,  479 


INDEX. 


681 


Libinia,  436,  437 
Lice,  491 
Lice,  bird,  479 
Lice,  book,  479 
Life,  origin  of,  140 
Ligula,  286 
Ligulidae.  286 
Limacidaa,  383 
Limacinidae,  382 
Limax,  383 

Limbs  of  vertebrates,  527 
Limicola,  315 
Limitaiis  cxterna,  540 
Limitans  interna,  '540 
Limnadia,  417 
Limnaea,  383 
Limnaeidse,  383 
Limnocnida,  239 
Limnocodium,  239 
Limnoria,  441,  442 
Limnothrips,  479 
Limpets,  379 
Limulus,  444 
Linckia,  334 

Linear  nervous  system,  122 
Linerges,  250 
Lines  of  growth,  358 
Lineus,  289,  292 
Lingual  ribbon,  355,  373 
Linguatulida,  454 
Lingula,  328 
Linin,  65 

Linnsean  system,  10 
Linnaeus,  lo 
Liobunum,  451 
Lion,  647 
Liriope,  239,  242 
Lithistidse,  226 
Lithobiidse,  461 
Lithobius,  461 
Lithodidae,  437 
Lithodomus,  367 
Littorina,  379 
Littorinidse,  380 
Liver,  106 
Liver  fluke,  276 
Lizards,  598 
Lizzia,  240,  242 


Lobatae,  264 

Lobate  foot,  614,  615 

Lobi  inferiores,  563 

Lobosa,  189 

Lobster,  435 

Lobster,  spiny,  436 

Locomotion,  12 1 

Locustidae,  481 

Locusts,  481,  489 

Loggerhead,  596 

Loligo,  384,  395 

Loligo,  cleavage  of,  155 

Longipennes,  615 

Loons,  615 

Lophobranchii,  578 

Lophodont  teeth,  626 

Lophogastridae.  429 

Lophophore,  324 

Lophopoda,  324 

Lophopus,  324 

Lophs  of  teeth,  626 

Lorica,  200 

Loricata,  435,  577,  601,  636 

Loris,  649 

Lota,  578 

Love  dart,  376 

Loven's  larva,  309 

Loxia,  616 

Loxosoma,  322 

Lucernariae,  250 

Luciae,  510 

Lumbricus,  315,  316 

Lumbricus,  anatomy  of,  118 

Lunatia,  379,  380 

Lung  book,  443 

Lung  fishes,  579 

Lungs,  109,  547 

Lung  sac,  443 

Lung  sacs  of  birds,  609 

Lutra,  647 

Lycosa,  453 

Lyell,  23,  24 

Lygseidae,  489 

Lymph,  88,  90 

Lymph  corpuscles,  90 

Lymph  glands,  550 

Lymph  System,  550 

Lymph  vessels,  114 


682 


INDEX. 


Lymphoid  gland,  331 
Lyonet,  13 
Lyre  birds,  616 
Lyriform  organs,  445 
Lytta,  484 

Macacus,  651 

Macaques,  651 

Machilis,  458,  477 

Mackerel,  577 

Mackerel  shark,  571 

Macoma,  368 

Macrsesthete,  357 

Macrobdella,  321 

Macrobiotus,  455 

Macrochelys,  596 

Macrochiroptera,  638 

Macrodrila,  315 

Macrogamete,  185 

Macronucleus,  206 

Macropodidse,  633 

Macropus,  634 

Mactra,  359 

Mactridse,  368 

Macrura,  434 

Madrepora,  261 

Madreporaria,  260 

Madreporite,  330 

Maioidea,  437 

Malaclemmys,  596 

Malacobdella,  291 

Malacoderma,  259 

Malacopoda,  456 

Malacopteri,  574 

Malacostraca,  426 

Malagassy  region,  178 

Malapterurus,  576 

Malar  bone,  526,  620 

Malaria,  217,  492 

Maldanidse,  313 

Malleus,  525,  544 

Mallophaga,  479 

Malpighi,  13 

Malpighian  body,  117 

Malpighian  tubes,  438,  445,  459,  461 

Mammalia,  617 

Mammals,  617 

Mammoth,  644 


Man,  651 

Manatee,  644 

Manatus,  644 

Mandible,  401 

Mandibles,  463,  464 

Mandibular  arch,  524 

Mandibular  cartilage,  524 

Mandrils,  651 

Manicina,  261 

Manis,  636 

Manna,  489 

Manubrium,  235 

Mantidse,  480 

Mantis,  480 

Mantis  shrimp,  429 

Mantle,  351,  505 

Mantle  cavity,  352 

Manyplies,  642 

Manyunkia,  313 

Margarita,  379 

Margelis,  144,  241 

Marginal  plates,  595 

Marine  faunae,  179 

Marmosets,  651 

Marsipobranchii,  555 

Marsupialia,  632 

Marsupial  bones,  631,  632 

Marsupium,  632 

Marten,  647 

Mastax,  294 

Mastigamceba,  188,  201 

Mastigophora,  200 

Mastodon,  644 

Maturation  of  egg,  146 

Maturation  and  Fertilization,  147 

Matuta,  437 

Maxilla,  401,  463 

Maxillary  bone,  525 

Maxillary  sinus,  624 

Maxillipeds,  401 

May  flies,  479 

Measly  meat,  284,  285 

Measuring  worms,  494 

Meckel,  14 

Meckelia,  292 

Mecoptera,  483 

Mediastinum,  546 

Medulla  oblongata,  534 


INDEX. 


683 


Medullary  plate,  501 
Medullary  sheath,  96 
Medusae,  144,  230,  234 
Megalops,  434 
Megalonyx,  636 
Megalosphaeres,  198 
Megatherium,  636 
Megascolex,  316 
Megastoma,  202 
Meissner's  corpuscles,  126 
Melanoplus,  481 
Meleagrina,  367 
Meleagris,  614 
Melitta,  346 
Meloidse,  484 
Melolontha,  484 
Melonites,  345 
Melophagus,  493 
Melopsittacus,  616 
Membracidse,  490 
Membrane  bones,  515 
Membranipora,  324 
Membranellse,  209 
Membranous  cranium,  519 
"Menopoma,  587 
Mentum,  464 
Menuridse,  619 
Mephitis,  647 
Meridional  furrows,  151 
Mermithidoe,  304 
Meroblastic  cleavage,  153 
Meroblastic  eggs,  152,  154 
Meryhippus,  643 
Mesectoderm,  222 
Mesencephalon,  533 
Mesenchyme,  157 
Mesenterial  filaments,  252 
Mesenteries,  109,  545 
Mesenteron,  105 
Mesethmoid  bone,  522 
Mesites,  613 
Mesoblast,  157 
Mesobronchus,  609 
Mesoderm,  104,  157 
Mesoglcea,  230 
Mesohippus,  643 
Mesonemertini,  291 
Mesonephros,  550 


Mesonephric  duct,  550 
Mesopterygium,  529 
Mesorchium,  546 
Mesotroche,  309 
Mesosternum,  462 
Mesothelium,  158 
Mesothorax,  462 
Mesozoic  era,  180 
Mesovarium,  546 
Metabolism,  172 
Metacarpal  bones,  529 
Metagenesis,  144 
Metameres,  137,  305 
Metamerism,  137 
Metamorphosis,  161 
Metamorphosis  of  insects,  473 
Metanemertini,  291 
Metanephric  duct,  550 
Metanephros,  550 
Metapodium,  369 
Metapterygium,  529 
Metastoma,  430 
Metatarsal  bones,  529 
Metathorax,  462 
Metazoa,  221 
Metencephalon,  533 
Methona,  48 
Metridium,  259 
Miastor,  492 
Mice,  639 
Micraesthete,  357 
Microcentrum,  481 
Microchiroptera,  638 
Microconodon,  632 
Microcotyle,  274 
Microdrilaa,  315 
Microgametes,  185 
Microlepidoptera,  494 
Microlestes,  632 
Micronucleus,  206 
Micropterus,  577 
Micropylar  apparatus,  148 
Microsphaeres,  198 
Microstomidae,  271 
Microthelyphonida,  448 
Micrura,  292 
Midas,  651 
Mid  brain,  533 


684 


INDEX. 


Middle  Ages,  Zoology  in,  9 
Mid  gut,  105 

Miescher's  corpuscles,  218 
Migration  of  birds,  612 
Migration  theory,  52 
Miliola,  197,  198 
Milk  teeth,  625 
Millepora,  233,  241 
Mimicry,  46 
Mink,  647 
Miocene  181 
Miohippus,  643 
Miracidium,  2"6 
Mites,  453 
Mitosis,  68 
Mixipterygium,  570 
Mnemiopsis,  264 
Moccasin,  601 
Modiola,  366.  367 
Molar  teeth.  625 
Mole  cricket,  481 
Moles,  637 
Molgula,  510 
Molgulidae,  510 
Mollusca,  351 
Molpadia,  349 
Monactinellidae,  227 
Monadina,  202 
Monascidise,  510 
Monaxial  symmetry,  135 
Monera,  189 
Moniezia.  287,  289 
Monitor,  599 
Monkeys,  651 
Monocaulis,  240,  241 
Monocystis,  215 
Monodelphia,  634 
Monodon,  646 
Monogenea,  273 
Monogony,  140 
Monomyaria,  367 
Monops,  271 
Monophyodont,  625 
Monopneumonia,  579 
Monopylea,  196 
Monorhina,  556 
Monoscelis,  271 
Monospermy,  148 


Monostomum,  275 

Monothalamia,  198 

Monotocardia,  379 

Monotremata,  631 

Moose,  642 

Morphology,  2 

Morphology,  development  of,  12 

Mosaic  vision,  406 

Mosasaurus,  600 

Moschidae,  642 

Moschus,  642 

Mosquitos,  492 

Mosquitos  and  malaria,  217 

Moths,  494 

Mouse,  639 

Mud  crab,  437 

Mud  puppy,  587 

Mud  turtle.  596 

Muller,  Fritz,  24 

Mtiller,  J.,  18 

Muller,  O.  F.,  13 

Miillerian  duct,  551 

Muller's  fibres,  540 

Mule.  641 

Multicellular  glands,  77 

Multicellularity,  70 

Multinuclearity,  70 

Multituberculata,  632 

Muricidse,  380 

Mus,  639 

Musca,  492,  493 

Muscariae,  493 

Muscidae,  493 

Muscle  cells,  92 

Muscle  fibres,  91 

Muscular  tissue,  91 

Musculature,  12 1 

Musk  deer,  642 

Musk  ox,  642 

Musk  rat,  639 

Mussels,  367 

Mustela,  647 

Mustelidse.  647 

Mustelus,  571 

Mya,  368 

Mycetozoa,  198 

Myctodera,  587 

Myelin.  96 


INDEX. 


685 


My  gale,  453 
Mygalidae,  453 
Mygnimia,  49 
Myidae,  368 
Mylodon,  636 
Myocommata,  531 
Myomerism,  523 
Myomorpha,  639 
Myopsida,  395 
Myosepta,  531 
Myotomes,  531 
Myrianida,  310,  313 
Myriapotla,  408.  459,  496 
Myriothelia,  241 
Myriotrochus,  349 
Mynnecocystus,  488 
Myrmecophaga,  636 
Myrmecophily,  169 
Myrmeleo,  481,  483 
Mysididse,  429 
Mysis,  428,  429 
Mysticetae,  645,  646 
Mytilus,  363 
Myxicolida,  313 
Myxidium,  213,  217 
Myxine,  557 
Myxobolus,  217 
Myxomycetes,  198 
Myxospongiae,  225,  227 
Myxosporida,  217 
Myzobdella,  315 
Myzontes,  557 

Nacre,  361 
Nageli.  24,  54,  55 
Naiadse,  367 
Naididse,  315 
Nails,  618 
Nais,  307 
Naja.  601 
Nandu,  613 
Nanomia,  244.  245 
Narcomedusae,  239,  242 
Narwal,  646 
Nasal  bone,  523 
Nassa,  cleavage  of,  154 
Nassellaria,  196 
Natatores,  614 


Naticidae,  380 
Natural  selection,  44 
Nauplius,  37,  413 
Nauplius  eye,  412 
Nausithog,  134,  250 
Nautilidae,  394 
Nautilus,  387,  394 
Nearctic  region,  176.  178 
Nebalia,  427 
Nectonema,  304 
Nectonemertes,  291 
Necturus,  587 
Nectocalyx,  243 
Needham's  sac,  392 
Nematocysts,  229 
Nematoda,  298 
Nemathelminthes,  298 
Nematophora,  228 
Nematus,  486 
Nemerteans,  289 
Nemertini,  289 
Nemocera,  492 
Nemognatha,  483 
Nemopsis,  242 
Neocrinoidea,  342 
Neogaea,  177 
Neomenia,  358 
Neotropical  region,  176,  177 
Nephilis,  321 
Nephridia,  116,  308 
Nephrostome,  116 
Nepidae,  489 
Neptunus,  437 
Nereidae,  313 
Nereis,  311,  312,  313 
Nerve-end  buds,  537 
Nerve  fibres,  94 
Nerve  hillock,  537 
Nerve  roots,  533 
Nerves  of  vertebrates,  535 
Nervous  system,  122 
Nervous  tissue,  94 
Nettle  bodies,  205 
Nettle  cells,  229 
Neural  arch,  516 
Neural  plates,  594 
Neural  spine,  517 
Neurapophysis,  517 


686 


INDEX. 


Neurenteric  canal,  502,  532 
Neurites,  94 
Neuropodium,  308,  312 
Neuropore,  503,  532 
Neuroptera,  481 
Never ita,  380 
New  Zealand,  177 
Nictitating  membrane,  541 
Nidamental  glands,  392 
Night  hawks,  616 
Nipple,  619 
Nirmus,  479 
Noctiluca,  201,  203 
Noctuina,  495 
Nodes  of  Ranvier,  96 
Nomarthra,  636 
Nomenclature,  binomial,  10 
Non-Ruminantia,  641 
Nosema,  218 
Nothria,  313 
Notochord,  501,  515 
Notochordal  sheath,  516 
Notodelphys,  585 
Notodelphidse,  422 
Notogsea,  176 
Notonectidae,  489 
Notopodium,  308,  312 
Nototrema,  585 
Notum,  462 
Nuclear  plate,  595 
Nuclear  fragmentation,  70 
Nuclear  spindle,  68 
Nuclear  substance,  65 
Nuclein,  65 
Nucleolus,  66 
Nucleus.  58,  64 
Nucleus,  cleavage,  149 
Nucleus,  egg,  146 
Nucleus  in  fertilization,  149 
Nucleus  of  Salpa,  511 
Nucleus,  significance  of,  67 
Nucleus,  somatic,  208 
Nucleus,  sperm,  149 
Nucleus,  substance  of,  65 
Nucleus,  structure  of,  65 
Nucula,  365,  367 
Nuculidae,  367 
Nuda,  264 


Nudibranchia,  382 

Nummulites,  198 

Nurse,  144 

Nutrition  and  reproduction,  64 

Nyctotherus,  210 

Nymphon,  456 

Obelia,  240,  242 

Obisium,  450 

Obturator  foramen,  622 

Occipitalia,  521 

Occipital  bone,  521,  619 

Occiput,  462 

Ocellatae,  239 

Ocellus,  129,  403 

Ocneria,  119,  495 

Octocoralla,  258 

Octopoda,  395 

Octopodidse,  395 

Octopus,  384,  390,  394,  395 

Ocular  plate,  335 

Oculina,  261 

Oculomotor  nerve,  536 

Odonata,  479 

Odontoholcae,  612 

Odontophore,  373 

Odontormse,  612 

Odontornithes,  612 

QScanthus,  481 

CEcology,  457,  164 

QEdipoda,  481 

OZdogonium,  173 

CEgopsida,  394 

CEsophageal  ring,  124 

Oesophagus,  106,  546 

OZstridae,  493 

OZstrus,  193 

Oikopleura,  506,  507 

Oil  bottle,  484 

Olfactory  organs,  126 

Olfactory  organs  of  vertebrates,  538 

Olfactory  lobe,  534 

Olfactory  nerve,  536 

Oligocene,  181 

Oligochaetae,  314 

Oligosoma,  599 

Oligotrochus,  349 

Olividse,  380 


INDEX. 


687 


Olynthus,  222 

Omasum,  642 

Omentum,  546 

Ommastrephes,  388,  395 

Ommatidium,  405 

Oncosphaera,  283 

Oniscidae,  442 

Oniscus,  442 

Ontogeny,  3,  160 

Oospore,  185 

Ootype,  272 

Opalina,  209 

Opercular  bones,  562,  566 

Opercularella,  242 

Operculum,  210,  323,  371,  440,  566 

Ophidia,  600 

Ophidiaster,  334 

Ophiocoma,  338 

Ophiocnida,  338 

Ophioglypha,  338 

Ophiopholis,  338 

Ophiothelia,  338 

Ophisaurus,  599 

Ophisthotic  bone,  522 

Ophiuroidea,  337 

Opisthobranchia,  381 

Opisthocoelous,  519 

Opisthopatus,  458 

Opossums,  633 

Opossum  shrimp,  428 

Opoterodonta,  601 

Optic  ganglion,  129 

Optic  lobes,  534 

Optic  nerve,  536 

Optic  stalk,  541 

Optic  thalami.  534 

Optic  vesicle,  541 

Oralia,  330,  340 

Orang-utan,  651 

Orange  scale  insect,  490 

Orbitelarise,  453 

Obitosphenoid  bone,  522 

Orchestia,  438,  439 

Order,  10 

Organic  bodies,  133 

Organisms,  origin  of,  139 

Organ-pipe  coral,  259 

Organs,  99 


Organs,  animal,  101,  121 

Organs,  of  assimilation,  102 

Organs,  auditory,  127 

Organs,  of  Bojanus,  363 

Organs,  circulatory,  109 

Organs,  of  Corti,  543 

Organs,  digestive,  103 

Organ,  dorsal,  341 

Organs,  electric,  563 

Organs  of  equilibrium,  128 

Organs,  excretory,  115 

Organs,  excretory,  of  vertebrates,  550 

Organ  of  Jacobson,  539 

Organs  of  hearing,  127 

Organs,  lateral  line,  537 

Organs,  olfactory,  126 

Organs,  pearl,  558 

Organs,  respiratory,  107 

Organs,  sensory,  125 

Organs,  sexual,  117 

Organs,  sexual,  of  vertebrates,  550 

Organs  of  smell,  126 

Organs,  systems  of,  100 

Organs,  tactile,  125 

Organs,  of  taste,  126 

Organs,  of  touch,  537 

Organs,  vegetative,  101,  102 

Oriental  region,  176,  178 

Orioles,  616 

Ornithodelphia,  631 

Ornithorhynchidae,  632 

Ornithorhynchus,  631,  632 

Orohippus,  643 

Oronasal  groove,  538 

Orthis,  328 

Orthoceras,  394 

Orthonectida,  220 

Orthoneurous,  374 

Orthopoda,  597 

Orthoptera,  480 

Orycteropus,  636 

Oscarella,  227 

Oscines,  616 

Osculum,  222,  225 

Os  en  ceinture,  581 

Ossein,  86 

Os  transversum,  590 

Os  turbinale,  620 


688 


INDEX. 


Osmerus,  576 
Osphradium,  354 
Ossicle,  auditory,  127 
Ostariophysi,  575 
Osteoblasts,  88 
Ostracoda,  422 
Ostracodermi,  557,  578 
Ostracoteuthis,  388 
Ostraeidae,  367 
Ostium  tubae,  552 
Ostrich,  613 
Otaria,  648 
Otic  ganglion,  537 
Otica,  521 
Otis.  615 
Otocysts,  236 
Otoliths,  127 
Otter,  647 
Ovibos,  642 
Ovicells,  322 
Ovidae,  642 
Oviducts,  120 
Oviparous,  161 
Ovis,  642 
Ovoid  gland,  331 
Ovoviviparous,  161 
Owen,  18 
Owlet  moths,  495 
Owls.  617 
Ox  warble.  493 
Oxy  haemoglobin,  89 
Oxyrhyncha,  437 
Oxystomata,  437 
Oxyuris,  301 
Oyster  crab,  437 
Oysters,  367 

Pachydermata,  641 
Pachydrilus,  315 
Pachylemuridae,  649 
Paddle  fish,  573 
Psedogenesis,  142,  472 
Paguridea,  436 
Palaearctic  region,  176,  178 
Palaemon,  400.  434 
Palsemonetes,  434 
Palaemonidae,  434 
Palseocrinoidea,  342 


Palaeotherium,  643 
Palaeozoic  era,  180 
Palate,  539 
Palatine  bone,  525 
Paleacrita,  494 
Palechinoidea,  345 
Paleontology,  4 
Paleozoology,  4 
Pali,  256 
Palinuridae,  435 
Palinurus,  436 
Pallial  line,  359 
Pallial  sinus,  360 
Pallium,  351,  534 
Palmate  foot,  614,  615 
Palm  crab,  436 
Palolo,  311 
Palpi,  labial,  362 
Palpus,  430,  463 
Paludicella  324 
Paludinidse,  380 
Pancreas.  1 06 
Pandalus,  434,  435 
Pandion,  617 
Pandionidae,  617 
Pangolin,  636 
Panopeus,  437 
Panorpa,  483 
Panorpidae,  483 
Pantopoda,  456 
Paper  nautilus,  395 
Papilio,  496 
Parachordals,  520 
Paractinopoda,  349 
Paradidymis.  552 
Paradisea,  50 
Paradiseidae,  616 
Paradoxides,  415 
Paraglossa,  464 
Paragnath,  430 
Paramaecium,  206,  207,  209 
Paranuclein,  66 
Paranucleus,  206 
Parapodium,  312,  369 
Parapophysis,  518 
Parapterium,  604 
Paraquadrate  bone,  581 
Parasita,  422 


INDEX. 


689 


Parasitism,  167 

Parasphenoid  bone,  523 

Parasuchia,  602 

Paraxon  gland,  331 

Parietal  bone,  523 

Parietal  foramen,  590 

Parietal  ganglia,  354 

Parietal  organ,  535 

Parostoses,  519 

Parotid  gland,  584 

Parrots,  616 

Parthenogenesis,  142,  145,  472 

Partial  cleavage,  152,  153 

Partridge,  614 

Parypha,  241 

Passer,  616 

Passeres,  616 

Patagium,  637 

Patellidae,  378 

Pathetic  nerve,  536 

Paunch,  641 

Pauropida,  497 

Pauropus,  497 

Pearl  organs,  558 

Pearl  oysters,  367 

Pearls,  361 

Pearls,  artificial,  558 

Pebrine,  218 

Peccaries,  641 

Pecora,  641 

Pecten,  366 

Pecten  of  eye,  611 

Pectinatella,  324 

Pectines,  447 

Pectinibranchia,  379 

Peclinidse,  367 

Pectoral  fin,  562 

Pectoral  girdle,  527 

Pedal  cords,  354 

Pedal  ganglia.  353 

Pedata,  349 

Pedes  spurii,  475 

Pedicellina,  322,  330 

Pediculati,  575 

Pediculus,  491 

Pedipalpi,  448 

Pedipalpus,  445 

Pelagia,  246.  250 


Pelecanus,  615 

Pelecypoda,  358 

Pelicans.  615 

Pelmatozoa,  338 

Pelobatidse,  588 

Pelomyxa,  189 

Peltogaster,  426 

Pelvic  fin,  526,  562 

Pelvic  girdle,  527 

Pen,  389 

Peneidse,  434 

Penella,  422 

Peneus,  434 

Penguin,  615 

Penis,  120 

Pennaria,  241 

Pennatula,  259 

Pennatulidae,  259 

Pentacrinus,  339,  342 

Pentacta,  349 

Pentadactyle  appendage,  529 

Pentamera,  484 

Pentamerus,  328 

Pentastomum,  169,  445 

Pentatomidse,  489 

Pentatoma,  489 

Pentremites,  342 

Perameles,  633 

Peramelidse,  633 

Perca,  574,  577 

Perch,  577 

Percidae,  577 

Perdix,  614 

Pereiopoda,  401 

Perforata,  197,  198 

Peribranchial  chamber,  503,  505 

Pericardial  sinus,  470 

Pericardium,  in,  546 

Perichaeta,  316 

Perichondrium,  86 

Pericolpa,  250 

Peridinium,  203 

Perilymph,  543 

Periosteum,  87 

Peripatidse,  456 

Peripatopsis,  458 

Peripatus,  456,  458 

Peripharyngeal  band,  506 


690 


INDEX. 


Peripheral  nervous  system,   122 

Periphylla,  250 

Periplaneta,  480 

Periproct,  343 

Peripylea,  195 

Perisarc,  233 

Perissodactyla,  640,  641 

Peristome,  209,  343 

Peritoneal  cavity,  546 

Peritoneum,  109,  546 

Periwinkle,  380 

Perla,  479 

Perlidse,  479 

Permian,  180 

Perennibranchiata,  587 

Peromedusse,  250 

Perophora,  510 

Peropoda,  601 

Perradius,  246 

Petaurus,  634 

Petiole,  485 

Petoscolex,  315 

Petromyzon,  557 

Petromyzon,  cleavage  of  egg,  154 

Petromyzontes,  557 

Petrosal  bone,  522 

Phacellse,  246 

Phoenicopterus,  615 

Phoeodaria,  196 

Phseodium,  196 

Phaethon,  615 

Phagocata,  271 

Phalanges,  529 

Phalangida,  450 

Phalangistidse,  634 

Phalangium,  451 

Phallusia,  509 

Pharyngeal  bones,  560,  576 

Pharyngognathi,  576 

Pharynx,  106,  506,  546 

Phascalosoma,  316,  317 

Phascolomyidae,  633 

Phascalomys,  633 

Phascolion,  317 

Phasianella,  379 

Phasianidse,  614 

Phasianus,  614 

Phasmidae,  480 


Phasmomantis,  480 

Pheasants,  614 

Phenacodon,  639 

Phenacodontidse,  643 

Phidippus,  453 

Philichthys,  38 

Philine,  381 

Philonexidse,  395 

Phlegethontias,  495 

Phoca,  648 

Phocidse,  648 

Pholadidse,  368 

Phoronidea,  325 

Phoronis,  325 

Phoxichilidium,  456 

Phragmocone,  389 

Phronima,  439 

Phryganea.  483 

Phrynicus,  448 

Phrynoidea,  448 

Phrynosoma,  599 

Phrynus,  448 

Phthirius,  491 

Phylactolsemata,  324 

Phyllium,  48 

Phyllodactylus,  598 

Phyllopoda,  415 

Phyllosoma,  434,  436 

Phyllostomidse,  638 

Phylloxera,  491 

Phylogeny,  4,  31 

Physalia,  245 

Physeter,  646 

Physiological  character  of  species.  27 

Physiologus,  9 

Physiology,  3 

Physoclisti,  567 

Physonectse,  244 

Physophora,  244 

Physophorse,  244 

Physopoda,  479 

Physostomi,  567,  575 

Phytoflagellata,  202 

Phytophaga,  633 

Picariae,  616 

Picas,  639 

Pickerel,  576 

Picus,  616 


INDEX. 


691 


Pieris,  496 

Pieris,  cleavage  of,  155 
Pigeons,  26,  27,  614 
Pigmented  epithelium,  540 
Pike,  576 
Pilidium,  290,  291 
Pill  bug,  442 
Pineal  eye,  535 
Pinealis,  535 
Pinniped  ia,  647 
Pinnotheres,  437 
Pinnulae,  340 
Pin  worm,  301 
Pipa,  585,  588 
Pipe  fish,  578 
Pisces,  557 
Piscicola,  321 
Pisidium,  368 
Pituitary  body,  535 
Placenta,  634 
Placentalia,  634 
Placoid  scale,  515,  558 
Placophora,  356 
Plagiaulax,  632 
Plagiotremata,  597 
Plagiostomi,  569 
Planaria,  271 
Planarians,  268 
Planipennia,  482 
Plankton,  179 
Planorbis,  383 
Plantigrade,  647 
Plant  lice,  490 
Plants  and  animals,  171 
Planula,  237 
Plasma,  blood,  88 
Plasmic  products,  64.  72 
Plasmodium,  198,  21 6 
Plastin,  66 
Plastogamy,  184 
Plastron,  594 
Platanista,  645 
Plathelminthes,  267 
Platyonichus.  436,  437 
Platyrrhinoe,  651 
Plecoptera,  479 
Plectognathi,  578 
Pleistocene,  181 


Pleopoda,  401,  402 
Plesiosauria,  594 
Plethodon,  587 
Pleura,  414,  462,  546 
Pleuracanthus,  572 
Pleural  cavity,  546 
Pleural  cords,  354 
Pleural  ribs,  518 
Pleurobrachia,  262,  264 
Pleurocercoid,  283 
Pleurodira,  596 
Pleurodont  teeth,  599 
Pleuronectidae,  578 
Pleuroperitoneal  cavity,  546 
Plictolophus,  616 
Pliocene,  181 
Pliohippus,  643 
Pliny,  8 
Plover,  615 
Plumularia,  242 
Plumatella,  324 
Pluteus,  332 
Pneumatic  duct,  567 
Pneumaticity  of  bones,  608 
Pneumatophore,  243 
Pneumodermon,  382 
Pneumogastric  nerve,  536 
Podocoryne,  241 
Podophrya,  68,  212 
Podophthalmia,  427 
Podura,  477 
Poikilothermous,  115 
Polar  bodies,  146 
Pole  field,  263 
Poles  of  egg,  147,  151 
Polian  vesicles,  331 
Polistotrema,  557 
Polybostrichus,  311 
Poly ch setae,  311 
Polychoerus,  269,  271 
Polycladidea,  269,  271 
Polyclinum,  510 
Polyclonia,  247,  251 
Polycystidse,  214 
Polydesmidoe,  497 
Polyergus,  488 
Polygordius,  309.  314 
Polymorphism,  165 


692 


INDEX. 


Polynesia,  177 
Polynoe,  313 
Polynqidae,  313 
Polyodon,  573 
Polyodontidae,  573 
Polyp,  230 
Polyphemidas,  417 
Polypid,  322 
Polypodium,  239,  241 
Polyprododonta,  633 
Polypterus,  573 
Polypterus  tail,  41 
Polyscelis,  271 
Polyspermy,  148 
Polystomeae,  273 
Polystomella,  198 
Polystomum.  273,  274 
Polythalamia,  196,  198 
Polytroche,  309 
Polyzoa,  321 
Poneridge,  487 
Pons  Varolii,  623 
Pontobdella,  321 
Pontodrilus,  308 
Pontella,  421 
Porcellanidae,  437 
Porcellain  crabs,  437 
Porcellio,  442 

Torcellio,  nervous  system  of,  124 
Porcupines,  639 
Pore'lla,  324 
Pori  abdominales,  546 
Porifera,  221 
'Porites.  261 
Porpita,  245 
Portal  vein,  548 
Portuguese  man-of-war,  245 
Portunidae,  437 
Porus  branchialis,  503 
Postabdomen.  401 
Postfrontal  bone,  526,  590 
Postorbital  bone,  590 
Postpermanent  dentition,  625 
Potato  beetle,  485 
Powder  down,  603 
Praeclavia,  528.  622 
Praecoces,  612 
Prairie  dogs,  639 


Praya,  166,  244 
Prefrontal  bone.  526,  590 
Prelacteal  dentition,  625 
Premaxillary  "bone,  525 
Premolar  teeth,  626 
Presphenoid  bone,  522 
Priapuloidea,  317 
Priapulus,  317 
Primaries,  604 
Primary  bone,  519 
Primary  yolk,  80 
Primates,  649 
Primnoa,  259 
Primordial  cranium,  521 
Principal  tissue,  99 
Priodon,  635 
Pristidae,  572 
Pristis,  572 
Proboscidia,  643 
Proboscis,  373 
Procoelous,  519 
Procoracoid,  528 
Proctodaeum,  104 
Procyon,  647 
Proechidna,  631,  632 
Proglottids,  278 
Profeet,  466 

Progression,  principle  of,  55 
Prolegs,  475 
Promorphology,  133 
Pronephric  duct,  550 
Pronephros,  550 
Prong  horn,  643 
Pronotum,  462 
Pronucleus,  149 
Proofs  of  phylogeny,  32 
Proostracum,  389 
Prootic  bone,  522 
Propodium,  369 
Propterygium,  529 
Prorostomus,  644 
Prosencephalon,  533 
Prosimiae,  648 
Prosobranchia,  378 
Prosternum,  528 
Prostoma,   156 
Protamoeba,    189 
Proteroglypha,  601 


INDEX. 


693 


Proteroglyphic  tooth,  600 
Proteus,  587 
Prothorax,  462 
Protobranchiata,  365 
Protista,  186 
Protocaris,  416 
Protocerebrum,  462,  468 
Protoconchise,  365 
Protodonta,  632 
Protohydra,  239,  241 
Protomerite,  214 
Protonemertini,  291 
Protonephridia,  115 
Protoplasm,  61,  80 
Protoplasm,  discovery  of,  59 
Protoplasm,  movement  of,  62 
Protopterus,  579 
Prototheria,  631 
Protovertebrse,  531 
Protozoa,  183 
Prortacheata,  408,  456 
Protula,  313 
Proventriculus,  467 
Psammonyx,  198 
Pseudelectric  organs,  563 
Pseudobranch,  570 
Pseudocuticula,  597 
Pseudolamellibranchiata,  365 
Pseudonavicellae,  215 
Pseudoneuroptera,  477 
Pseudopodia,  187 
Pseudoscorpii,  450 
Pseudosuchia,  602 
Psittaci,  616 
Psittacus,  616 
Psocidae,  479 
Psolus,  349 
Psorosperms,  217 
Pteranodon,  602 
Pteraspis,  557 
Pterichthys,  557 
Pterodactylia,  602 
Pteronarcys,  479 
Pteropoda,  382 
Pteropod  ooze,  382 
Pteropus,  638 
Pterosauria,  602 
Pterotic  bone,  522,  560 


Pterotracheidae,  380 
Pterygoid  bone,  525 
Pterygoid  process,  622 
Pterygoquadrate,  524 
Pterygotus,  444 
Pterylae.  603 
Pubic  bone,  528 
Pugettia,  437 
Pulex,  493,  494 
Pulmonata,  383 
Pulmonary  artery,  549 
Pulmonary  circulation,  549 
Pulmonary  vein,  549 
Pulp  cavity,  515 
Pulvilla,  493 
Puma,  647 
Pupa,  383 
Pupae,  474 
Pupipara,  493 
Purpura,  379,  380 
Putorius,  647 
Pycnogonida,  456 
Pygidium,  414 
Pyloric  caeca,  565 
Pylorus,  546 
Pyrosoma,  510 
Pyrula,  373 
Python,  601 
Pythonaster,  337 
Pythonomorpha,  600 

Quadrula,  196,  198 
Quadrumana,  650 
Quadrate  bone,  525 
Quahog,  368 
Quail,  614 
Quaternary,  181 

Raccoon,  647 
Rachis,  603 
Racemose  glands,  77 
Radial  canals,  235,  331 
Radial  symmetry,  135 
Radiale,  529 
Radialia,  330,  340,  527 
Radiata,  228,  329 
Radiolaria,  192 
Radius,  529 


694 


INDEX. 


Radula,  355,  373 

Raia,  571,  572 

Raiidae,  572 

Rail,  615 

Rainey's  corpuscles,  218 

Rallus,  615 

Rana,  588 

Ranatra,  489 

Rangifer,  642 

Ranvier,  nodes  of,  96 

Raptores,  616 

Raptorial  foot,  614 

Rasorial  foot,  6*4 

Rasor  clam,  368 

Rathke,  1 8 

Ratitse,  612 

Rats,  639 

Rat-tail  larva,  493 

Rattlesnake,  601 

Ray,  10,  20 

Reamur,  13 

Receptaculum  seminis,  120,  471 

Rectrices,  604 

Rectum,  461 

Red  coral,  256 

Redia,  276 

Reduviidae,  489 

Regulares,  345 

Reindeer,  642 

;Remak,  18 

Remiges,  604 

Remora,  577 

Renilla,  258,  259 

Reproduction,  asexual,  140,  143 

Reproduction,  sexual,  142 

Reptilia,  588 

Respiratory  organs,  107 

.Respiratory      organs     of     vertebrates, 

547 

Reticularia,  196 
Reticulum,  642 
Retina,  129,  131 
Retina  of  vertebrates,  540 
Retinaculum,  494. 
Retinula,  405 
Retitelarise,  453 
Rhabdites,  270 
Rhabditis,  300 


Rhabdoccelida,  269,  271 
Rhabdom,  129,  405 
Rhabdonema,  145,  300 
Rhabdopleura,  514 
Rhachiglossa,  380 
Rhachis,  414 
Rhamphastos,  616 
Rhea,  613 
Rhegmatodes,  242 
Rhinoceros,  641 
Rhinocerotidse,  641 
Rhinoderma.  585 
Rhizocephala,  426 
Rhizocrinus,  342 
Rhizopoda,  187 
Rhizostomeae,  250 
Rhopalocephalus,  213 
Rhopalocera,  495 
Rhopalonema,  234 
Rhynchobdellidae,  321 
Rhynchobothrium,  286 
Rhynchocephalia,  596 
Rhynchonella,  325,  328 
Rhynchophora,  485 
Rhynchota,  489 
Rhytina,  645 
Rib,  517,  518 
Right  whale,  646 
Ring  canal,  331 
Rocky  Mountain  sheep,  643 
Rodentia,  638 
Rods  and  cones,  129,  540 
Root  barnacles,  426 
Rorqual,  646 
R5sel  von  Rosenhofen,  13 
Rossia,  395 
Rostellum,  280 
Rostrum,  389,  424,  465 
Rotalia,  188,  198 
Rotatoria,  293 
Rotifera,  293 
Round  worms,  298 
Rove  beetles,  484 
Rudistidse,  368 
Rugosa,  258 
Rumen,  641 
Ruminantia,  641 
Rupicapra,  642 


INDEX. 


695 


Sabellidse,  313 
Sabinea,  435 
Sable,  647 
Sacconereis,  311 
Sacculina,  426 
Sacculus,  128,  542 
Saccus  vasculosus,  563 
Sacral  ribs,  528 
Sagartia,  259 
Sagitta,  296 
Sagitta  (ear  bone),  564 
Sagitta.  development  of,  158 
St.  Hilaire,  14,  22 
Salamandra,  585,  587 
Salamindrina,  587 
Salinella,  220 
Salivary  glands,  106 
Saltatoria,  480 
Saltigrada,  453 
Salmo,  576 
Salmon,  576 
Salmonidae,  576 
Salpa,  510,  512 
Salpceformes,  510 
Sand  dollar,  345 
Sand  saucers,  380 
San  Jose  scale  insect,  490 
Sapajous,  651 
Sapphirina,  421 
Sarcocystis,  213,  218 
Sarcode,  60 
Sarcolemma,  93 
Sarcophaga,  493 
Sarcophilus,  633 
Sarcopsylla,  494 
Sarcoptes,  454 
Sarcosepta,  255 
Sarcosporida,  218 
Sarsia,  241 
Saurii,  598 
Sauropsida,  588 
Saururae,  612 
Savigny,  14 
Savigny's  law,  401 
Sawfish,  572 
Sawflies.  485,  486 
Saxicava,  367 
Saxicavidae,  368 


Scala  media,  543 
Scala  tympani,  543 
Scala  vestibuli,    543 
Scale  insects,  490 
Scale,  placoid,  515 
Scales  of  fishes,  515,  558 
Scales  of  reptiles,  597 
Scallops,  637 
Scalpellum,  424 
Scansores,  616 
Scansorial  foot.  614 
Scape,  603 
Scaphander,  381 
Scapharca,  367 
Scaphiopus,  588 
Scaphognathite,  431 
Scaphopoda,  369 
Scapula,  528 
Scarabseidse,  484 
Schafter,  13 
Schizodont  hinge,  359 
Schizopoda,  428 
Schizopodal  appendages,  409 
Schizosomi,  437 
Sclerophylla,  261 
Schleiden-Schwann  theory,  58 
Schwann,  sheath  of,  96 
Scincidse,  599 
Sciuridse,  639 
Sciuromorpha,  639 
Sciuropterus,  639 
Sciurus,  639 
Sclera.  131,  539 
Scleral  bones,  611 
Sceleporus,  599 
Scelrophyllia,  257 
Sclerosepta,  255 
Sclerotic,  539 
Sclerotic  bones,  593 
Sclerotic  coat,  130,  131 
Sclerotomes,  531 
Scolex,  278 
Scollops,  367 
Scolopax,  615 
Scolopendra,  460,  461 
Scolopendrelta,  497 
Scolopendridse,  461 
Scomber,  577 


696 


INDEX. 


Scombridse,  577 

Scops,  617 

Scorpionida,  447 

Sculpin,  577 

Scutellum,  489 

Scutibranchia,  378 

Scutigera,  461 

Scutigeridse,  461 

Scutum,  424 

Scyphomedusse,  245 

Scyphopolyp,  230 

Scyphostoma,  245,  246 

Scyphozoa,  245 

Sea  anemones,  251,  259 

Sea  cucumbers,  346 

Sea  fans,  259 

Seahorse,  578 

Sea  lion,  648 

Sea  pens,  259 

Sea  otter,  647 

Sea  snakes,  601 

Sea  squirts,  505,  508 

Sea  urchin,  fertilization  of,  149 

Sea  urchins,  343 

Sea  whips,  259 

Seals,  648 

Secodont  teeth,  626 

Secondaries,  604 

Secondary  bones,  515 

Secreta,  73 

Sedentaria,  313,  453 

Segmental  organs,  116,  308 

Segmentation  cavity,  155 

Segmentation  of  egg,  149,  151 

Segments  of  head,  536 

Selachii,  570 

Selection,  artificial,  43 

Selection,  natural,  44 

Selection,  sexual,  46 

Selenodont  teeth,  626 

Semaeostomae,  250 

Semicircular  canals,  128,  542 

Semilunar  valves,  567 

Semipalmate  foot,  614 

Semiplumes,  604 

Sensations,  125 

Sense  organs  of  vertebrates,  537 

Senses,  125 


Sensory  epithelium,  73,  82 

Sensory  organs,  125 

Sepia,  386,  388,  395 

Septibranchiata,  368 

Septum,  306 

Sericteria,  494 

Serosa,  473 

Serpulidae,  313 

Serranidse,  577 

Serripes,  368 

Sertularia,  242 

Serum,  blood,  88 

Sesiidae,  495 

Seventeen-year  locust,  489,  490 

Sexual  cells,  143 

Sexual  epithelium,  78 

Sexual  glands,  78,  80 

Sexual  organs,  80,  117 

Sexual  organs  of  vertebrates,  550 

Sexual  reproduction,  142,  145 

Sexual  selection,  49 

Shad,  576 

Shagreen,  569 

Sharks,  571 

Sheath  of  Schwann,  96 

Sheep,  642 

Sheep  tick,  493 

Shell  gland,  411 

Shell,  layers  of,  361 

Ship  worms,  368 

Shore  crab,  437 

Shoulder  blade,  528 

Shoulder  girdle,  527 

Shrews,  637 

Shrimp,  mantis,  429 

Shrimp,  opossum,  428 

Siala,  616 

Sialidse,  482 

Sialis,  482 

Sicyonia,  434 

Siderone,  47 

von  Siebold,  18 

Silenia,  368 

Silicispongise,  226 

Siliqua,  367 

Silkworms,  495 

Silurian,  180 

Siluridae,  576 


IJSDEX. 


691 


Silverfish,  477 
Simia,  651 
Simiidse,  651 
Simuliidge,  493 
Sinupalliata,  368 
Sinus  frontalis.  539 
Sinus,  sphenoid,  539 
Siphon,  345,  387,  360 
Siphonaptera,  493 
Siphonophora,  240,  243 
Siphonophores,  166 
Siphonostomate,  371 
Siphonostomata,  422 
Siphuncle,  388 
Sipunculoida,  317 
Sipunculus,  317 
Siredon,  36 
Siren,  586 
Sirenia,  644 
Sirex,  485 
Siricidse,  486 
Sixth  sense,  125,  538 
Skalis,  571 
Skin,  76 
Skull,  519 

Skull  of  mammals,  619 
Skunk,  647 
Skylark,  616 
Slime  animals,  198 
Slime  eels,  557 
Slime  moulds,  198 
Sloths,  636 
Smell,  organs  of,  126 
Smelt,  576 
Snakes,  600 
Snapping  turtle,  596 
Snout  beetles,  485 
Snow  flea,  477 
Social  animals,  167 
Soft- shell  crab,  437 
Soft-shelled  turtle,  596 
Solasteridse,  337 
Sole,  578 
Solemyidse,  367 
Solen,  368 
Solenoconchse,  369 
Solenogastres,  358 
Solenoglypha,  60 1 


Solenoglyphic  tooth,  600 

Solenidse,  368 

Solidungula,  641 

Solifugae,  449 

Solpuga,  450 

Solpugida,  449 

Somatic  cells,  143 

Somatic  layer,  159 

Somatopleure,  159 

Somites,  305 

Song  birds,  616 

Sorex,  637 

Soricidse,  637 

Sowbug,  442 

Spadella,  298 

Spadix,  238 

Spanish  flies,  484 

Span  worms,  494 

Spatangoidea,  346 

Species,  10 

Species,  nature  of,  19,  25 

Species,  physiological  characters  of,  27 

Spelerpes,  587 

Speotyto,  617 

Spermaceti,  646 

Spermatophore,  392 

Spermatozoa,  81 

Spermatozoids,  202 

Sperm  nucleus,  149 

Sperm  whale,  646 

Sphaeridia,  330 

Sphasrogastrida,  451 

Sphseroma,  442 

Sphaeromidae,  442 

Sphaerophrya,  212 

Sphaerozoidae,  195 

Sphargis,  595 

Sphenethmoid  bone,  581 

Sphenodon,  596 

Sphenoidalia,  521 

Sphenoid  bone,  523,  620 

Sphenoid  sinus,  539,  624 

Sphenopalatine  ganglion,  537 

Sphenotic  bone,  522 

Spherical  animals,  135 

Sphingina,  495 

Sphyranura,  274 

Spicula,  300 


698 


INDEX. 


Spicules  of  sponges,  225 
Spider  crab,  436,  437 
Spider  monkeys,  651 
Spiders,  451,  452 
Spinal  canal,  517 
Spinal  ganglion,  533 
Spindle,  directive,  146 
Spindle  fibres,  69 
Spindle,  nuclear,  68 
Spinnerets,  452 
Spinous  process,  517 
Spiny  ant  eaters,  632 
Spiny  lobster,  436 
Spiracle,  459,  524,  544 
Spiral  valve,  565 
Spirifer,  328 
Spirorbis,  313 
Spirobolus,  497 
Spirula,  388,  394 
Spirulidse,  394 
Spittle  bug,  489 
Splanchnic  layer,  159 
Splanchnopleure,  159 
Spleen,  550 
Splenial  bone,  582 
Splint  bones,  640 
Spondyhdse,  367 
Sponge,  fresh-water,  133 
Sponges,  221 
Spongida,  221 
Spongilla,  133,  221,  227 
Spongillidae,  227 
Spongioplasm,  61 
Spontaneous  generation,  31 
Sporangia,  199 
Spores,  213,  215 
Sporoblasts,  185,  213,  215 
Sporocyst,  276 
Sporosacs,  238 
Sporozoa,  213 
Sporozoites,  185,  213,  215 
Springtails,  477 
Sprinkling-pot  shell,  368 
Spumellaria,  195 
Squali,  571 
Squalus,  571 
Squamata,  597,  636 
Squamosal  bone,  526 


Squid,  395 
Squilla,  429 
Squirrels,  639 
Staggers,  493 
Stapes,  525 
Staphylinidse,  484 
Starfish,  333 
Statoblasts.  227,  323 
Statoliths,  128 
Stauromedusse,  250 
Steganopodes,  615 
Stegocephali,  586 
Stegosaurs,  597 
Stellate  ganglion,  390 
Stelmatopoda,  323 
Stemma,  403 
Stenops,  649 
Stenson*s  duct,  539 
"Stentor,  209 
Stephalia,  244 
Stephanocyphus,  250 
Sterna,  615 
Sternaspis,  314 
Stercoral  pocket,  445 
Sternum,  462,  518 
Sticklebacks,  577 
Stigmata,  459 
Sting,  472,  486 
Sting  rays,  572 
Stipes,  463 
Stink  bug,  489 
Stolo  prolifer,  512 
Stomach,  106 
Stomatopoda,  429 
Stomodaeum,  104 
Stomolophus,  251 
Stone  canal,  330 
Stone  flies,  479 
Storks,  615 

Stratified  epithelium,  73,  74 
Stratum  corneum,  76,  514 
Stratum  Malpighi,  76,  514 
Streaming  of  protoplasm,  62,  188 
Strepsiptera,  483 
Streptoneury,  373 
Stridulating  organs,  469 
Striges,  617 
Strix,  617 


INDEX. 


699 


Strobila,  249,  278 

Strongylidse,  301 

Strongyloides,  300 

Strongylocentrotus,  345 

Strongylosoma,  497 

Struggle  for  existence,  44 

Struthio,  613 

Struthiones,  613 

Sturgeon,  573 

Sturgeon,  tail  of,  41 

Stylaster,  241 

Style,  crystalline,  364 

Stylochus,  271 

Stylohyoid  ligament,  621 

Styloid  process,  621 

Stylommatophora,  383 

Stylonychia,  211,  212 

Stylopidse,  483 

Stylops,  483 

Subcutaneous  tissue,  514 

Subintestinal  ganglion,  374 

Subintestinal  vein,  548 

Submentum,  464 

Subneural  gland,  509 

Subumbrella,  234 

Suckers,  576 

Suck  fish,  577 

Suctoria,  212 

Suidse,  641 

Sulci,  535 

Summer  eggs,  416 

Sun  animalcules,  190 

Supporting  cells,  83 

Supporting  layer,  230 

Supraintestinal  ganglion,  374 

Supraoccipital  bone,  522 

Supracesophageal  ganglion,  123 

Suprascapula,  528 

Surf  perch,  577 

Sus,  641 

Swallows,  616 

Swallow  tails,  496 

Swammerdam,  13 

Swans,  615 

Swarm  spores,  185,  195 

Sweat  glands,  618 

Swell  fish,  578 

Swim  bladder,  547 


Swimming  bell,  243 
Swimming  birds,  614 
Swine.  641 
Sword  fish,  577 
Sycandra,  222,  225 
Sycon,  223,  225 
Sy cones,  225 
Syllidse,  313 
Syllis,  311 
Sylvicolidse,  616 
Sylvius.  12 
Symbiosis,  169 
Symmetry,  134 
Sympathetic  coloration,  46 
Sympathetic  system,  537 
Symplectic  bone,  561 
Symphyla.  497 
Synapta,  349 
Synapticula,  257 
Synascidise,  510 
Syncitia,  71 
Synccelidium,  269,  271 
Syncoryne,  241 
Synentognathi,  575,  576 
Syngamus.  301 
Syngnathus,  578 
Syringopora,  259 
Syrinx,  608 
Syrphidse,  493 
Systems  oi  organs,  100 
Systemic  circulation,  549 
Systemic  heart,  391 

Tabanidae,  493 

Tabulae,  257 

Tabulatse,  257 

Tactile  bristles,  126 

Tactile  corpuscle,  537 

Tactile  organs,  125 

Tadpole,  586 

Tadpoles  of  Rana  temporaria,  35 

Taenia.  169,  279,  282 

Tseniadas,  287 

Tseniolse,  246 

Tsenioglossa,  380 

Talpa,  637 

Talpidse,  637 

Tanais,  442 


700 


INDEX. 


Tanystoma,  492 

Tapetum  nigrum,  131,  540 

Tape  worms,  285 

Tapiridae,  641 

Tapirs,  641 

Tapirus,  641 

Tarantula,  453 

Tardigrada,  455,  636 

Tarsal  bones,  529 

Tarsus,  463 

Tarsiidse,  649 

Tarsius,  649 

Tarso-metatarsus,  607 

Tasmanian  devil, '633 

Taste,  organs  of,  126 

Taste  organs  of  vertebrates,  538 

Tatusia,  636 

Tautoga,  576 

Taxidea,  647 

Taxodont  hinge,  359 

Tectibranchia,  381 

Tectrices.  604 

Teeth,  dermal,  515 

Teeth  of  mammals,  624 

Teeth  of  vertebrates,  547 

Tejidse,  599 

Tejus,  599 

Telea,  495 

Teleostei,  574 

Teleostomi,  569 

Tellina,  368 

Tellinidse,  368 

Telolecithal  eggs,  152 

Telson,  427 

Telotroche,  309 

Temporal  bone,  620 

Temperature  of  mammals,  630 

Tendinous  tissue,  85 

Tenebrionidse,  484 

Tentacles,  gastral,  246 

Tentaculata,  264 

Tent  caterpillars,  495 

Tenthredinidse,  486 

Terebella,  108,  313 

Terebellidae,  313 

Terebra,  486 

Terebrantia,  486 

Terebratulina,  328 


Teredo,  368 
Teredidae,  368 
Tergum,  424 
Termes,  478 
Termitidse,  478 
Terns,  615 
Terrapin,  596 
Terricola,  315 
Tertiary,  181 
Tessellata,  342 
Tesseridoe,  250 
Testicardines,  328 
Testudo,  596 
Testudinata,  594 
Testudinidae,  596 
Tethyoidea.  508 
Tetrabothrium,  286 
Tetrabranchia,  394 
Tetracoralla.  258 
Tetractinellidae,  227 
Tetramera,  484 
Tetraonidae,  614 
Tetrapneumones,  453 
Tetrapoda,  555 
Tetrarhynchidse,  286 
Tetrarhynchus,  281,  286 
Tetrastemma,  290,  291 
Tetrasticta,  453 
Tetraxonia,  226 
Tettix,  481 
Thalamophora,  196 
Thalamus,  534 
Thalassicola,  192 
Thalassicolidce,  195 
Thalassima,  317 
Thalassochelys,  596 
Thaliacea,  510 
Thamnocnida.  242 
Thaumantia,  242 
Theca,  255,  338 
Thecasomata,  382 
Thelepus,  313 
Thelyphonida,  448 
Thelyphonus,  448 
Theridium,  453 
Theriodonta,  594 
Theromorpha,  594 
Thoracici,  562 


INDEX. 


701 


Thoracic  fin,  526,  562 
Thoracostraca,  427 
Thread  cells,  229 
Thrips,  479 
Thrushes,  616 
Thylacinus,  633 
Thymus  gland,  547,  577 
Thyone,  349 
Thyroid  gland,  547 
Thysanoptera,  479 
Thysanozoon,  271 
Thysanura,  477 
Tiara,  236 
Tiaris.  242 
Tibia,  463,  529 
Tibiale,  529 
Tibio-tarsus,  607 
Ticks,  454 
Tick,  sheep,  493 
Tiedemann's  vesicles,  331 
Tiger,  647 
Tiger  beetles,  484 
Tillodontia,  643 
Tim  a,  242 
Tinea,  494 
Tineidse,  494 
Tipulidae,  492 
Tissues,  71 

Tissues,  accessory,  99 
Tissues,  classification  of,  72 
Tissues,  connective,  83 
Tissues,  elastic,  85 
Tissues,  epithelial,  73 
Tissues,  muscular,  91 
Tissues,  nervous,  94 
Tissues,  principal,  99 
Tissues,  tendinous,  85 
Toad,  horned,  599 
Toads,  588 
Tobacco  worm,  495 
Tocogony,  140 
Tomato  worm,  495 
Tongue  bone,  524 
Toothed  birds,  612 
Tooth  shells,  369 
Tornaria,  513 
Torpedinidse,  572 
Torpedo,  572 


Tortoises,  594 
Tortoise  shell,  596 
Tortricidse,  494 
Total  cleavage,  152 
Totipalmate  foot,  614,  615 
Toucans,  616 
Toxiglossa,  380 
Toxodontia.  643 
Toxopneustes,  345 
Trabeculae,  520 
Trachea.  109,  443,  458,  547 
Tracheal  gills,  469 
Trachydermon,  357 
Trachymedusse,  239,  242 
Trachynema,  242 
Tractus  olfactorius,  534 
Tragulidse,  642 
Tragulus,  642 

Transverse  commissure,  123 
Transverse  process,  518 
Trapdoor  spider,  453 
Tree  cricket,  481 
Tree  hoppers,  490 
Tree  toads,  588 
Trematoda,  271 
Triarthus,  415 
Triassic,  180 
Triaxonia,  226 
Trichechidae.  648 
Trichechus,  648 
Trichina,  302 
Trichocephalus,  302 
Trichocysts,  205 
Trichodectes,  479 
Trichomonas,  202 
Trichoplax,  220 
Trichoptera,  483 
Trichotrachelidse,  302 
Triclalidse,  269,  271 
Triconodont  teeth,  626 
Tridacna,  367 
Trigeminal  nerve,  536 
Trilobitse,  414 
Trimera,  485 
Trionychia,  596 
Tristicta,  453 
Tristoma,  274 
Tritocerebrum,  462,  468 


7.02 


INDEX. 


Triton,  sections  of  embryo,  39 

Tritonidse,  382 

Tritubercular  teeth,  626 

Tritylodon,  632 

Trivium,  334 

Trochal  disc,  293 

Troc banter,  463 

Trochidse,  379 

Trochilida;,  616 

Trochilus,  616 

Trochlear  nerve,  536 

Trochophore,  306 

Trochosa,  453 

Trochus,  379 

Troctes,  479 

Troglodytidse,  616 

Troglodytes,  651 

Trombididae,  454 

Trophi,  294 

Tropic  birds,  615 

Tropidonotus,  601 

Trout,  576 

Trunk  fish,  578 

Trutta,  tail  of,  41 

Trygonidae,  572 

Tubicola,  313 

Tubificidae,  315 

Tubinares,  615 

Tubifex,  315 

Tubiporidse,  259 

Tubitelariae,  453 

Tubular  glands,  77 

Tubular  nervous  system,  124 

Tubulariae,  239,  241,  242 

Tubulipora,  324 

Tunic,  505 

Tunicata,  505 

Turbellaria,  268 

Turbinated  bone,  620 

Turbinidse,  379 

Turbo,  379 

Turbot,  578 

Turbidse,  616 

Turd  us,  616 

Turkey,  614 

Turkey  buzzard,  617 

Turritopsis.  240,  242 

Turtles,  594 


Twixt  brain,  534 
Tylenchus,  300 
Tylopoda,  643 
Tympanal  organ,  128,  468 
Tympanic  annulus,  544 
Tympanic  bone,  526 
Tympanic  cavity,  621 
Tympanic  membrane,  544 
Tympanum,  544 
Type  theory,  15 
Typhline,  599 
Typhlops,  60 1 
Tyrian  purple,  380 
Tyrannidae,  616 

Uca,  437 

Uintatherium,  643 

Ulmaris,  247 

Ulna,  529 

Ulnare,  529 

Umbilicus,  370,  554 

Umbo,  358 

Umbrella,  234 

Uncinate  process,  605 

Ungues,  618 

Ungulae,  618 

Ungulata,  639 

Unguligrade,  640 

Unicellular  glands,  77 

Unicorn,  646 

Unio,  367 

Unionidae,  367 

Ureter,  550 

Urinary  bladder,  552 

Urinator,  615 

Urinatores,  615 

Urnatella,  322 

Uroceridse,  486 

Urochorda,  505 

Urodela,  587 

Urogenital  sinus,  553 

Urogenital  system,  120 

Urogenital  system  of  vertebrates,  550 

Urosalpinx,  379,  380 

Ursidse,  647 

Ursus,  647" 

Use  and  Disuse,  55,  99 

Uterus,  120,  629 


INDEX. 


703  ; 


Utriculo-saccular  duct,  542 
Utriculus,  128.  542 

Varanus,  599 

Vacuole,  contractile,  183 

Vacuole,  food,  183 

Vagabundas,  453 

Vagina,  120,  629 

Vagus  nerve,  536 

Valkeria,  324 

Vampyre,  638 

Vanessa,  496 

Varan  id  oe,  599 

Variation,  25 

Vas  deferens,  120 

Vasa  Malpighii,  461 

Vascular  arches,  504 

Vater  Pacinian  corpuscles,  126 

Vegetative  organs,  101 

Vegetative  pole,  147,  151 

Veins,  112 

Velella,  245 

Veliger,  355,  364 

Velum,  235,  356 

Veneridae,  368 
Ventral  aorta,  548 

Ventral  fin,  562 

Ventral  nerve  cord,  123 

Ventricles  of  brain,  534 

Ventricle  of  heart,  in,  548 

Venous  sinus,  567 

Venus,  368 

Venus'  flower  basket,  226 

Venus'  girdle,  264 

Vermiform  appendix,  627 

Vermilinguia,  599,  636 

Vermes,  535 

Vertebra,  518 

Vertebra  (of  ophiuroids),  337 

Vertebral  column,  516 

Vertebrata,  514 

Vertex,  462 

Vesal,  12 

Vesicle,  auditory,  127 

Vesicle,  blastodermic,  155 

Vesicle,  germinal,  146 

Vesicle,  Polian,  331 

Vesicle,  Tiedemann's.  331 


Vesicularia,  324 
Vesicula  seminalis,  120 
Vespariae,  487 
Vesperlilionidse,  638 
Vesperugo,  638 
Vibracularia,  323 
Vibrissae,  618 
Viperidce,  601 
Visceral  ganglia,  353 
Visceral  sac,  351 
Visceral  skeleton,  523 
Vitellarium,  267 
Vitreous  body,  130 
Vitrodentine,  558 
Viviparous,  161 
Vogt.  24 
Volutidae,  380 
Volvocina,  202 
Volvox,  202 
Vomer,  525 

Vortex,  anatomy  of,  I2O 
Vorticella.  211 
Vorticellidae,  210 

Wading  birds,  615 

Wagner,  24 

Waldheimia,  326 

Walking  stick,  480 

Wallace,  24 

Wallace's  line,  177 

Walrus,  648 

Warblers,  616 

Warm-blooded  animals,  114 

Wasps,  487 

Water  bears.  455 

Water  beetles,  484 

Water  scorpion,  489 

Water  snake,  601 

Water-vascular  system,  115,  330 

Weasel,  647 . 

Weevils,  485 

Weismann,  24 

Whalebone,  645 

Whales,  645 

Whelks,  380 

White  ants,  478 

White  fish,  576 

White  matter,  124,  532 


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